Interfacial pH at electrically charged lipid monolayers investigated by the lipoid pH-indicator method

Ultrasensitive Fluorescence Detection of Peroxymonosulfate Based on a Sulfate Radical-Mediated Aromatic Hydroxylation

Recently, peroxymonosulfate (PMS)-based advanced oxidation processes have exhibited broad application prospects in the environment field. Accordingly, a simple, rapid, and ultrasensitive method is highly desired for the specific recognition and accurate quantification of PMS in various aqueous solutions. In this work, SO₄^•--induced aromatic hydroxylation was explored, and based on that, for the first time, a novel fluorescence method was developed for the PMS determination using Co²⁺ as a PMS activator and benzoic acid (BA) as a chemical probe. Through a suite of spectral, chromatographic, and mass spectrometric analyses, SO₄^•- was proven to be the dominant radical species, and salicylic acid was identified as the fluorescent molecule. As a result, a whole radical chain reaction mechanism for the generation of salicylic acid in the BA/PMS/Co²⁺ system was proposed. This fluorescence method possessed a rapid reaction equilibrium (<1 min), an ultrahigh sensitivity (detection limit = 10 nM; quantification limit = 33 nM), an excellent specificity, and a wide detection range (0-100 μM). Moreover, it performed well in the presence of possible interfering substances, including two other peroxides (i.e., peroxydisulfate and hydrogen peroxide), some common ions, and organics. The detection results for real water samples further validated the practical utility of the developed fluorescence method. This work provides a new method for the specific recognition and sensitive determination of PMS in complex aqueous solutions.

Quantitative Monitoring of Microphase Separation Behaviors in Cationic Liposomes Using HHC, DPH, and Laurdan: Estimation of the Local Electrostatic Potentials in Microdomains

Microphase separation behaviors of cationic liposomes have been investigated using a pH-sensitive fluorescent probe with 4-heptadecyl-7-hydroxycoumarin (HHC), 1,6-diphenyl-1,3,5-hexatriene, and 6-lauroyl-2-dimethylaminonaphthalene, and to estimate localized electrostatic potentials. Shifts of the apparent pKa values of HHC were observed in cationic liposomes in proportion to the amount of cationic lipids. Two pKa values were obtained with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/3β-[N(N',N'-dimethylaminoethane)-carbamoyl] cholesterol hydrochloride (DC-Ch) liposomes, while only one pKa value was generated with either DOPC/1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or DOPC/dimethyldioctadecylammonium-bromide (DODAB) liposomes. The physicochemical membrane property analyses, focusing on membrane fluidity and membrane polarity, revealed heterogeneity among DOPC/DC-Ch liposomes. By analyzing the pH titration curves using sigmoidal fitting, the localized electrostatic potentials were estimated. For DOPC/DOTAP = (7/3), the membrane was in the liquid-disordered phase and the density of cationic molecules was 0.41 cation/nm(2). For DOPC/DC-Ch = (7/3), the membrane was heterogeneous and the densities of cationic molecules in liquid-disordered and liquid-ordered phases were 0.25 and 1.24 cation/nm(2), respectively. We thereby conclude that the DC-Ch molecules can form nanodomains when these molecules are concentrated to 59%.

Protolytic equilibrium in lyophilic nanosized dispersions: Differentiating influence of the pseudophase and salt effects

The so-called apparent ionization constants of various acids (mainly indicator dyes) in versatile organized solutions are analyzed. Aqueous micellar solutions of colloidal surfactants and related lyophilic colloidal systems display a strong differentiating influence on the acidic strength of indicators located in the dispersed pseudophase, i.e., non-uniform changes of pKa on going from water to the given system. This concept allows the influence of such media on acid-base properties of dissolved reagents to be rationalized. It is demonstrated that the differentiating phenomenon is the main reason for limitation of the common electrostatic model of acid-base interactions, and is the principal hindrance to exact evaluations of the interfacial electrical potentials of ionic micelles by means of acid-base indicators. Salt effects, i.e., the influence of supporting electrolytes on the apparent ionization constants of acid-base indicators in the Stern region of ionic micelles, are considered. These salt effects can be conventionally divided into two kinds, namely, general (normal) and special (specific) effects. While the first type adds up to screening of the surface charge, the second one consists in micellar transitions caused by hydrophobic counterions.

A novel pH-controlled transfer process of 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin in CTAB micelles

By analysis of the UV-visible and fluorescence spectra of 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl)porphyrin (P) in different microenvironments of micelle and solvent solutions, a novel pH-controlled transfer process of P in CTAB micelle was reported. In neutral CTAB micelles, porphyrins may locate at the inner layers of micelles. With pH increases to 11.19, the porphyrin can be completely deprotonated and transfers to the outer surface of CTAB micelle. The investigation of kinetics of porphyrin complexing with Cu(II) indicates that the metallation rate of porphyrins in CTAB micelles could also be controlled by changing pH.

UV-visible and fluorescence spectral study on a pH controlled transfer process of an amphiphilic porphyrin in nonionic micelle

5,15-di(4-hydroxyphenyl)-10,20-di(hexadecyloxyphenyl) porphyrin P was solubilized in nonionic polyoxyethylene(9.5) octylphenol (Triton X-100 or TX-100) micelle solutions. By means of analyzing the UV-visible and fluorescence spectra of the synthesized amphiphilic porphyrin P in different solvent environments, and the relationship between the solubilizing location of the porphyrins in TX-100 micelle and the microenvironment polarity, P is shown to involve in a transfer process for the porphyrin moiety from inner to the outer surface of TX-100 micelle as the pH is increased. The kinetic study of porphyrin incorporate with Cu(II) shows that metalation rate of porphyrin increases with the pH increasing, indicating that metalation rate could be controlled by changing pH.

Maintenance and regulation of the pH microclimate at the luminal surface of the distal colon of guinea-pig

1. The fluorescent dye 5-N-hexadecanoyl-aminofluorescein (HAF) was used to study the mechanisms involved in maintaining a relatively constant luminal surface pH (pHs) in the distal colon of the guinea-pig. The fatty acyl chain of the HAF molecule inserts into the apical membrane of epithelial cells. This allows a continuous measurement of the surface pH for several hours. 2. The localization of HAF was confirmed by confocal laser-scanning microscopy and by using monoclonal antibodies against fluorescein. The insertion of HAF into the apical membrane of the colonocytes did not change the transepithelial conductance or the short-circuit current of the epithelium. 3. With the HAF method a pH microclimate was confirmed at the colonic surface. Although the pH of the bulk luminal solution was decreased in bicarbonate-containing solution from 7.4 to 6.4 the pHs changed only in the range 7.54-6.98. 4. In the absence of bicarbonate pHs almost followed changes of bulk luminal pH. In the presence of bicarbonate there was a decrease in pHs after removal of chloride from the luminal side and an increase in pHs after addition of butyrate to the luminal solution. This suggests the involvement of a bicarbonate-anion exchange in bicarbonate secretion: a Cl--HCO3- as well as a short-chain fatty acid--HCO3- exchange. 5. The apical K+-H+-ATPase in the distal colon of guinea-pig has little influence on pHs in the presence of physiological buffer concentrations. 6. Our findings indicate that bicarbonate plays a major role in maintaining the pH microclimate at the colonic surface.

Probing biomembrane interfacial potential and pH profiles with a new type of float-like fluorophores positioned at varying distance from the membrane surface

Fluorophores of a new type were synthesized to probe the electrostatic potential or pH profiles in the external interface of biomembranes. The probes consist of the pH-sensitive fluorophore 7-hydroxycoumarin, coupled to a tetradecyl (myristyl) tail by a spacer group of varying length. A positively charged group is included between the tetradecyl and spacer groups to encourage a float-like alignment in the membrane head-group region. Three probes of this type were compared with 4-heptadecyl-7-hydroxycoumarin the fluorophore of which is embedded in the lipid head-group domain. Thus, a ruler-type positioning of the fluorophores was obtained at about 0.2, 0.6, 1.0, and 1.3 nm from the surface. The membrane-bound probes were tested in well-defined liposomes prepared by extrusion with different surface charge densities and size. The predicted positioning of the float-like probes is supported by their binding behavior in liposomes and by steady-state and nanosecond time-resolved fluorescence anisotropy, as well as by their accessibility to different quenchers. The interfacial electrostatic potential (psi d) and pH (pHd) values were derived from the observed apparent pKa shifts of the probes. The obtained psi d and pHd profiles as function of the surface potential (psi 0) and distance from the membrane surface are in good harmony with predictions from nonlinear Gouy-Chapman theory. The electrokinetic potentials (zeta) of the liposome series, measured by Doppler-electrophoretic frequency shift of laser light scattering, are in good proportion to the probe data. When bound to yeast cells, these probes monitor interfacial changes in parallel with glucose-induced medium acidification.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phospholipid headgroup composition on the transfer of fluorescent long-chain free fatty acids between membranes

The transfer of long-chain anthroyloxy-labeled-free fatty acids (AOffa) between small unilamellar vesicles (SUV) was studied using a fluorescence energy transfer assay. Donor SUV were labeled with AOffa, and acceptor SUV contained the nonexchangeable quencher NBD-phosphatidylethanolamine. Donor and acceptor membranes were mixed using a stopped-flow apparatus, and intermembrane transfer was monitored by the decrease in AO fluorescence with time. The effect of donor membrane phospholipid headgroup composition on AOffa transfer was examined by incorporating phosphatidylethanolamine (PE), phosphatidic acid (PA), or phosphatidylglycerol (PG) into donor SUV otherwise composed of phosphatidylcholine (PC). Addition of 25 mol% of either of the negatively charged phospholipids (PA or PG) resulted in an increase in the rate of AOffa transfer, whereas addition of zwitterionic PE had no effect on transfer rate. The transfer kinetics were in all cases best described by a biexponential process, and it was found that the addition of PA caused an increase in the fraction of AOffa which transfer at the fast rate. This was likely due in large part to the asymmetric distribution of AOffa in these vesicles, with more fatty acid in the outer hemileaflet. This in turn may be due to the asymmetric distribution of PA between the inner and outer hemileaflets. Thus the increased AOffa transfer rate from negatively charged vesicles may be caused by charge repulsion between ffa and negatively charged headgroups. This increase in transfer rate was maximized at pH 9 as compared to pH 7, further suggesting that the increased rate of intermembrane transfer may arise because of charge repulsion. Finally, it was shown that decreasing the membrane surface potential by increasing the ionic strength caused the rate of AOffa transfer from PA-containing vesicles and PC vesicles to become identical. The results demonstrate that the ionic character of the donor membrane bilayer is an important determinant of the transfer rate of long-chain fatty acids between membranes.

Quantitative treatments of protonation equilibria shifts in micellar systems

The paper reviews the theoretical models and equations used for the interpretation of weak acid dissociation equilibria in the presence of micellar systems. The models and the equations of the literature can be rewritten and reinterpreted in terms of the generalized pseudophase model, for which the basic equations are reported. Some new relationships for the titration of weak acids in micellar systems are presented.

Effect of pH on interlamellar spacing in rat sciatic myelin

Tissue sections of rat sciatic nerves were incubated at various pH ranges to study the effects of proton titration on myelin fine structure by transmission electron microscopy. It was found that the major dense line of the sheath can open and close reversibly in response to proton titration at a pH range compatible with in vivo conditions. Opened and closed major dense lines coexisted in the sheath, but the number of opened lines increased with increasing pH. There was a pattern of periodicity in the opening of dense lines in that either one, two, or three opened lines alternated with one closed line. The local opening or closing of a dense line, furthermore, was often associated with reciprocal changes in the two adjacent dense lines, highly suggestive of the interaction of electric fields between the myelin membranes. These observations show that energy can be stored within the myelin leaflet.

Effect of the chaotropic anions thiocyanate and perchlorate on the entry of ricin into Vero cells

The effect of different anions on the sensitivity of Vero cells to ricin was investigated. The cells were equally sensitive to ricin when NaCl was replaced by NaBr, NaI, Na2SO4 or with iso-osmotic concentrations of mannitol. In contrast, NaSCN and NaClO4 strongly protected against ricin at pH 7.2, but not at pH 7.6. The possibility that the protective effect is due to the ability of chaotropic anions to decrease the pH close to the membrane is discussed.

Location and dynamics of a membrane-bound fluorescent hapten. A spectroscopic study

In the preceding paper (Petrossian, A. and Owicki, J.C. (1984), Biochim. Biophys. Acta 776, 217-227), we describe the binding of a monoclonal anti-fluorescein antibody to a membrane bound fluorescein-lipid hapten. Those results suggest that some of the hapten fluorescein moiety is extended away from the membrane surface and is available for antibody binding, while some of the hapten is sequestered and not immediately available for antibody binding. In this paper, we carry out a spectroscopic study of the membrane-bound hapten and show that there is more than one physically distinct fluorophore environment, with the sequestered hapten associated with the phospholipid headgroup region. The amount of membrane-associated fluorophore depends upon the membrane lipid composition: most of the fluorophore is associated when the lipid is unsaturated or branched-chain phosphatidylcholines (PC), whereas the hapten is largely extended for PC/cholesterol mixtures. The effect of cholesterol on the availability of membrane-bound hapten to antibody binding is not unique to this system. The conversion between sequestered and extended hapten is slow (minutes).

The reconstituted ADP/ATP carrier from mitochondria is both inhibited and activated by anions

Anions were found to have a number of different effects on the reconstituted ADP/ATP carrier from mitochondria. (1) Binding of adenine nucleotides to the active site of the translocator is competitively inhibited by various anions. These anions can be arranged in a sequence of increasing competitive effect due to their order in a lyotropic series, and also due to increasing charge. (2) Apart from this competition effect, the presence of a sufficiently high concentration of anions turned out to be absolutely essential for functional ADP/ATP exchange in the reconstituted system. The activating anions too can be arranged in sequence, similar to that of the competition effect. The adenine nucleotide transport shows sigmoidal dependence on the stimulating anions with a Hill coefficient of n = 2. Addition of anions does not change the basic amount of functionally active translocator molecules. (3) The different effects of anions, i.e., inhibition and activation, were shown to take place at different sites and to be due to different mechanisms. Anions compete with substrates both at the outer (cytosolic) and at the inner (matrix) active site, whereas anion activation is observed solely by interaction with the cytosolic side of the translocator protein. (4) Activation of the reconstituted ADP/ATP exchange by anions could be discriminated from an activating influence of anionic phospholipids in the surroundings of the carrier protein.

Interaction of membrane surface charges with the reconstituted ADP/ATP-carrier from mitochondria

Various modulating influences of negative and positive membrane charges on binding and transport properties of the reconstituted ADP/ATP carrier from mitochondria were investigated. The results are interpreted in terms of functional and structural asymmetries of the adenine nucleotide carrier embedded in the liposomal membrane. The surface potential of liposomes was measured directly either by potential-dependent adsorption of the fluorescent dye 2-p-toluidinylnaphthalene 6-sulfonate (TNS) or by the pK shift of the lipophilic pH indicator pentadecylumbelliferone. These results were correlated with the following observations. (1) Negative surface potentials increase the apparent dissociation constant, Kd, for binding of the negatively charged inhibitor carboxyatractylate to the reconstituted carrier protein. (2) Surface potentials modulate the apparent transport affinity, Km, of the reconstituted adenine nucleotide carrier for ADP and ATP. The interaction of surface charges with the transport function was investigated with carrier proteins oriented both right-side-out and inside-out. Thus the influence of the surface potential on the function of the ADP/ATP carrier could be determined for the internal and external active sites of the translocator on the outer side of the membrane. Large discrepancies were observed not only between the potentials measured directly (fluorescent dyes) and those measured indirectly (binding and transport affinities), but also between the different surface potentials determined from the influence on the alternatively oriented carrier proteins. The effect of surface charges was rather weak on the cytosolic side of the translocator, whereas there was a strong influence of surface charges on the active site at the matrix side. The most obvious explanation, i.e., screening of negative membrane charges by positively charged amino acid residues at the protein surface, could be ruled out. Besides the modulation of binding affinities for substrates and inhibitors, an additional side-specific effect of surface charges on the transport velocity was observed. Again, the influence on the internal active site of the ADP/ATP carrier was found to be much higher than that on the cytosolic site. The observed effects can be explained by a definite structural asymmetry of the carrier embedded in the liposomal membrane. That site which is physiologically exposed to the cytosol is located at a considerable distance from the plane of the membrane, whereas the opposite site seems to be in close proximity to the membrane surface. Moreover, a spatial equivalence of carboxyatractylate binding site and nucleotide binding site at the external side of the carrier protein was concluded.

Electrostatic control by lipids upon the membrane-bound (Na+ + K+)-ATPase. II. The influence of surface potential upon the activating ion equilibria

Electrostatic influences upon the enzymatic activity of the (Na+ + K+)-ATPase from ox brain (EC 3.6.1.3) have been studied. (1) The characteristics of the temperature dependence of the activity - the slopes and inflection temperature, Ti, of the Arrhenius plots - have been shown to depend on the total concentration, but not on the specific properties of added monovalent ions. (2) The enzymatic activity has been shown to be subject simultaneously to unspecific and specific influences of alkali-metal ions or NH+4. Ion-specific effects result from different binding constants of complexation between activating ions and enzyme. These stability constants are affected by the formation of an electrical double layer at the membrane surface. With increasing electrostatic screening, the complex formation is destabilized and, as a consequence, the enzymatic activity decreases. (3) This interaction between ion binding and surface electrostatics enables the enzyme to adapt its activity to the actual ionic conditions. This gives rise to a complex net dependence of the enzymatic activity upon the concentrations of activating ions. Such dependencies are analyzed, and an 'activity surface' has been constructed which represents the enzymatic activity as a function of simultaneously varying concentrations of sodium and potassium. The shape of this activity surface is determined by the relations between ion concentrations, surface potential and the resulting stability of the complexation between the activating ions and the enzyme. By means of three-dimensional representation it is demonstrated that the adaptability of the stability constants is of great importance with respect to the maintenance of the optimal ionic concentrations within the living cell. Therefore, by means of the surrounding membrane, the ATPase is provided with a quality, in addition to its substrate specificity and catalytic ability, which is necessary for its function as a transport enzyme.

Determination of surface potential in liposomes

The ability of the fluorescent pH indicator 4-heptadecylumbelliferone to detect the electrical potential at the surface of negatively charged liposomes, was investigated. The vesicles were prepared from mixtures of egg lecithin and dicetyl phosphate at different molar ratios in NaCl solutions of various concentrations. It has been found that the dependence of the experimental surface potential in the proportion of charged lipid in the vesicles and on the salt concentration in the aqueous phase, was very similar to the predictions of the Gouy-Chapman equation as calculated by assuming a reasonable value for the mean molecular area of the lamellar lipids. In view of the good correlation obtained between the experimental and theoretical results, it is concluded that 43-heptadecylumbelliferone is quantitatively sensitive to changes in double-layer potential at the surface of lipid vesicles.

The effect of fatty acids on the surface potential of phospholipid vesicles measured by condensed phase radioluminescence

(1) The carboxyl group of fatty acids has a very low pK value, which is shifted into the physiological pH range when they are incorporated in a phospholipid membrane. As a result of a pH increase the surface charge and surface potential of the membrane increase. (2) The titration of the carboxyl group was observed with condensed phase radioluminescence. This technique uses the electron emitted by the tritiated membrane probe (oleic acid or cholesterol) to excite a fluorophore also incorporated in the bilayer. (3) The phase transition of dimyristoyl phosphatidylcholine vesicles labelled within 12-(9-anthroyloxy)stearic acid was measured by condensed phase radioluminescence at different pH values. (4) We related the condensed phase radioluminescence signal to the asymmetrical distribution of the fluorophore between the inner and outer layer of the lipid membrane which is induced by the repulsion of the negatively charged fatty acids. (5) We showed that the condensed phase radioluminescence signal is proportional to the protonation of the carboxyl group. On this basis, the broadening of the titration curve can be explained as an effect of the self-induced membrane potential calculated using the Gouy-Chapman theory. (6) Ca2+ drastically reduces the flip-flop rate of fatty acids across the membrane and also caused a decrease in the asymmetric distribution. (7) We concluded that a fatty acid can act as a membrane surface buffer. The pK value of 12-(9-anthroyloxy)stearic acid in a dimyristoyl phosphatidylcholine membrane is 7.0 +/- 0.3. (8) We discuss the results with respect to aggregation, fusion and clustering.

Probing the micelle/water interface by a rapid laser-induced proton pulse

The laser-induced pH jump (Gutman, M. and Huppert, D.J. (1979) Biochem. Biophys. Methods 1, 9-19) has a time resolution capable of measuring the diffusion-controlled rate constant of proton binding. In the present study we employed this technique for measuring the kinetics of protonation-deprotonation of surface groups of macromolecules. The heterogeneous surface of proteins excludes them from serving as a simple model, therefore we used micelles of a neutral detergent (Brij 58) as a high molecular weight structure. The charge was varied by the addition of a low concentration of sodium dodecyl sulfate and the surface group with which the protons react was an adsorbed pH indicator (bromocresol green or neutral red). The dissociation of a proton from adsorbed bromocresol green is slower than that from free indicator. This effect is attributed to the enhanced stabilization of the acid form of the indicator in the pallisade region of the micelle. The pK shift of bromocresol green adsorbed on neutral micelles is thus quantitatively accounted for by the decreased rate of proton dissociation. Indicators such as neutral red, which are more lipid soluble in their alkaline form, do not exhibit such decelerated proton dissociation in their adsorbed state nor a pK shift on adsorption to neutral micelles. The protonation of an indicator is a diffusion-controlled reaction, whether it is free in solution or adsorbed on micelles. By varying the electric charge of the micelle this rate can be accelerated or decelerated depending on the total charge of the micelle. The micellar charge calculated from this method was corroborated by other measurements which rely only on equilibrium parameters. The high time resolution of the pH jump is exemplified by the ability to estimate the diffusion coefficient of protons through the hydrated shell of the micelle.

Electrostatic interactions at charged lipid membranes

A quantitative study of calcium-ion binding by the negatively-charged phospholipid methylphosphatidic acid is presented. Experimental results are compared with the predictions of the Gouy-Chapman theory, taking into account both the ions bound at the membrane surface and the ions held in the diffuse layer. This theory suffices to explain the titration of the calcium/lipid system, but fails to explain completely the behaviour of the ordered-fluid transition temperature, which shows a splitting that according to electrostatic theory alone should not occur. The dependence of the calcium-lipid binding constant. upon 1: 1 electrolyte concentration is correctly predicted by the theory; the latter however gives equations which can only be solved numerically. A simple, approximate equation is therefore given (in the text, eq. 34) for the prediction of the degree of calcium binding to a negatively-charged lipid membrane.

Pyranine as a sensitive pH probe for liposome interiors and surfaces. pH gradients across phospholipid vesicles

Pyranine is shown to be a convenient and sensitive probe for reporting pH values, pHi, at the interior of anionic and at the outer surface of cationic liposomes. It is well shielded from the phospholipid headgroups by water molecules in the interior of anionic liposomes, but it is bound to the surface of cationic liposomes. Hydrogen ion concentrations outside the liposomes, 'bulk pH values', pHo, were measured by a combination electrode. While pHi = pHo for neutral, pHi less than pHo for anionic and pHi greater than pHo for cationic liposomes prepared in 5.0 . 10(-3) M phosphate buffers. pKa values for the ionization of pyranine were 7.22 +/- 0.04 and 6.00 +/- 0.05 in water and at the external surface of cationic liposomes. The surface potential for cationic liposomes containing dipalmitoyl-DL-alpha-phosphatidylcholine, cholesterol and octadecylamine in the molar ratio of 1.00 : 0.634 : 1.01, were calcuated to be +72.2 mV. Proton permeabilities were measured for single and multicompartment anionic liposomes. Transfer of anionic liposomes prepared at a given pH to a solution of different pH resulted in a pH gradient if sodium phosphate or borate were used as buffers. In the presence of sodium acetate proton equilibration is promptly established.

Electrostatic interactions at charged lipid membranes. Measurement of surface pH with fluorescent lipoid pH indicators

The 5-dimethylaminonapthalene-1-sulfonyl (dansyl) chromophore attached to the polar head groups of lipids has been used as a fluorescent lipoid pH indicator to evaluate the interfacial pH in lipid-water lamellar systems prepared from negatively charged lipids. The pH in the vicinity of the charged lipid bilayers is different from the pH of the bulk aqueous phase and the difference is a function of the electrolyte concentration in the aqueous phase and of the lipid packing in the bilayer. At a fixed electrolyte concentration in the aqueous phase, the observed interfacial pH is 0.6 to 0.7 pH units lower above the thermal phase transition of the lipid than it is below this temperature. A quantitative interpretation of the results is given on the basis of the Gouy-Chapman theory. The results indicate that the dansyl chromophore is located in front of the charged surface and its distance from this surface increases with a decrease in lipid packing.

An ESR study of the anchoring of spin-labeled stearic acid in lecithin multilayers

In egg lecithin-water lamellar phases, spin-labeled stearic acid gives two superimposed ESR spectra which are only well resolved when the temperature is greater than 30 degrees C. These two spectral components are attributed to the dissociated and non-dissociated forms of the fatty acid carboxylic group, anchored at two different positions in the polar interface constituted by the hydrated lipid polar heads. Results on such interactions of other functional groups (spin-labeled fatty ester and fatty alcohol) are also presented.

Electrostatic interactions at charged lipid membranes. I. Effects of pH and univalent cations on membrane structure

Electrostatic interactions at charged lipid membranes make a significant contribution to the free energy of the system, and can be varied within a wide range by alteration either of the membrane's surface charge density or of the concentration of electrolytes in the surrounding medium. Changes in the charged membrane's structure, such as the ordered in equilibrium fluid phase transition, can thus be induced at constant temperature by variations in pH and salt concentration. An adequate quantitative description of these phenomena is obtained from the Gouy--Chapman theory. The good agreement between theory and experiment confirms that the expression derived for the electrostatic free energy especially in respect of its positive sign is correct. The classical expression derived for the electrostatic free energy, especially in respect of its positive sign, is correct. The classical expression for the "free energy of the double layer" derived by Verwey and Overbeek, which has a negative sign, is not applicable to lipid membranes with ionizable polar groups.