Microwave dielectric study on hydration of moist collagen

Protein dielectrophoresis: Key dielectric parameters and evolving theory

Globular proteins exhibit dielectrophoresis (DEP) responses in experiments where the applied field gradient factor ∇E² appears far too small, according to standard DEP theory, to overcome dispersive forces associated with the thermal energy kT of disorder. To address this a DEP force equation is proposed that replaces a previous empirical relationship between the macroscopic and microscopic forms of the Clausius-Mossotti factor. This equation relates the DEP response of a protein directly to the dielectric increment δε⁺ and decrement δε^- that characterize its β-dispersion at radio frequencies, and also indirectly to its intrinsic dipole moment by way of providing a measure of the protein's effective volume. A parameter Γ_pw , taken as a measure of cross-correlated dipole interactions between the protein and its water molecules of hydration, is included in this equation. For 9 of the 12 proteins, for which an evaluation can presently be made, Γ_pw has a value of ≈4600 ± 120. These conclusions follow an analysis of the failure of macroscopic dielectric mixture (effective medium) theories to predict the dielectric properties of solvated proteins. The implication of a polarizability greatly exceeding the intrinsic value for a protein might reflect the formation of relaxor ferroelectric nanodomains in its hydration shell.

Electric-field penetration depth and dielectric spectroscopy observations of human skin

Background

The dynamic behavior of water molecules remains an important subject for understanding human skin. The change in the dynamics of water molecules from those in bulk water can be effectively observed by dielectric spectroscopy. To study water in the human skin in vivo, non‐invasive and non‐destructive measurements are essential. Since many unknowns remain from previous research, in this report we employ a two‐layer dielectric model to evaluate the penetration depth of the electric field and use the results in measurements on human skin.

Materials and Methods

We used open‐ended coaxial probes with different diameters to perform time‐domain reflectometry (TDR) measurements for an acetone‐Teflon double‐layer model and for human skin from various parts of the body.

Results

The electric‐field penetration depth obtained from model measurements increases with the increasing outer diameter of open‐ended coaxial electrodes. For skin measurements, the relaxation strength corresponding to the water content shows a clear dependence on the epidermal thickness of the measured body parts.

Conclusion

We determined the depth distribution of the water content of skin from results of dielectric measurements obtained using electrodes with various electric‐field penetration depths. We found exponential decays with the thickness of the epidermis of each body part for several examinees. This study suggests an effective method for detailed evaluations of human skin.

Physical Meanings of Fractal Behaviors of Water in Aqueous and Biological Systems with Open-Ended Coaxial Electrodes

The dynamics of a hydrogen bonding network (HBN) relating to macroscopic properties of hydrogen bonding liquids were observed as a significant relaxation process by dielectric spectroscopy measurements. In the cases of water and water rich mixtures including biological systems, a GHz frequency relaxation process appearing at around 20 GHz with the relaxation time of 8.2 ps is generally observed at 25 °C. The GHz frequency process can be explained as a rate process of exchanges in hydrogen bond (HB) and the rate becomes higher with increasing HB density. In the present work, this study analyzed the GHz frequency process observed by suitable open-ended coaxial electrodes, and physical meanings of the fractal nature of water structures were clarified in various aqueous systems. Dynamic behaviors of HBN were characterized by a combination of the average relaxation time and the distribution of the relaxation time. This fractal analysis offered an available approach to both solution and dispersion systems with characterization of the aggregation or dispersion state of water molecules. In the case of polymer-water mixtures, the HBN and polymer networks penetrate each other, however, the HBN were segmented and isolated more by dispersed and aggregated particles in the case of dispersion systems. These HBN fragments were characterized by smaller values of the fractal dimension obtained from the fractal analysis. Some examples of actual usages suggest that the fractal analysis is now one of the most effective tools to understand the molecular mechanism of HBN in aqueous complex materials including biological systems.

Dielectric spectroscopy in the GHz region on fully hydrated zwitterionic amino acids

The complex dielectric permittivity of eight different amino acids in water solutions was determined in the frequency range from 0.2 to 20 GHz at room temperature, trying to span the whole range of solubility in each case. Two relaxations were observed at room temperature in this frequency range, which can be mainly assigned to the rotation of amino acids in the aqueous environment, and the reorientational motion of water molecules, respectively. Although the amino acids have a charged (zwitterionic) nature with huge dipole moments, the tendency towards dipolar alignment seems to be very weak, over the investigated concentration ranges. For these small bio-molecules, water screens solute-solute interactions and amino acids remain typically as isolated hydrated monomers. The dielectric results were used to estimate the number of water molecules restrained by each solute molecule. Finally, the comparison between the amino acid relaxation times made it possible to discuss the relationship between rotational dynamics and the structure and hydrodynamic coupling of the amino acid studied.

Biomimetic remineralization as a progressive dehydration mechanism of collagen matrices--implications in the aging of resin-dentin bonds

Biomineralization is a dehydration process in which water from the intrafibrillar compartments of collagen fibrils are progressively replaced by apatites. As water is an important element that induces a lack of durability of resin-dentin bonds, this study has examined the use of a biomimetic remineralization strategy as a progressive dehydration mechanism to preserve joint integrity and maintain adhesive strength after ageing. Human dentin surfaces were bonded with dentin adhesives, restored with resin composites and sectioned into sticks containing the adhesive joint. Experimental specimens were aged in a biomimetic analog-containing remineralizing medium and control specimens in simulated body fluid for up to 12 months. Specimens retrieved after the designated periods were examined by transmission electron microscopy for the presence of water-rich regions using a silver tracer and for collagen degradation within the adhesive joints. Tensile testing was performed to determine the potential loss of bond integrity after ageing. Control specimens exhibited severe collagen degradation within the adhesive joint after ageing. Remineralized specimens exhibited progressive dehydration, as manifested by silver tracer reduction and partial remineralization of water-filled microchannels within the adhesive joint, as well as intrafibrillar remineralization of collagen fibrils that were demineralized initially as part of the bonding procedure. Biomimetic remineralization as a progressive dehydration mechanism of water-rich, resin-sparse collagen matrices enables these adhesive joints to resist degradation over a 12-month ageing period, as verified by the conservation of their tensile bond strength. The ability of the proof of concept biomimetic remineralization strategy to prevent bond degradation warrants further development of clinically relevant delivery systems.

Tannic acid-stabilized pericardium tissue: IR spectroscopy, atomic force microscopy, and dielectric spectroscopy investigations

Infrared (IR) spectroscopy, atomic force microscopy (AFM), and dielectric spectroscopy methods were employed to study structural and dynamic changes in the tannic acid (TA)-stabilized pericardium tissue. Chemically stabilized pericardium tissue is widely used in construction of the tissue derived bioprostheses. IR spectra recorded in the range 400-4000 cm-1 allowed us to recognize different types of TA-collagen interactions. Formation of hydrogen bonds between amine as well as amide NH groups from collagen and hydroxyl groups of TA was analyzed. The AFM imaging showed that the stabilization procedure with TA introduces considerable changes in both surface topography and thickness of collagen fibrils as well as in fibril arrangement on the tissue surface. It was found, that these structural changes have an impact on the dielectric behavior of the TA-stabilized tissue. The dielectric spectra for the native and TA-stabilized tissues were measured in the frequency and temperature ranges of 10(-1) -10(7) Hz and 120-270 K, respectively. The dielectric spectra revealed the relaxation process due to orientation of bound water supplemented by the fluctuation of collagen polar side groups. At the temperatures above approximately 210 K, the relaxation due to ion migration process was observed. It was found that both relaxation processes were influenced by the TA-collagen interaction.

Free water content and monitoring of healing processes of skin burns studied by microwave dielectric spectroscopyin vivo

We have investigated the dielectric properties of human skin in vivo at frequencies up to 10 GHz using a time-domain reflectometry method with open-ended coaxial probes. Since gamma-dispersion results from the reorientation of free water molecules, the free water content of skin is quantitatively determined by dielectric measurements. The free water content of finger skin increased by about 10% after soaking in 37 degrees C water for 30 min, and it systematically decreased again through the drying process, as expected. Thus this analytical method has been applied to the study of skin burns. The free water content of burned human cheek skin due to hydrofluoric acid was significantly lower than that of normal skin, and the burned skin recovered through the healing process. In the case of a human hand skin burn due to heat, although the free water content was almost the same as that of normal skin at the beginning, it decreased during the healing process for the first 10 days, then began to increase. Although the number of test subjects was one for each experiment, it was shown that free water content is a good indicator for evaluating skin health and can be well monitored by dielectric spectroscopy.

Ability of Polyvinylpyrrolidone and Polyacrylic Acid to Inhibit the Crystallization of Amorphous Acetaminophen

The inhibition of crystallization of amorphous acetaminophen (ACTA) by polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) was studied using amorphous solid dispersions prepared by melt quenching. Co-melting with PVP and PAA decreased the average molecular mobility, as indicated by increases in glass transition temperature and enthalpy relaxation time. The ACTA/PAA dispersion exhibited much slower crystallization than the ACTA/PVP dispersion with a similar glass transition temperature value, indicating that interaction between ACTA and polymers also contributed to the stabilizing effect of these polymers. The carboxyl group of PAA may interact with the hydroxyl group of ACTA more intensely than the carbonyl group of PVP does, resulting in the stronger stabilizing effect of PAA. Dielectric relaxation spectroscopy showed that the number of water molecules tightly binding to PVP per monomer unit was larger than that to PAA. Furthermore, a small amount of absorbed water decreased the stabilizing effect of PVP, but not that of PAA. These findings suggest that the stronger stabilizing effect of PAA is due to the stronger interaction with ACTA. The ability of PAA to decrease the molecular mobility of solid dispersion was also larger than that of PVP, as indicated by the longer enthalpy relaxation time.

Dielectric study of heat-denatured ovalbumin in aqueous solution by time domain reflectometry method

The dielectric behavior of native and heat-denatured ovalbumins (OVAs) from three avian species in aqueous solution was examined over a frequency range of 100 kHz to 20 GHz, using the time domain reflectometry (TDR) method. For the native OVA solutions, three kinds of relaxation processes were observed at around 10 MHz, 100 MHz, and 20 GHz, respectively; these could be assigned to the overall rotation of protein molecules, the reorientations of the bound water, and the free water molecules, respectively. For the heat-denatured samples, three relaxation processes were also observed. However, the relaxation process at approximately 100 MHz originated via a different mechanism other than the reorientation of bound water, namely, the micro-Brownian motion of peptide chains of heat-denatured protein. From the observed relaxation process at approximately 100 MHz, the relaxation strength of heat-denatured OVA solution for duck was higher than that of OVA solutions for hen and guinea fowl and showed the pH dependency from pH 7.0 to 8.0 for OVAs obtained from all three species. Furthermore, the results demonstrated that the relaxation strength was closely related to surface hydrophobicity of protein molecules and gel rheological properties. It was suggested that the difference in the surface hydrophobicity of protein influenced the dielectric behavior of water around denatured protein, whereas the dielectric behavior of denatured protein could be an indication of the gel rheological properties. Such studies can aid in the understanding of the different network structures of OVA gels from three avian species.

Evidence that collagen fibrils in tendons are inhomogeneously structured in a tubelike manner

The standard model for the structure of collagen in tendon is an ascending hierarchy of bundling. Collagen triple helices bundle into microfibrils, microfibrils bundle into subfibrils, and subfibrils bundle into fibrils, the basic structural unit of tendon. This model, developed primarily on the basis of x-ray diffraction results, is necessarily vague about the cross-sectional organization of fibrils and has led to the widespread assumption of laterally homogeneous closepacking. This assumption is inconsistent with data presented here. Using atomic force microscopy and micromanipulation, we observe how collagen fibrils from tendons behave mechanically as tubes. We conclude that the collagen fibril is an inhomogeneous structure composed of a relatively hard shell and a softer, less dense core.

Ordering in aqueous polysaccharide solutions. I. Dielectric relaxation in aqueous solutions of a triple-helical polysaccharide schizophyllan

Deuterium oxide solutions of a triple-helical polysaccharide schizophyllan, undergoing an order-disorder transition centered around 17 degrees C, were studied by the time-domain reflectometry (TDR) to obtain dielectric dispersions in the solution and frozen states. In the solution state, the dispersion below the transition temperature is resolved in three dispersions (relaxation times at 0 degrees C) ascribed to side chain glucose residue (1; 102 ns), structured water (s; 2.0 ns) and bulk water (h), respectively, from low to high frequencies. Bulk water is divided into slow water (h2; 0.04 ns) and free or pure water (h1; 0.02 ns). Above the transition temperature structured water almost disappears and is compensated by slow water. Structured water is similar to bound water for proteins but different from it because of this transition behavior. Another dispersion (l) seen at the lowest frequency is assigned to the rotation of side-chain glucose residue coupled with hydrated water. Parts of this dispersion and structured water are suggested to constitute bound water. In the frozen state were observed a major dispersion (h; 0.14 ns) and a minor one (m; 28 ns), which were ascribed to considerably mobile and less mobile waters. They are similar to but not exactly the same as that for unfreezable water in bovine serum albumin solutions argued by Miura et al. (Biopolymers, 1995, Vol. 36, p. 9). Water is molded into different structures by the triple helix.

Globule-coil transition of denatured globular protein investigated by a microwave dielectric technique

A mechanism for the gel-glass transition of denatured globular protein has been explained from the viewpoint of the globule-coil transition with microwave dielectric measurements using a time domain reflectometry (TDR) method. Boiled egg white, which is an aqueous gel of egg white prepared by heat treatment at 100 degrees C, becomes a glass on drying. In the gel state, the relaxation processes corresponding to the orientation of bulk water and the micro-Brownian motion of peptide chains of denatured protein were observed around 10 GHz and 10 MHz, respectively. When the gel-glass transition occurred, the relaxation strength for bulk water decreased rapidly as evaporation and breaking of water structure occurred. Simultaneously, the relaxation strength for micro-Brownian motion increased abruptly, as the structure of globular protein varied from globule state to coiled state. It is considered that the protein molecule spreads out and takes up a coiled state by reductions of hydrophobic and hydrophilic interactions of the globular protein. These reductions occur through a decrease in the amount of water.

Molecular dynamics of hinge-bending motion of IgG vanishing with hydrolysis by papain

We have performed dielectric relaxation measurements via a time domain reflectometry (TDR) method to study dynamic behaviors of the segmental flexibility of immunoglobulin G (IgG) in aqueous solution without antigen binding. In general, an intermediate relaxation process due to bound water is observed around 100 MHz at 25 degrees C for common proteins between two relaxation processes due to overall rotation and reorientation of free water. However, the intermediate process observed around 6 MHz for IgG was due to both bound water and hinge-bending motion. The apparent activation energy of 33 kJ/mol was larger than 27 kJ/mol for only bound water, and the relaxation strength was about five times as large as expected for bound water. The shape of the relaxation curve was very broad and asymmetric. These characteristic differences arising from the hinge-bending motion of IgG disappeared for fragments decomposed from IgG hydrolyzed by papain, since the hinge-bending motion did not exist in this case. We have separated the relaxation processes due to hinge-bending motion and bound water for IgG and obtained the Fab-Fab angle of IgG as about 130 degrees by Kirkwood's correlation parameter and the activation energy of 34 kJ/mol for hinge-bending motion.

Microwave Dielectric Relaxation of Bound Water to Silica, Alumina, and Silica-Alumina Gel Suspensions

By the use of a time domain reflectomery method, dielectric measurements were carried out on silica, alumina, and silica-alumina gel suspensions (five types with composition varying between 0.3 < Si/Al atomic ratio <0.8) in the frequency range of 100 kHz to 20 GHz. For all the gels, a relaxation peak due to bound water was observed. This peak locates at around 1-10 MHz, indicating that the peak is a decade or 10 decades lower than those of biological polymers such as an aqueous DNA solution. The silica and alumina gels have a different bound water structure, judging from the fact that the peaks are different in their locations and shapes between the two gels. The silica-alumina gels exhibit two different peaks other than the peak of bulk water. The sum of the relaxation strength on the two peaks is proportional to the monolayer capacity obtained from water vapor isotherms. The shape of the one peak holds that of the silica gel, whereas the other retains that of alumina gel, and furthermore the ratio of the relaxation strength on the former peak to that on the latter depends on Si/Al atomic ratio. It is suggested that both peaks are caused by the orientation of bound water molecules. Copyright 1999 Academic Press.

Microwave dielectric analysis of human stratum corneum in vivo

The dielectric properties of the human skin stratum corneum (SC) in the frequency range higher than 107 Hz are not well understood because of the difficulty in selective scanning of the SC area in vivo. The present study was carried out to make clear factors responsible for the dielectric properties using a measuring system specially developed for the study of SC [S. Naito, M. Hoshi, S. Mashimo, Anal. Biochem. 251 (1997) 163-172]. We found that the dielectric properties of SC can be expressed by the linear combination of two relaxation processes and d.c. conduction. The faster relaxation is that of free water. The slower relaxation and d. c. conduction were analyzed using a model assuming interfacial polarization between dissimilar materials. We concluded that the polarization is the origin of the slower relaxation process because the experimental data could be well interpreted according to the above mechanism. We also concluded that the polarization of swelled SC locates at the interface between SC cells and the intercellular lipid layer, or at the interface between the lipophilic and the hydrophilic part of the lamellar structured intercellular lipid layer.

Raman spectral evidence for hydration forces between collagen triple helices

Hydration forces are thought to result from the energetic cost of water rearrangement near macromolecular surfaces. Raman spectra, collected on the same collagen samples on which these forces were measured, reveal a continuous change in water hydrogen-bonding structure as a function of separation between collagen triple helices. The varying spectral parameters track the force-distance curve. The energetic cost of water "restructuring," estimated from the spectra, is consistent with the measured energy of intermolecular interaction. These correlations support the idea that the change in water structure underlies the exponentially varying forces seen in this system at least over the 13-18-A range of interaxial separations.

In vivo dielectric analysis of free water content of biomaterials by time domain reflectometry

A method of in vivo analysis of the free water content in living organisms by dielectric analysis in the time domain is described. Human skin is chosen as an example of living tissue. The cells suitable for the measurement of various layers of human skin and calculation procedures for the waveform reflected from the probe end are described. The approach was confirmed to be effective for the determination of the water content through measurement of the standard samples, keratin-water mixtures. This method was also applied to human skin in vivo. Water content data measured with a probe specially designed for surface layer analysis were sensitive to humidity around the subject. The formula expressing the relation between the electrical field character of the probe, the permittivity depth profile, and the measured permittivity was used to analyze the water content profile as a function of the depth from the skin surface. The use of several kinds of probes, differing in their electric field characteristics, permitted evaluation of the water content depth profile of human skin. This procedure is easy and applicable to any sample due to its simplicity. The measurement needs only a touch of the probe on a sample spot. It is therefore a promising method of physicochemical research on living organisms and biomaterials.

X-ray diffraction analysis of tendon collagen at ambient and cryogenic temperatures: role of hydration

Equatorial (d) and meridional (D) spacings of native rat tail tendon (RTT) and unmineralized native turkey leg tendon (UTLT) were measured at ambient and liquid-nitrogen temperatures, using X-ray diffraction. Cooling of air-dried RTT or UTLT caused little change in d, which was approximately equal (1.1 nm) in the two tissues before and after cooling. For fully hydrated RTT or UTLT, cooling caused the familiar broad equatorial diffraction pattern to increase in sharpness to more resemble the pattern seen in dehydrated tissue. The d-spacings of hydrated RTT and UTLT fell by 0.12 nm (8.5%) and 0.19 nm (13%), respectively to values seen (at ambient temperature) when the tissue water content is the maximum possible in the absence of unbound water (0.5 g water/g dry collagen). These results can be explained by the movement of water within the fibril. In tissue with a water content of greater than 0.5 g/g dry collagen the spacings reflecting the lateral packing of the axially-linked tropocollagen molecules comprising a collagen fibril are determined partly by the unbound component of intermolecular hydration. As the bulk water between the fibrils freezes, this mobile component remains initially unfrozen as the tissue is cooled below zero degree C. It diffuses from the intermolecular spaces into the interfibrillar spaces where it also rapidly freezes. This allows the d-spacing to decrease to the value appropriate for the presence of bound intermolecular water only. The mechanism is likely to be an energetically favourable relaxation of the lateral positions of the tropocollagen molecules from a quasi-hexagonal arrangement to hard-disk liquid-like packing. The results and methods of this study may be applicable in the elaboration of more complex collagenous systems.

Solvent hydrogen-bond network in protein self-assembly: solvation of collagen triple helices in nonaqueous solvents

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers in reconstituted films is controlled by the concentration of soluble polymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between the triple helices as a function of the polymer concentration is measured by x-ray diffraction. Exponential-like changes in the spacing with increasing osmotic stress, qualitatively similar to the forces previously found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming three-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen-bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some threshold where removal of the solvent becomes energetically favorable. No solvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2-methoxyethanol, or N-methylformamide, the change in spacing with polymer concentration is intermediate between exponential-like and hard-wall. These results provide direct evidence that the exponential repulsion observed between collagen helices at 0-8-A surface separations in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen surfaces.

Water mobility in poly(ethylene glycol)-, poly(vinylpyrrolidone)-, and gelatin-water systems, as indicated by dielectric relaxation time, spin-lattice relaxation time, and water activity

The mobility of water molecules present in poly(ethylene glycol) (PEG)-, poly(vinylpyrrolidone) (PVP)-, and gelatin-water systems was determined by dielectric relaxation and 17O NMR spectroscopy. Water activity was also measured. Dielectric relaxation spectra indicate that all the polymer systems studied contained water exhibiting a dispersion at a frequency > 10(9) Hz; in other words, water with high mobility close to that of bulk water. The dielectric relaxation time of the highly mobile water increased as polymer concentration increased. The PVP- and gelatin-water systems also contained water exhibiting a dispersion at a frequency < 10(9) Hz, which can be considered to be "bound water" with a restricted mobility because of its association with polymer molecules. Dielectric relaxation spectroscopy was used to determine water mobility separately for the populations of highly mobile water and bound water, whereas NMR relaxation spectroscopy was used to determine the average mobility of both populations. The spin-lattice relaxation time of water in these polymer-water systems showed a deviation from the isotropic two-state model. Dielectric relaxation data indicate that this deviation can be ascribed to variations in the relaxation time of highly mobile water caused by a change in polymer concentration. The dielectric relaxation time of highly mobile water in the gelatin system did not change with a change in polymer concentration to the extent that it did in the PEG and PVP systems. This result is consistent with a slight change in water activity of the gelatin system with increasing polymer concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Dielectric relaxation spectroscopy and some applications in the pharmaceutical sciences

With a few exceptions, dielectric relaxation spectroscopy (DRS) has been largely neglected by pharmaceutical scientists, despite the potential for this technique as a noninvasive and rapid method for the structural characterization and quality control of pharmaceutical materials. DRS determines both the magnitude and time dependency of electrical polarization (i.e. the separation of localized charge distributions) by either measuring the ability of the material to pass alternating current (frequency domain DRS) or by investigating the current that flows on application of a step voltage (time domain DRS). DRS is thus (i) sensitive to molecular mobility and structure, (ii) non-invasive, and (iii) employs only mild stresses (a weak electromagnetic field) in order to measure the sample properties. The technique covers a broad-band frequency window (from 10(-5) to 10(11) Hz) and therefore enables the investigation of a diverse range of processes, from slow and hindered macromolecular vibrations and restricted charge transfer processes (such as proton conductivity in nearly dry systems) to the relatively fast reorientations of small molecules or side chain groups. The dielectric response provides information on (i) structural characteristics of polymers, gels, proteins, and emulsions, (ii) the interfacial properties of molecular films, (iii) membrane properties, (iv) water content and states of water (and the effects of water as a plasticizer), and (v) lyophilization of biomolecules. This review article details the basis of dielectric theory and the principles of measuring dielectric properties (including a comprehensive account of measurement artifacts), and gives some applications of DRS to the pharmaceutical sciences.

Thermal stability and folding of the collagen triple helix and the effects of mutations in osteogenesis imperfecta on the triple helix of type I collagen

Osteogenesis imperfecta (OI) is an inherited disease in which 90% of the cases result from mutations in the 2 genes, pro alpha 1 and pro alpha 2, coding for type I collagen. Type I collagen is a trimeric molecule, (alpha 1)2 alpha 2, which is dominated both structurally and functionally by the 300 nm triple-helical domain. Most OI mutations occur in this domain and almost all point mutations result in the substitution of other amino acids for the obligate glycine which occurs at every third residue. The phenotypic effects of these mutations are frequently attributed in part to alterations in the stability and rate of folding of the triple helix. In order to better understand the relationship between glycine substitutions and stability we review current concepts of the forces governing triple helical stability, denaturational and predenaturational unfolding, and the techniques of measuring stability. From observations on the stability of several collagen types as well as synthetic tripeptides, we present a model for stability based on the contribution of individual and neighboring tripeptide units to the local stability. Although in preliminary form, this empirical model can account for the observed shifts in the Tm of many of the point mutations described. The folding of the triple helix is reviewed. The involvement of peptidyl prolyl cis-trans isomerase in this process in vivo is demonstrated by the inhibition of collagen folding in fibroblasts by cyclosporin A. An hypothesis based on the relationship between the thermal stability at the site of mutation and the propensity for renucleation of folding is proposed.

Effect of water mobility on drug hydrolysis rates in gelatin gels

The stability of drugs incorporated in gelatin gels was studied, with a focus on the water mobility in the gels. Trichlormethiazide hydrolysis and kanamycin-catalyzed flomoxef hydrolysis in gelatin gels were chosen as models for apparent first-order and second-order hydrolysis, respectively. The mobility of water in gelatin gels was determined by NMR, ESR, and dielectric relaxation spectroscopies. The amount of bound water in the gels was determined from dielectric relaxation spectra. Spin-lattice relaxation time of water determined by 17O NMR and rotational correlation time of an ESR probe determined by an ESR probing method were useful in determining the microviscosity of the gels. The hydrolysis rate of trichlormethiazide in the gels was found to depend on the amount of free water available for the reaction, while that of flomoxef depended on the microviscosity of the gels, which reflected the mobility of water molecules. Thus the dependence of hydrolysis rates on the water mobility was influenced by the hydrolysis mechanism.

Sequential hydration-dehydration studies of lysozyme by the thermally stimulated depolarization currents (TSDC) technique

To investigate the physical state of water in hydrating biological macro-molecules, the dielectric properties of water in hen egg lysozyme pellets with various moisture contents were studied using the thermally stimulated depolarisation currents technique. The water dipoles appeared to be directly involved in the relaxation processes, such that, by increasing the content of water of sorption from ho = 0.075 to ho = 0.29, the current density recorded increased abruptly at moisture content above 0.075. At a fixed starting hydration level, the time evolution of water content was also studied by isothermal sample aging in dynamic vacuum: the TSDC spectra changed in both intensity and position of their main peaks (TM = 245 K, 190 K, 150 K) with moisture content and showed hysteresis. The complex behaviour of the TSDC response can be compared with the results obtained with the same technique on other biological macromolecules and suggests possible models for water configurations and rearrangements.

Bound water in the collagen-like triple-helical structure

The ir amide bands of the triple-helical polytripeptides and collagens upon hydration of films are investigated. On the basis of our assignment of the amide I components, the formation of hydrogen bonds between the peptide backbone and structural water is studied. The C1O1--HOH hydrogen bonds are found more ordered than the C3O3--HOH hydrogen bonds. The specific incorporation of water in the triple helix is followed by multistep conformational changes and by increasing of the interpeptide hydrogen-bond strength. The formation of the polypeptide hydrate structure depending on the amino acid composition and the chain length is examined.