The interactions of water with cellulose- and starch-derived pharmaceutical excipients

Water associated with polymeric pharmaceutical excipients derived from cellulose and starch can have a profound effect on the properties of the excipient and on the other ingredients making up a solid dosage form. Important questions which need to be addressed include How much water will be sorbed or desorbed at various relative humidities and temperatures? and What is the thermodynamic state of water associated with the solid as a function of moisture content? A critical review of the literature is presented to demonstrate the most likely answers to these questions. It appears that water exists in at least three thermodynamic states in starch, cellulose, and their derivatives: (1) water directly and tightly bound, with a stoichiometry of one water molecule per anhydroglucose unit; (2) water in a relatively unrestricted form, approaching the properties of bulk or pure liquid water; and (3) water in an intermediate state or states, with properties reflecting a much higher level of structure than bulk water but less than that of tightly bound water. Some implications of such behavior for pharmaceutical systems are discussed.

A study of amorphous molecular dispersions of indomethacin and its sodium salt

Amorphous solid dispersions of indomethacin (IMC) and sodium indomethacin (NaIMC) over a range of compositions were prepared by physically mixing amorphous IMC and amorphous NaIMC, as well as by coprecipitation from methanol solution. Measurement of glass transition temperatures, T(g), for the physical mixtures revealed two values indicating, as expected, phase separation. In contrast, all samples of coprecipitated materials exhibited one value of T(g), which was greater than that predicted for ideal miscibility in the formation of a molecular dispersion. Such nonideality suggests a stronger acid-salt interaction in the amorphous state than that between acid-acid and salt-salt. FTIR spectroscopic analysis provides evidence for interactions between NaIMC and IMC through a combination of hydrogen bonding and ion-dipole interactions between the carboxylic group of the acid and the carboxylate anion of the salt. The inhibition of isothermal crystallization of IMC by NaIMC only when in molecular dispersion is believed to result from the interaction between the acid and the salt, which prevents the formation of hydrogen-bonded carboxylic acid dimers for IMC, required for the formation of crystal nuclei and crystallization.

Water vapor absorption into amorphous sucrose-poly(vinyl pyrrolidone) and trehalose-poly(vinyl pyrrolidone) mixtures

Previous studies from this laboratory suggested that a solution model (Flory-Huggins equation) modified by a free volume model (Vrentas equation) could satisfactorily describe water absorption into an amorphous solid composed of a sugar or a polymer. This paper has extended the studies of single solutes to binary mixtures of trehalose-and sucrose-poly(vinyl pyrrolidone) (trehalose-PVP and sucrose-PVP, respectively) either co-lyophilized or individually lyophilized and then physically mixed. Water vapor absorption isotherms of the binary mixtures were determined at 30 degrees C. Co-lyophilized PVP-sugar mixtures take up essentially the same amount of water as predicted by the weight average of individual isotherms, whereas sugar crystallization is significant retarded in the molecular dispersions. The sugar-PVP interaction, as reflected in the Flory-Huggins chi interaction parameter, was estimated by fitting the high relative pressure (p/p(0)) region of the isotherm, at which the system is in a liquid state, with a three-component Flory-Huggins-type model. The estimated sugar-water PVP-water, and sugar-PVP interaction parameters suggest that the solute-water interactions are not significantly affected by the sugar-PVP interaction; that is, the solute-water interaction parameters in a binary solute system are similar to those in the corresponding single solute systems. Based on these interaction parameters, the sucrose-PVP interaction appears to be stronger than that of trehalose-PVP. Manipulation of the interaction parameters suggest that the water vapor absorption isotherm is not a sensitive indicator of possible sugar-PVP interactions. Density, glass transition temperature, T(g), and the heat capacity change, DeltaC(p), at T(g) were determined to estimate the excess water absorption energy due to the plasticizing effect of water using the structural relaxation model, as described by Vrentas. Results suggest that PVP is a better antiplasticizer for sucrose than for trehalose. Consequently, the excess free energy arising from structural relaxation was disproportionally reduced by the presence of PVP in these molecular dispersions. Finally, the entire isotherms of co-lyophilized sugar-PVP mixtures are reasonably described with an extended three-component Flory-Huggins model and Vrentas glass structural relaxation model.

Fourier transform Raman spectroscopic study of the interaction of water vapor with amorphous polymers

Water associated with amorphous polymers is known to affect their chemical and physical properties. The purpose of this study was to investigate the nature of water-polymer interactions for some polymers of pharmaceutical interest. Using Raman spectroscopy, polymer-water hydrogen bond interactions were probed for two molecular weight grades of poly(vinylpyrrolidone), namely PVP K90 and PVP K12, and also for poly(vinylacetate) and poly(vinyl pyrrolidone-co-vinyl acetate). Water vapor absorption isotherms were obtained for the polymers, and the effect of the absorbed water on the glass transition temperature was determined. A knowledge of the water content and physical state of the polymer was used to aid interpretation of Raman spectral changes. The strength of the hydrogen bond formed with water was found to depend on the chemistry of the polymer, with the pyrrolidone group interacting more strongly than the acetate group. However, minor differences were also observed between the degree of interaction of water and polymer for PVP K12 and PVP K90 at some water contents. This result is attributed to differences in the structural relaxation changes accompanying plasticization by water for the two molecular weight grades. Using principal components analysis of the spectral data, it was also possible to differentiate between samples in the rubbery state and samples in the glassy state. In conclusion, water sorbed into polymers causes changes in the polymer Raman spectra not only because of hydrogen bonding, but also as a result of the plasticizing effect of water on polymer mobility.

Effects of lyophilization on the physical characteristics and chemical stability of amorphous quinapril hydrochloride

PURPOSE

To prepare amorphous quinapril hydrochloride (QHCl) by lyophilization and to compare its physical characteristics and chemical stability as a function of the initial pH of the pre-lyophilized solution.

METHODS

Amorphous QHCl samples were prepared by lyophilization from aqueous solutions. Solid-state characteristics were evaluated by DSC, PXRD, and optical microscopy. Chemical degradation was monitored by an HPLC assay.

RESULTS

Amorphous QHCl samples obtained from lyophilization exhibited variable glass transition temperatures, depending on the pH and/or concentration of the starting aqueous solutions. Neutralized quinapril (Q) in the amorphous form, which has a Tg of 51 degrees C, lower than that of its HCl salt (91 degrees C), was significantly more reactive than QHCl at the same temperature. The Tg of lyophilized samples prepared at various initial pH values correlated well with values predicted for mixtures of QHCl and Q. Their different reaction rates were related to their glass transition temperature, consistent with the results from earlier studies obtained with amorphous samples made by precipitation from an organic solution and grinding of the crystal solvate.

CONCLUSIONS

Lyophilization of different QHCl solutions produces mixtures of amorphous QHCl and its neutralized form Q, with Tg values intermediate to the values of QHCl and Q. As the fraction of Q increases the overall rate of chemical degradation increases relative to QHCl alone, primarily due to the increase in molecular mobility induced by the plasticizing effects of Q.

The relationship between "BET" and "free volume"-derived parameters for water vapor absorption into amorphous solids

Water vapor absorption isotherms for amorphous solids with the same chemical composition but differing in molecular weight (i.e., PVP-90, PVP-30, and PVP-12), and for glucose, trehalose, and two molecular weight grades of dextran were obtained at 30 degrees C and analyzed using the Brunauer-Emmett-Teller (BET) equation to obtain the parameters, W(m) and C(B). Similar analyses were carried out for the same molecule (e.g., glucose or fructose) at -10 and 40 degrees C. Within each chemical group, W(m), the apparent BET-like parameter that is generally referred to as the "monolayer-limit of absorption", changed very little. In contrast, C(B), a measure of the free energy of absorption, significantly increased with increasing molecular weight or decreasing temperature, leading to a shift from a Type III to a Type II isotherm. The shift in isotherm shape correlates directly with the glass transition temperature, T(g), of the dry sample relative to the operating temperature, T (i.e., Type III when T > T(g) and Type II when T < T(g). These results are shown to be consistent with the combined Flory-Huggins solution model and Vrentas structural relaxation model; wherein Type II isotherm behavior, observed for T < T(g), reflects nonideal volumetric contributions to the overall free energy of absorption due to plasticization by water, as described by Vrentas, whereas Type III behavior only reflects the Flory-Huggins solution model. These volumetric free energy changes within each chemical group are shown to be correlated to the values of the "BET" parameter C(B).

Acid-catalyzed inversion of sucrose in the amorphous state at very low levels of residual water

PURPOSE

Factors affecting the solid-state acid-catalyzed inversion of amorphous sucrose to glucose and fructose in the presence of colyophilized citric acid, with less than 0.1% w/w residual water, have been studied.

METHODS

Samples of citric acid and sucrose were lyophilized at a weight ratio of 1:10 citric acid:sucrose from solutions with initial pH values of 1.87, 2.03, and 2.43, as well as at a weight ratio of 1:5, at an initial pH of 1.87. Glass transition temperatures, Tg, were measured by DSC and the presence of any possible residual water was monitored by Karl Fischer Titrimetry. The inversion of sucrose was measured by polarimetric analysis after reconstitution of solid samples stored at 50 degrees C under P2O5.

RESULTS

Samples of 1:10 citric acid:sucrose at an initial pH of 1.87, 2.03, and 2.43 exhibited the same Tg. The initial rate of reactivity was affected at a 1:10 ratio by the solution pH before lyophilization in the order: 1.87 > 2.03 > 2.43 and by citric acid concentration at pH 1.87 in the order 1:5 > 1:10.

CONCLUSIONS

Sucrose, colyophilized with an acid such as citric acid, undergoes significant acid-catalyzed inversion at 50 degrees C despite the very low levels of residual water, i.e., <0.1% w/w. At the same ratio of citric acid to sucrose (1:10), and hence the same Tg, the rate of reaction correlates with the initial solution pH indicating that the degree of ionization of citric acid in solution is most likely retained in the solid state. That protonation of sucrose by citric acid is important is shown by the direct relationship between maximum extent of reaction and citric acid composition. It is concluded that colyophilization of acidic substances with sucrose, even in the absence of residual water, can produce reducing sugars capable of further reaction with other formulation ingredients susceptible to reaction with reducing sugars.

Physical characteristics and chemical degradation of amorphous quinapril hydrochloride

This study was designed to investigate the relationships between the solid-state chemical instability and physical characteristics of a model drug, quinapril hydrochloride (QHCl), in the amorphous state. Amorphous QHCl samples were prepared by rapid evaporation from dichloromethane solution and by grinding and subsequent heating of the crystalline form. Physical characteristics, including the glass transition temperature and molecular mobility, were determined using differential scanning calorimetry, thermogravimetric analysis, powder x-ray diffractometry, polarizing microscopy, scanning electron microscopy, and infrared spectroscopy. The amorphous form of QHCl, produced by both methods, has a T(g) of 91 degrees C. Isothermal degradation studies showed that cyclization of QHCl occurred at the same rate for amorphous samples prepared by the two methods. The activation energy was determined to be 30 to 35 kcal/mol. The rate of the reaction was shown to be affected by sample weight, dilution through mixing with another solid, and by altering the pressure above the sample. The temperature dependence for chemical reactivity below T(g) correlated very closely with the temperature dependence of molecular mobility. Above T(g), however, the reaction was considerably slower than predicted from molecular mobility. From an analysis of all data, it appears that agglomeration and sintering of particles caused by softening of the solid, particularly above T(g), and a resulting reduction of the particle surface/volume ratio play a major role in affecting the reaction rate by decreasing the rate of removal of the gaseous HCl product.

Physical properties of solid molecular dispersions of indomethacin with poly(vinylpyrrolidone) and poly(vinylpyrrolidone-co-vinyl-acetate) in relation to indomethacin crystallization

PURPOSE

To measure solid-state features of amorphous molecular dispersions of indomethacin and various molecular weight grades of poly(vinylpyrrolidone), PVP, and poly(vinylpyrrolidone-co-vinylacetate), PVP/VA, in relation to isothermal crystallization of indomethacin at 30 degrees C.

METHODS

The glass transition temperatures (Tg) of molecular dispersions were measured using differential scanning calorimetry (DSC). FT-IR spectroscopy was used to investigate possible differences in interactions between indomethacin and polymer in the various dispersions. The enthalpy relaxation of 5%w/w and 30%w/w polymer dispersions was determined following various aging times. Quantitative isothermal crystallization studies were carried out with pure indomethacin and 5%w/w polymers in drug as physical mixtures and molecular dispersions.

RESULTS

All coprecipitated mixtures exhibited a single glass transition temperature. All polymers interacted with indomethacin in the solid state through hydrogen bonding and in the process eliminated the hydrogen bonding associated with the carboxylic acid dimers of indomethacin. Molecular mobility at 16.5 degrees C below Tg was reduced relative to indomethacin alone, at the 5%w/w and 30%w/w polymer level. No crystallization of indomethacin at 30 degrees C was observed in any of the 5%w/w polymer molecular dispersions over a period of 20 weeks. Indomethacin alone and in physical mixtures with various polymers completely crystallized to the y form at this level within 2 weeks.

CONCLUSIONS

The major basis for crystal inhibition of indomethacin at 30 degrees C at the 5%w/w polymer level in molecular dispersions is not related to polymer molecular weight and to the glass transition temperature, and is more likely related to the ability to hydrogen bond with indomethacin and to inhibit the formation of carboxylic acid dimers that are required for nucleation and growth to the gamma crystal form of indomethacin.

Solid-state characteristics of amorphous sodium indomethacin relative to its free acid

PURPOSE

Having previously studied the amorphous properties of indomethacin (IN) as a model compound for drugs rendered amorphous during processing, we report on the formation and characterization of its sodium salt in the amorphous state and a comparison between the two systems.

METHODS

Sodium indomethacin (SI) was subjected to lyophilization from aqueous solution, rapid precipitation from methanol solution, and dehydration followed by grinding to produce, in each case, a completely amorphous form. The amorphous form of SI was analyzed using DSC, XRD, thermomicroscopy and FTIR. The method of scanning rate dependence of the glass transition temperature, Tg, was used to estimate the fragility of the SI system. Enthalpy relaxation experiments were carried out to probe the molecular mobility of the SI system below Tg.

RESULTS

The amorphous form of SI formed by different methods had a Tg equal to 121 degrees C at a scanning rate of 20 degrees C/min. This compares with a Tg for indomethacin of 45 degrees C. Estimation of fragility by the scanning rate dependence of Tg indicates no significant differences in fragility between ionized and unionized forms. Enthalpy relaxation measurements reveal very similar relaxation patterns between the two systems at the same degree of supercooling relative to their respective Tg values.

CONCLUSIONS

The amorphous form of SI made by various methods has a Tg that is about 75 degrees C greater than that of IN, most likely because of the greater density and hence lower free volume of SI. Yet, the change of molecular mobility as a function of temperature relative to Tg is not very different between the ionized and unionized systems.

The effects of absorbed water on the properties of amorphous mixtures containing sucrose

PURPOSE

To measure the water vapor absorption behavior of sucrose-poly(vinyl pyrrolidone) (PVP) and sucrose-poly(vinyl pyrrolidone co-vinyl acetate) (PVP/VA) mixtures, prepared as amorphous solid solutions and as physical mixtures, and the effect of absorbed water on the amorphous properties, i.e., crystallization and glass transition temperature, Tg, of these systems.

METHODS

Mixtures of sucrose and polymer were prepared by co-lyophilization of aqueous sucrose-polymer solutions and by physically mixing amorphous sucrose and polymer. Absorption isotherms for the individual components and their mixtures were determined gravimetrically at 30 degrees C as a function of relative humidity. Following the absorption experiments, mixtures were analyzed for evidence of crystallization using X-ray powder diffraction. For co-lyophilized mixtures showing no evidence of crystalline sucrose, Tg was determined as a function of water content using differential scanning calorimetry.

RESULTS

The absorption of water vapor was the same for co-lyophilized and physically mixed samples under the same conditions and equal to the weighted sums of the individual isotherms where no sucrose crystallization was observed. The crystallization of sucrose in the mixtures was reduced relative to sucrose alone only when sucrose was molecularly dispersed (co-lyophilized) with the polymers. In particular, when co-lyophilized with sucrose at a concentration of 50%, PVP was able to maintain sucrose in the amorphous state for up to three months, even when the Tg was reduced well below the storage temperature by the absorbed water.

CONCLUSIONS

The water vapor absorption isotherms for co-lyophilized and physically mixed amorphous sucrose-PVP and sucrose-PVPNA mixtures at 30 degrees C are similar despite interactions between sugar and polymer which are formed when the components are molecularly dispersed with one another. In the presence of absorbed water the crystallization of sucrose was reduced only by the formation of a solid-solution, with PVP having a much more pronounced effect than PVP/VA. The effectiveness of PVP in preventing sucrose crystallization when significant levels of absorbed water are present was attributed to the molecular interactions between sucrose, PVP and water.

Sugar-polymer hydrogen bond interactions in lyophilized amorphous mixtures

The objective of this work was to investigate hydrogen bonding interactions between a variety of glass-forming sugars and a model polymer, poly(vinylpyrrolidone) (PVP), in binary amorphous solid solutions, produced by lyophilization. The glass transition temperatures of the sugars and sugar-PVP colyophilized mixtures were assessed using differential scanning calorimetry. The hydrogen bonding interactions between each sugar and PVP were monitored using FT-Raman spectroscopy. Sucrose was found to hydrogen bond to a greater extent with PVP at a particular sugar:polymer ratio than the other disaccharides studied including trehalose and the trisaccharide raffinose. Maltodextrins showed a decreased tendency to hydrogen bond with the polymer compared to the lower molecular weight sugars. The extent of hydrogen bonding was found to correlate inversely with the glass transition temperature of the sugar, with the tendency to hydrogen bond decreasing as the Tg increased. The importance of hydrogen bonding interactions to the thermodynamics of mixing in amorphous solids is discussed.

Enthalpy relaxation in binary amorphous mixtures containing sucrose

PURPOSE

To compare the enthalpy relaxation of amorphous sucrose and co-lyophilized sucrose-additive mixtures near the calorimetric glass transition temperature, so as to measure the effects of additives on the molecular mobility of sucrose.

METHODS

Amorphous sucrose and sucrose-additive mixtures, containing poly(vinylpyrrolidone) (PVP), poly(vinylpyrrolidone-co-vinyl-acetate) (PVPNA) dextran or trehalose, were prepared by lyophilization. Differential scanning calorimetry (DSC) was used to determine the area of the enthalpy recovery endotherm following aging times of up to 750 hours for the various systems. This technique was also used to compare the enthalpy relaxation of a physical mixture of amorphous sucrose and PVP.

RESULTS

Relative to sucrose alone, the enthalpy relaxation of co-lyophilized sucrose-additive mixtures was reduced when aged for the same length of time at a comparable degree of undercooling in the order: dextran approximately PVP > PVPNA > trehalose. Calculated estimates of the total enthalpy change required for sucrose and the mixtures to relax to an equilibrium supercooled liquid state (deltaHinfinity) were essentially the same and were in agreement with enthalpy changes measured at longer aging times (750 hours).

CONCLUSIONS

The observed decrease in the enthalpy relaxation of the mixtures relative to sucrose alone indicates that the mobility of sucrose is reduced by the presence of additives having a Tg that is greater than that of sucrose. Comparison with a physically mixed amorphous system revealed no such effects on sucrose. The formation of a molecular dispersion of sucrose with a second component, present at a level as low as 10%, thus reduces the mobility of sucrose below Tg, most likely due to the coupling of the molecular motions of sucrose to those of the additive through molecular interactions.

Phase behavior of binary and ternary amorphous mixtures containing indomethacin, citric acid, and PVP

PURPOSE

To better understand the nature of drug-excipient interactions we have studied the phase behavior of amorphous binary and ternary mixtures of citric acid, indomethacin and PVP, as model systems.

METHODS

We have prepared amorphous mixtures by co-melting or coprecipitation from solvents, and have measured glass transition temperatures with differential scanning calorimetry.

RESULTS

Citric acid and indomethacin in the amorphous state are miscible up to 0.25 weight fraction of citric acid, equivalent to about 2 moles of citric acid and 3 moles of indomethacin. Phase separation, as reflected by two Tg values, occurs without crystallization leading to a saturated citric acid-indomethacin amorphous phase and one essentially containing only citric acid. PVP-citric acid and PVP-indomethacin form non-ideal miscible systems at all compositions. A ternary system containing 0.3 weight fraction of PVP produces a completely miscible system at all citric acid-indomethacin compositions. The use of 0.2 weight fraction of PVP, however, only produces miscibility up to a weight fraction of 0.4 citric acid relative to indomethacin. The two phases above this point appear to contain citric acid in PVP and citric acid in indomethacin, respectively.

CONCLUSIONS

Two components of an amorphous solid mixture containing citric acid and indomethacin with limited solid state miscibility can be solubilized as an amorphous solid phase by the addition of moderate levels of PVP.

Mixing behavior of colyophilized binary systems

The purpose of this study was to investigate the factors which govern the mixing of amorphous sucrose with trehalose, poly(vinylpyrrolidone) (PVP), dextran, and poly(vinylpyrrolidone-co-vinyl acetate) (PVP/VA). These materials were chosen as model systems to represent multicomponent freeze-dried pharmaceutical preparations. Mixtures were prepared by colyophilization of the components from aqueous solutions. The glass transition temperatures (Tg) of these mixtures were measured using differential scanning calorimetry (DSC) and were compared to predictions based on simple mixing rules. FT-Raman spectroscopy was used to probe selected mixtures for evidence of molecular interactions between components. Colyophilized mixtures were confirmed to be amorphous by X-ray powder diffraction. The Tg values of the various mixtures generally were lower than values predicted from free volume and thermodynamic models, indicating that mixing is not ideal. The FT-Raman spectra of colyophilized sucrose-PVP and sucrose-PVP/VA mixtures provided evidence for interaction between the components through hydrogen bonding. Hydrogen bonds formed between components in colyophilized sucrose-additive mixtures are formed at the expense of hydrogen bonds within sucrose and in some cases within the additive. A thermodynamic analysis of these mixtures indicates that mixing is endothermic, which is consistent with a net loss in the degree of hydrogen bonding on mixing. There is also a positive excess entropy of mixing which accompanies the net loss in hydrogen bonds. Despite this gain in excess entropy, the excess free energy of mixing is positive, consistent with the observed deviations in Tg from values predicted using models which assume ideal mixing.

The molecular mobility of supercooled amorphous indomethacin as a function of temperature and relative humidity

PURPOSE

To determine the relaxation times of supercooled indomethacin as a function of temperature and relative humidity above Tg, and to analyze the results in the context of being able to predict such behavior at various storage conditions.

METHODS

Dielectric relaxation times were measured in the frequency domain (12 to 10(5) Hz) for amorphous indomethacin equilibrated at 0, 56, and 83% relative humidity. The heating rate dependence of Tg for dry supercooled indomethacin was measured with differential scanning calorimetry and used to determine relaxation times. The results were compared with previously published shear relaxation times and enthalpy recovery data.

RESULTS

Very good agreement was observed between dielectric and shear relaxation times, and those obtained from the heating rate dependence of the Tg, for dry indomethacin as a function of temperature above Tg. The introduction of water lowered the dielectric relaxation times of supercooled indomethacin without significantly affecting its fragility. The relaxation times below Tg, found to be lower than those predicted by extrapolation of the data obtained above Tg, were analyzed in the context of the Adam-Gibbs-Vogel equation.

CONCLUSIONS

The relaxation times of amorphous indomethacin obtained from the heating rate dependence of Tg were in good agreement with those obtained from shear and dielectric measurements, thus validating a relatively simple approach of assessing molecular mobility. The significant molecular mobility of amorphous indomethacin observed below Tg, and the significant plasticizing effects of sorbed water, help to explain why amorphous indomethacin crystallizes well below Tg over relatively short time scales.

The quantitative analysis of crystallinity using FT-Raman spectroscopy

PURPOSE

To establish if FT-Raman spectroscopy can be used to quantitate the degree of crystallinity in a model compound.

METHODS

Mixtures containing different proportions of amorphous and crystalline indomethacin were prepared. Using the peak intensity ratio 1698 cm(-1) (crystalline) to 1680 cm(-1) (amorphous), a correlation curve was prepared. This correlation curve was validated by testing further samples of known composition. Partially crystalline indomethacin was prepared by milling crystalline indomethacin.

RESULTS

A linear correlation curve was obtained across the entire range of 0-100% crystallinity. Using this method, it was possible to detect down to either 1% amorphous or crystalline content. The largest errors were found to result from inhomogeneities in the mixing of the calibration and validation samples. The spectra of the mechanically processed samples were similar to the spectra of the calibration samples, and the degree of crystallinity could be estimated in these samples.

CONCLUSIONS

FT-Raman spectroscopy is a potentially useful method to complement existing techniques for the quantitative determination of crystallinity.

Water vapor sorption by peptides, proteins and their formulations

The interactions of pharmaceutical peptides, proteins and their formulations with environmental water vapor are reviewed. Particular attention is paid to the importance of the physical structure and chemical diversity of peptides and proteins, and comparisons are made with the mechanisms of water vapor sorption by synthetic macromolecular systems. The influences of formulation processes and additives are also considered and suggestions made for future areas of research.

Effects of a cationic and hydrophobic peptide, KL4, on model lung surfactant lipid monolayers

We report on the surface behavior of a hydrophobic, cationic peptide, [lysine-(leucine)4]4-lysine (KL4), spread at the air/water interface at 25 degrees C and pH 7.2, and its effect at very low molar ratios on the surface properties of the zwitterionic phospholipid 1,2-dipalmitoylphosphatidylcholine (DPPC), and the anionic forms of 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) and palmitic acid (PA), in various combinations. Surface properties were evaluated by measuring equilibrium spreading pressures (pi(e)) and surface pressure-area isotherms (pi-A) with the Wilhelmy plate technique. Surface phase separation was observed with fluorescence microscopy. KL4 itself forms a single-phase monolayer, stable up to a surface pressure pi of 30 mN/m, and forms an immiscible monolayer mixture with DPPC. No strong interaction was detected between POPG and KL4 in the low pi region, whereas a stable monolayer of the PA/KL4 binary mixture forms, which is attributed to ionic interactions between oppositely charged PA and KL4. KL4 has significant effects on the DPPC/POPG mixture, in that it promotes surface phase separation while also increasing pi(e) and pi(max), and these effects are greatly enhanced in the presence of PA. In the model we have proposed, KL4 facilitates the separation of DPPC-rich and POPG/PA-rich phases to achieve surface refinement. It is these two phases that can fulfill the important lung surfactant functions of high surface pressure stability and efficient spreading.

The interaction of lung annexin I with phospholipid monolayers at the air/water interface

Lung annexin I (LAI), a calcium-ion-dependent phospholipid-binding protein, has been shown earlier to cause aggregation and fusion of bilayered vesicles containing phospholipids found in lung surfactant, and to be a very likely factor in the assembly of lung surfactant into the lamellar bodies stored in the Type II cell. In this study, we have measured the accumulation of LAI into spread monolayers of some major lipid components of lung surfactant, dipalmitoyl-phosphatidylcholine (DPPC), dipalmitoyl-phosphatidylglycerol (DPPG), palmitoyl-oleyoyl-phosphatidylglycerol (POPG), and selected mixtures, as a function of calcium-ion concentration and surface concentration (degree of packing) of the phospholipid monolayer. The ability of LAI to significantly penetrate such monolayers was calcium-ion-dependent and only occurred in the presence of DPPG or POPG. The relative extent of penetration into DPPG and POPG was directly related to the available free area in the monolayer, penetration being greater with POPG. Fluorescence microscopy measurements revealed that DPPC mixed with either DPPG or POPG caused a change in surface phase behavior in a manner believed to be related to certain types of bilayer fusion. A chemical breakdown product of LAI, LAI-bp, previously found not to cause aggregation and fusion of bilayers, did not exhibit comparable monolayer penetration or surface phase separation to LAI.

Properties of citric acid at the glass transition

To better understand the properties of citric acid when used in solid dosage forms as an acid-base buffer, we initiated a study of its properties in the amorphous state. Such a state often arises during processes such as lyophilization and wet granulation. In view of inconsistencies in the literature concerning the glass transition temperature, Tg, of citric acid in the dry and hydrated states, we measured the Tg of samples formed by a melt-quench cool sequence in a DSC. We also used DSC to measure Tg', the glass transition temperature of the maximally freeze-concentrated solution. It was shown that dry citric acid has a Tg of 11 degrees C, while that containing 8.6% water (equimolar) has a value of -25 degrees C. The Tg' of a frozen solution of citric acid is -53 degrees C. Measuring Tg for the dry and hydrated samples at various scanning rates allowed measurement of the activation energy for enthalpy relaxation at Tg and enabled estimation of the degree of fragility (or the strength parameter) for both samples. It was shown that citric acid is a fairly fragile liquid expected to exhibit non-Arrhenius dynamic behavior and that the presence of residual water at a level of 8.6% causes a decrease in fragility.

Spectroscopic characterization of interactions between PVP and indomethacin in amorphous molecular dispersions

PURPOSE

To study the molecular structure of indomethacin-PVP amorphous solid dispersions and identify any specific interactions between the components using vibrational spectroscopy.

METHODS

Solid dispersions of PVP and indomethacin were prepared using a solvent evaporation technique and IR and FT-Raman spectra were obtained.

RESULTS

A comparison of the carbonyl stretching region of gamma indomethacin, known to form carboxylic acid dimers, with that of amorphous indomethacin indicated that the amorphous phase exists predominantly as dimers. The hydrogen bonding of alpha indomethacin is not as dimers. Addition of PVP to amorphous indomethacin increased the intensity of the infrared band assigned to non-hydrogen bonded carbonyl. Concomitantly, the PVP carbonyl stretch appeared at a lower wavenumber indicating hydrogen bonding. Model solvent systems aided spectral interpretation. The magnitude of the spectral changes were comparable for an indomethacin-PVP solid dispersion and a solution of indomethacin in methylpyrrolidone at the same weight percent.

CONCLUSIONS

Indomethacin interacts with PVP in solid dispersions through hydrogen bonds formed between the drug hydroxyl and polymer carbonyl resulting in disruption of indomethacin dimers. PVP may influence the crystallisation kinetics by preventing the self association of indomethacin molecules. The similarity of results for solid dispersions and solutions emphasises the "solution" nature of this binary amorphous state.

Molecular mobility of supercooled amorphous indomethacin, determined by dynamic mechanical analysis

PURPOSE

To determine the viscosity and the frequency-dependent shear modulus of supercooled indomethacin as a function of temperature near and above its glass transition temperature and from these data to obtain a quantitative measure of its molecular mobility in the amorphous state.

METHODS

Viscoelastic measurements were carried with a controlled strain rheometer in the frequency domain, at 9 temperatures from 44 degrees to 90 degrees C.

RESULTS

The viscosity of supercooled indomethacin shows a strong non-Arrhenius temperature dependence over the temperature range studied, indicative of a fragile amorphous material. Application of the viscosity data to the VTF equation indicates a viscosity of 4.5 x 10(10) Pa.s at the calorimetric Tg of 41 degrees C. and a T0 of -17 degrees C. From the complex shear modulus and the Cole-Davidson equation the shear relaxation behaviour is found to be non-exponential, and the shear relaxation time at Tg is found to be approximately 100 sec.

CONCLUSIONS

Supercooled indomethacin near and above its Tg exhibits significant molecular mobility, with relaxation times similar to the timescales covered in the handling and storage of pharmaceutical products.

Effects of sorbed water on the crystallization of indomethacin from the amorphous state

The water sorption isotherm and the crystallization rates for amorphous indomethacin were determined at 30 degrees C as a function of relative humidity (RH), along with the effects of water content on the glass transition temperature (T(g)). Below 43% RH, only the stable gamma crystal form appears, whereas at higher RH, only the metastable alpha crystal form appears. The tendency for the alpha crystals to form at higher RH is consistent with the Ostwald step rule. The crystallization rate of the alpha form continuously increased with increasing RH due to increasing molecular mobility. The crystallization mechanism of the gamma form changed from surface-initiated to bulk-initiated crystallization at 21% RH, and although crystallization rates of the gamma form increased with increasing RH in both cases, they were higher when crystallization was surface-initiated. The complex crystallization behavior of the gamma form is explained by the higher water content and molecular mobility of the surface relative to the bulk and the general effect of water on alpha or gamma crystal form selection as described by the Ostwald step rule.

Characteristics and significance of the amorphous state in pharmaceutical systems

The amorphous state is critical in determining the solid-state physical and chemical properties of many pharmaceutical dosage forms. This review describes the characteristics of the amorphous state and some of the most common methods that can be used to measure them. Examples of pharmaceutical situations where the presence of the amorphous state plays an important role are presented. The application of our current knowledge to pharmaceutical formulation problems is illustrated, and some strategies for working with amorphous character in pharmaceutical systems are provided.

Inhibition of indomethacin crystallization in poly(vinylpyrrolidone) coprecipitates

Differential scanning calorimetry and powder X-ray diffraction studies have been carried out with amorphous coprecipitates of indomethacin and poly(vinylpyrrolidone), PVP, to measure the glass transition temperature, Tg, as a function of mixture composition and the nonisothermal and isothermal crystallization of the indomethacin. Values of Tg as a function of mixture composition followed the ideal Gordon-Taylor equation up to about 50% w/w PVP. Inhibition of crystallization occurred at levels as low as 5% PVP and very significant inhibition was observed at and above 20% PVP. Inhibition of crystallization of indomethacin in the absence of PVP required a storage temperature 40-50 degrees C below Tg, whereas comparable inhibition with PVP was observed at storage temperatures 5 degrees C above Tg. This suggests that the inhibition of indomethacin crystallization by PVP may involve mechanisms other than just the general antiplasticizing effect (raising Tg) by PVP.

Molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures

PURPOSE

To measure the molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures (Tg), using indomethacin, poly (vinyl pyrrolidone) (PVP) and sucrose as model compounds.

METHODS

Differential scanning calorimetry (DSC) was used to measure enthalpic relaxation of the amorphous samples after storage at temperatures 16-47 K below Tg for various time periods. The measured enthalpy changes were used to calculate molecular relaxation time parameters. Analogous changes in specimen dimensions were measured for PVP films using thermomechanical analysis.

RESULTS

For all the model materials it was necessary to cool to at least 50 K below the experimental Tg before the molecular motions detected by DSC could be considered to be negligible over the lifetime of a typical pharmaceutical product. In each case the temperature dependence of the molecular motions below Tg was less than that typically reported above Tg and was rapidly changing.

CONCLUSIONS

In the temperature range studied the model amorphous solids were in a transition zone between regions of very high molecular mobility above Tg and very low molecular mobility much further below Tg. In general glassy pharmaceutical solids should be expected to experience significant molecular mobility at temperatures up to fifty degrees below their glass transition temperature.

Hydration and dehydration of crystalline and amorphous forms of raffinose

The trisaccharide raffinose was prepared in its crystal pentahydrate, anhydrous methanolate, and amorphous forms and evaluated with regard to dehydration and hydration properties at various temperatures and relative humidities. The pentahydrate, when stored at relative humidities (RHs) of < 60% but > 10%, showed no loss of water after 3 months of storage at 30 degrees C. When stored below 10% RH, only one water molecule could be removed over a period of 3 months, whereas within 24 h at 30 degrees C in a vacuum oven, two water molecules were removed with no change in crystal structure. Increasing the temperature to 60 degrees C progressively removed the remaining three molecules, causing the crystal, however, to collapse into an amorphous form identical to one prepared by lyophilization. Rehydration at 30 degrees C, which was sufficient to reduce the glass transition temperature to < 30 degrees C, rapidly restored the pentahydrate crystal structure. Rehydration of the methanolate also restored the pentahydrate structure. The significant amount of water accommodated by raffinose in both the crystalline and amorphous forms would appear to make it a potentially useful water scavenger in certain types of dosage forms.

Crystallization of indomethacin from the amorphous state below and above its glass transition temperature

The solid state crystallization of amorphous polymers, sugars, and inorganic glasses is often thought to be restricted to the region above the glass transition temperature, Tg, because insufficient molecular mobility (high viscosity) exists below Tg for nucleation and crystal growth. Here we report on the isothermal and nonisothermal crystallization of dry amorphous indomethacin in the temperature range of 20 degrees C above and below its Tg. These studies were carried out with two amorphous samples having different degrees of metastability relative to the crystalline state. It was shown that in both samples significant crystallization to the most stable polymorphic form occurred over several days when stored below Tg, and in some cases this process was preceded by the relaxation of one amorphous form to the other. At storage temperatures near to and above Tg the rates of crystallization increased as expected but a second less thermodynamically stable polymorph also appeared with the more stable crystal form. This behavior was explained by the possible relationship between the degree of metastability relative to the crystalline state of each amorphous form and the interfacial energy existing at the respective nucleation sites, in accord with the Ostwald step rule.

Non-isothermal and isothermal crystallization of sucrose from the amorphous state

The crystallization of a model compound, sucrose, from the amorphous solid state has been studied non-isothermally using differential scanning calorimetry to determine crystallization temperature, Tc, and isothermally at 30 degrees C by subjecting samples to 32.4% relative humidity and gravimetrically monitoring water vapor uptake and subsequent loss with time due to crystallization. From the measurement of glass transition temperature, Tg, and melting temperature, Tm, for sucrose alone and in the presence of absorbed water it was possible to predict Tc and thus to directly relate the plasticizing effects of water to its tendency to promote crystallization. Colyophilization of sucrose with lactose, trehalose, and raffinose, all having Tg values greater than that of sucrose, increased Tc significantly, even at levels as low as 1-10% w/w. In the isothermal studies the time required for crystallization to commence, due to the plasticizing effects of water, i.e., the induction time, assumed to be mostly affected by rates of nucleation, was greatly increased by the presence of the additives at these low levels, with raffinose producing a greater effect than lactose and trehalose. Similarly, these additives reduced the rate of water loss, i.e., the rate of crystal growth, but now no significant differences were noted between the three additives. The possible relationships of nucleation and crystal growth and the effects of additives on molecular mobility are discussed.

The relationship between the glass transition temperature and the water content of amorphous pharmaceutical solids

The glass transition temperature of an amorphous pharmaceutical solid is a critical physical property which can dramatically influence its chemical stability, physical stability, and viscoelastic properties. Water frequently acts as a potent plasticizer for such materials, and since many amorphous solids spontaneously absorb water from their surroundings the relationship between the glass transition temperature and the water content of these materials is important. For a wide range of amorphous and partially amorphous pharmaceutical solids, it was found that there is a rapid initial reduction in the glass transition temperature from the dry state as water is absorbed, followed by a gradual leveling off of the response at higher water contents. This plasticization effect could generally be described using a simplified form of the Gordon-Taylor/Kelley-Bueche relationships derived from polymer free volume theory. Most of the systems considered showed a nearly ideal volume additivity and negligible tendency to interact. This is consistent with the hypothesis that such mixtures behave as concentrated polymer solutions and indicates that water acts as a plasticizer in a way similar to that of other small molecules and not through any specific or stoichiometric interaction process(es).

The use of solution theories for predicting water vapor absorption by amorphous pharmaceutical solids: a test of the Flory-Huggins and Vrentas models

The limitations of traditional gas adsorption models for describing water vapor sorption by amorphous pharmaceutical solids are described and an alternative approach based on polymer solution theories is proposed. The approach is tested by comparing a priori predicted isotherms with literature data for the poly(vinylpyrrolidone)(PVP)-water system. The well-known Flory-Huggins model is able to describe the water vapor sorption isotherm only when the PVP-water mixture is in the rubbery state (i.e., above its glass transition temperature). However, a newer model developed by Vrentas and coworkers, which takes into account the plasticizing effect of water on the polymer, is able to describe the entire form of the isotherm. Consideration of the parameters in this model allows a number of critical variables to be identified and also enables the characteristic shape of the water vapor sorption isotherm to be explained.

Molecular mobility in mixtures of absorbed water and solid poly(vinylpyrrolidone)

Poly(vinylpyrrolidone) (PVP) was used as model system to examine molecular mobility in mixtures of absorbed water with solid amorphous polymers. Water vapor absorption isotherms were determined, along with diffusion and proton NMR relaxation measurements of absorbed water. Concurrently, measurements of glass transition temperatures (Tg) and carbon-13 NMR relaxation times for PVP were determined as a function of water content. Two water contents were used as reference points: Wm, obtained from the fit of water absorption isotherms to the BET equation, corresponding to the first shoulder in the sigmoid isotherm; and Wg, the amount of water necessary to depress Tg to the isotherm temperature. Translational diffusion coefficients of water, along with proton T1 relaxation time constants, show that both the translational and the rotational mobility of the water is hindered by the presence of the solid polymer and that the absorbed water is most likely represented by two or more populations of water with different modes or time scales of motion. The presence of "tightly bound" or immobilized water at levels corresponding to Wm, however, is unlikely, since water molecules maintain a high degree of mobility, even at the lowest levels of water. Above Wg, water shows an increase in mobility with increasing water content, but it is always less mobile than bulk water. With increasing water content, carbon-13 T1 relaxation time constants for PVP, measured under the same conditions as above, indicate a major increase in the molecular mobility of carbon atoms associated with the pyrrolidone side chains.

The surface properties of lung 36 kDa Ca(2+)-dependent phospholipid-binding protein

The intrinsic surface activity of a 36 kDa rabbit lung calcium-dependent phospholipid-binding protein (PLBP), a member of the annexin family of such proteins, at the air/water interface has been determined from measurements of surface tension of aqueous solutions, and surface concentration of 14C-labeled PLBP adsorbed from aqueous solution in the absence and presence of Ca2+. It was also possible to spread insoluble monolayers of PLBP to determine surface pressure vs. surface concentration isotherms, as well as surface elasticity and surface viscosity as a function of frequency from electrocapillary wave diffraction measurements. PLBP has been shown to exhibit significant intrinsic surface activity at the air/water interface, comparable to a variety of other hydrophobic proteins known to be quite surface active. In all cases, surface properties were enhanced by the presence of Ca2+, particularly the degree of surface viscoelasticity at close-packing in the monolayer. This is believed to reflect changes in protein conformation at the surface.

Solid-state phase transitions initiated by water vapor sorption of crystalline L-660,711, a leukotriene D4 receptor antagonist

Significant water vapor sorption at room temperature by the crystalline and lyophilized forms of L-660,711, a potent, selective leukotriene D4 receptor antagonist, has been measured and shown to produce increasingly more nonflowing, semisolid masses with increasing relative humidity. Thermal analysis, SEM, powder X-ray diffraction, solid-state NMR, and thermomicroscopic measurements reveal that water vapor sorbed at room temperature converts the crystalline form to a noncrystalline form resembling the lyophilized sample. Evidence is presented to indicate that L-660,711 has surface active properties with a critical micellization concentration of approximately 1 x 10(-4) M and an ability to form thermotropic and lyotropic mesomorphic phases when the crystal is heated above 80 degrees C in the anhydrous state; it is lyophilized from aqueous solution, and it is exposed to relative humidities at and above 12%, at room temperature.

Lung calcium-dependent phospholipid-binding proteins: structure and function

Distinct peptide maps of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins (PLBPs), 36,000 and 33,000, were generated by cyanogen bromide (CNBr) cleavage, trypsin or Staphylococcus aureus V8 proteinase digestion. The amino acid sequence of a CNBr-cleaved peptide of the 36,000 PLBP was aligned to the amino terminus of human lipocortin I with more than 77% identity, but had no identity with the known amino terminal sequence of other known annexins. Partial amino acid sequence of a 33,000 PLBP peptide demonstrated a close (56%) relationship to endonexin II, human placental anticoagulant protein, and porcine intestine protein II, but shared only 32% identity with lipocortin I, 30% with lipocortin II. Antiserum generated against purified 36,000 PLBP reacted strongly with the 33,000 PLBP, but did not react with any other rabbit lung cytosolic proteins. Both PLBPs inhibited the phospholipase A2 reaction when dioleoyl phosphatidylcholine and phosphatidylglycerol vesicles or monolayers were used as substrates. In the vesicle assay, the phospholipase A2 reaction was inhibited at lower substrate phospholipid concentrations but not at nearly saturating substrate concentrations. In the monolayer assay, the phospholipid-binding proteins did not inhibit phospholipase A2 at a low phospholipid surface concentration of 3.8.10(-3) molecules/A2, but they did at higher surface concentrations between 1.1 x 10(-2) and 3.8 x 10(-2) molecules/A2. The inhibition of phospholipase A2 by rabbit lung phospholipid-binding proteins is most likely due to the prevention of penetration by phospholipase A2 into the interface, a requirement for the enzyme to act on the substrate.

The use of a multichannel scaling method to detect radioactive species in spread monolayers

This study describes a simple and inexpensive method for monitoring radioactive species spread as monolayers at the air/water interface. The combination of a discriminator and multichannel scalar counter with a personal computer functions to unify all measurements, to simplify the operational process and data acquisition, and to provide a real-time display of the data. Its use is demonstrated by following the hydrolysis of L-alpha-[1-14C]dioleoyl phosphatidylcholine by the enzyme, phospholipase A2, isolated from the porcine pancreas.

The relationship between the glass transition temperature and water vapor absorption by poly(vinylpyrrolidone)

Water associated with amorphous solids is known to affect significantly the physical and chemical properties of dosage form ingredients. An analysis of water vapor absorption isotherms of poly(vinylpyrrolidone) measured in this and other laboratories, over the range -40 to 60 degrees C, along with the measurement of the glass transition temperature of poly(vinylpyrrolidone) as a function of water content is reported. It is observed that the amount of water vapor absorbed at a particular relative humidity increases with decreasing temperature, along with a significant change in the shape of the isotherm. It is also shown that at any temperature, along with a significant change in the shape of the isotherm. It is also shown that at any temperature the state of the solid changes from a highly viscous glass to a much less viscous rubber in the region where absorbed water appears to enter into a "solvent-like" state. Further, the apparent "tightly bound" state, observed at low relative humidities, appears to exist when the polymer enters into a very viscous glassy state. It is concluded that the apparent states of water and polymer are interrelated in a dynamic manner and, therefore, that they cannot be uncoupled by simple thermodynamic analyses based only on a water-binding model.

Uptake of L-carnitine by rat jejunal brush border microvillous membrane vesicles. Evidence of passive diffusion

We have previously described apparent active transport of carnitine into rat intestinal mucosa with intracellular accumulation against a concentration gradient in a process dependent upon the presence of sodium ions, oxygen, and energy. In the work described here, we sought to define the interaction between carnitine and the brush border membrane, which we presumed contained the transport mechanism. Using isolated rat jejunal brush border microvillous membrane vesicles, we found evidence of passive diffusion alone. We found no evidence of carrier-mediated transport--in particular no saturation over a concentration range, inhibition by structural analogs, transstimulation phenomenon, and no influence of sodium ions, potential difference or proton gradients. We conclude that a carnitine transporter does not exist in the brush border membrane of enterocytes and that other cellular mechanisms are responsible for the apparent active transport observed.

Equilibrium uptake of D-glucose by osmotically stressed unilamellar phospholipid vesicles

The assumptions inherent in the use of osmotic manipulation to determine the extent of solute binding to brush border membrane vesicles (the ideal osmotic responsiveness of the vesicles and the independence of solute binding from the incubation medium osmotic pressure) were examined in a model system (large unilamellar lipid vesicles). The equilibrium uptake of D-glucose by unilamellar vesicles composed of egg lecithin (PC), phosphatidic acid (PA), and cholesterol (Chol) was measured as a function of the osmotic concentration of the incubation medium. The variation of the encapsulated aqueous volume of PC:PA and PC:PA:Chol vesicles with the osmotic stress was directly determined by a fluorescence self-quenching technique. Encapsulated volume changes of both PC:PA and PC:PA:Chol vesicles were found to be resistant to the osmotic stress, exhibiting positive deviations from ideal behavior. Equilibrium uptake experiments with these vesicles showed that glucose was taken up in excess of that amount predicted on the basis of the encapsulated volume when the vesicles were subjected to osmotic stress less than 0.25 osmol/kg. At osmotic stresses greater than 0.75 osmol/kg, equilibrium uptake could be predicted solely on the basis of the encapsulated volume. These results, based on a model vesicle system, strongly suggest that osmotic manipulation may be an inappropriate method to assess the extent of solute binding to natural membrane vesicle preparations, such as brush border membrane vesicles, without more direct evidence.

The association of D-glucose with unilamellar phospholipid vesicles

The equilibrium uptake of hydrophilic solutes, D-glucose and L-carnitine, by large unilamellar phospholipid vesicles composed of egg lecithin (PC), phosphatidic acid (PA), and various concentrations of cholesterol (Chol) has been measured. Calculation of the encapsulated volume of PC-PA and PC-PA-Chol vesicles, based on electron-microscopy data, agreed with the values directly measured by fluorescence techniques. Likewise, vesicle surface areas determined directly and from electron microscopy were in good agreement. Equilibrium uptake experiments by these well-characterized vesicles showed that glucose was taken up in excess of that amount predicted on the basis of the encapsulated aqueous volume. In contrast, the equilibrium uptake of carnitine can be predicted solely on the basis of the vesicle encapsulated volume. Each excess glucose molecule was found to be associated with from 7 to 5200 phospholipid molecules for 100 and 0.1 mM glucose, respectively. Uptake of glucose by PC-PA-Chol vesicles is independent of the cholesterol concentration and is similar to that observed in PC-PA vesicles. The cholesterol concentration independence and oil/buffer partitioning studies with octane and octanol, coupled with previous studies, strongly suggest that excess glucose is located in the vicinity of the phospholipid head group. A probable mechanism would have phospholipid, water and glucose all involved in the interaction rather than a competition between water and glucose for the phospholipid surface, as has been suggested in the literature.

Water vapor sorption of water-soluble substances: studies of crystalline solids below their critical relative humidities

Water vapor sorption on unground and ground samples of sodium chloride and sodium salicylate at relative humidities below RHo, that at which deliquescence is initiated, has been measured. Sorption isotherms, expressed as the amount sorbed per unit area of solid surface, were different for unground and ground samples. Measurement of specific surface area for samples previously exposed to various relative humidities revealed no change with unground samples but a significant reduction with ground samples beyond about 20% relative humidity. Correcting isotherms for this change in area brings the results with ground and unground samples into closer agreement. These studies reveal that relatively low levels of water vapor sorption on crystalline water-soluble solids, below RHo, can give rise to some form of "surface dissolution" when the solid had been subjected to various forms of mechanical disturbance.