X-ray diffraction study of in vitro calcification of tendon collagen

Decalcified samples of turkey leg tendon were submitted to in vitro calcification in the presence of metastable solutions of calcium phosphate at different concentrations. The structural relationship between apatitic deposits and collagen fibrils was examined by high- and small-angle X-ray diffraction using conventional and synchrotron radiation sources. At high supersaturation the apatitic crystallites were deposited on the collagen fibrils with their crystallographic c-axis preferentially oriented parallel to the fibril axis. At lower supersaturation, a fraction of the apatitic crystallites also grew with the c-axis preferentially oriented parallel to the collagen fibril axis, whereas other exhibited a preferential orientation perpendicular to the fibril axis. The analysis of the small-angle X-ray diffraction data indicates that the deposition of the apatitic phase in the sample stored in solution at lower supersaturation induced modifications of the collagen electron density distribution in the axial direction, which can be attributed to the deposition of the inorganic crystallites inside the gap region of the collagen structure.

Small-angle X-ray scattering of kappa- and iota-carrageenan in aqueous and in salt solutions

Aqueous and saline (0.03-0.08 M NaCl) solutions of the Na(+)-salt of kappa- and iota-carrageenan (3-17 mg/ml) have been investigated using small-angle X-ray scattering (SAXS). Investigation of the SAXS curves normalized for concentration allows to detect the transition from the dilute to the semidilute regime. Some of the experimental curves exhibit a clear maximum. In the case of kappa-carrageenan, the position of this maximum is proportional to the cube root of polysaccharide concentration. The fluctuation theory of Borue and Erukhimovich (BE theory), which predicts a maximum in the scattering curves for low ionic strength, gives a good fit for all our experiments. An increase of the polymer concentration or the ionic strength causes the appearance of two subsystems in solution exhibiting a different characteristic screening scale of Coulombic interaction for ideal Gaussian chains (and two different segment lengths). The condition for an increase of the relative contribution of the subsystem with the larger segment length correlates with the one for the disorder-order transition of carrageenan. In the most concentrated (17.1 mg/ml) solution of Na(+)-iota-carrageenan, the increase of NaCl molarity above 0.06 M gives rise to additional scattered intensity at the smallest angles, indicating molecular association.

Structural model of the 50S subunit of E. coli ribosomes from solution scattering

The application of new methods of small-angle scattering data interpretation to a contrast variation study of the 50S ribosomal subunit of Escherichia coli in solution is described. The X-ray data from contrast variation with sucrose are analyzed in terms of the basic scattering curves from the volume inaccessible to sucrose and from the regions inside this volume occupied mainly by RNA and by proteins. From these curves models of the shape of the 50S and its RNA-rich core are evaluated and positioned so that their difference produces a scattering curve which is in good agreement with the scattering from the protein moiety. Based on this preliminary model, the X-ray and neutron contrast variation data of the 50S subunit in aqueous solutions are interpreted in the frame of the advanced two-phase model described by the shapes of the 50S subunit and its RNA-rich core taking into account density fluctuations inside the RNA and the protein moiety. The shape of the envelope of the 50S subunit and of the RNA-rich core are evaluated with a resolution of about 40 A. The shape of the envelope is in good agreement with the models of the 50S subunit obtained from electron microscopy on isolated particles. The shape of the RNA-rich core correlates well with the model of the entire particle determined by the image reconstruction from ordered sheets indicating that the latter model which is based on the subjective contouring of density maps is heavily biased towards the RNA.

Physicochemical characterization of protein-free low density lipoprotein models and influence of drug loading

PURPOSE

Drug free and drug loaded protein-free low density lipoprotein (LDL) models consisting mainly of phospholipids, cholesterol, cholesterol esters, and triglycerides in ratios found for physiological LDL have been prepared. Their physicochemical characteristics were compared with those of physiological LDL.

METHODS

Different characterization methods were used: photon correlation spectroscopy, transmission electron microscopy, X-ray solution scattering, and 1H nuclear magnetic resonance spectroscopy (NMR).

RESULTS

Particle sizes are highly dependent on the preparation method and in particular on the homogenization conditions. Electron microscopy indicates that the size distributions of model systems are much broader than those of physiological LDL. The X-ray solution scattering patterns of the model systems display a temperature dependent maximum near 3.8 nm similar to that found in the patterns of physiological LDL. NMR indicates a comparable mobility of the lipid molecules in model particles and in physiological LDL. The influence of drug loading is similar to that found earlier for physiological LDL. In particular, the incorporation of the anti-cancer drug WB 4291 seems to have a fluidizing effect on the lipids in the core region of the particles.

CONCLUSIONS

The preparation method of LDL model systems is of crucial importance as only the solvent evaporation method yielded systems in the size range of physiological LDL with acceptable high lipid concentrations. The fluidizing influence of temperature and drug incorporation (WB 4291) may be disadvantage in drug targeting.

Nonideality of water-hexafluoropropanol mixtures as studied by X-ray small angle scattering

Strong temperature dependent low angle X-ray scattering by trifluoroethanol- and hexafluoro-2-propanol-water mixtures was observed in conditions commonly used in NMR work on peptides. At least two types of molecular effects may contribute to the observed scattering: formation of clathrate hydrate-like aggregates of alcohol with water as have been proposed for the similar system tert-butanol-water (Iwasaki, K. and Fujiyama, T. (1976) J. Phys. Chem. 81, 1908-1912) and a further heterogeneity of the solution resulting from immiscibility of the two components.

Influence of phosphoenolpyruvate and magnesium ions on the quaternary structure of enzyme I of the phosphotransferase system from gram-positive bacteria

Solution X-ray scattering patterns of enzyme I of the phosphotransferase system from Staphylococcus carnosus indicate an increase in radius of gyration and molecular mass in the presence of Mg2+ or both Mg2+ and phosphoenolpyruvate, indicating a partial dimerization of enzyme I. Mg2+ ions are essential for both the dimerization and the activation, whereas the substrate phosphoenolpyruvate shifts the monomer--dimer equilibrium to the enzymatically active dimer by decreasing the dissociation rate of the phosphorylated dimer.

Small-angle X-ray solution scattering study on the dimerization of the FKBP25mem from Legionella pneumophila

The dimerization of the FK506-binding peptidyl-prolyl cis/trans-isomerase (PPIase) FKBP25mem (Mip (macrophage infectivity potentiator) protein) from Legionella pneumophila was studied by small-angle X-ray solution scattering. A value of 44 kDa, independent on the protein concentration between 2 and 13 mg/ml, confirming that FKBP25mem is a dimer was found for the molecular mass of the protein. The radius of gyration of the protein is 3.3 nm and the Porod volume 87 nm3. A model of the shape of FKBP25mem was evaluated from the scattering curve. Each monomer consists of a proximal and a peripheral domain, which are perpendicular to each other. The envelope of the crystallographic model of human FKBP12 fits well into the peripheral domain. The contact regions between the two monomers in the dimeric protein are probably located between the N-terminal parts of the monomers.

Ultrastructural and biochemical modifications of collagen from tissue of Morbus Dupuytren patients

Small angle X-ray diffraction and biochemical analyses were carried out on normal palmar aponeurosis and on tissue from patients suffering from Dupuytren contractures (MD). Pathological tissue exhibits a higher overall content of collagen III. Type I collagen extracted from pathological tissue has a melting point of 0.8 degrees C higher than that of normal collagen. The only chemical differences compared to normal collagen I are 50% overhydroxylation of lysyl residues and a reduced amount of diglycosylated hydroxylysine residues. Analysis of the electron density distribution inside the collagen repeating period of MD-samples reveals disordered molecular packing in MD samples compared to in normal collagen. The disorder, which is higher in the gap region, is considerably reduced upon stretching.

Behavior of cholesterol and spin-labeled cholestane in model bile systems studied by electron spin resonance and synchrotron x-ray

The behavior of mixed bile salt micelles consisting of sodium taurocholate, egg phosphatidylcholine, and cholesterol has been studied by ESR spin labeling and synchrotron x-ray scattering. Consistent with published phase diagrams, pure and mixed bile salt micelles have a limited capacity to incorporate and, hence, solubilize cholesterol. Excess cholesterol crystallizes out, a process that is readily detected both by ESR spin labeling using 3-doxyl-5 alpha-cholestane as a probe for cholesterol and synchrotron x-ray scattering. Both methods yield entirely consistent results. The crystallization of cholesterol from mixed bile salt micelles is indicated by the appearance of a magnetically dilute powder spectrum that is readily detected by visual inspection of the ESR spectra. Both the absence of Heissenberg spin exchange and the observation of a magnetically dilute powder spectrum provide evidence for the spin label co-crystallizing with cholesterol. In mixed bile salt micelles containing egg phosphatidylcholine, the solubility of cholesterol is increased as detected by both methods. With increasing content of phosphatidylcholine and increasing mole ratio cholesterol/phosphatidylcholine, the anisotropy of motion of the spin probe increases. The spin label 3-doxyl-5 alpha-cholestane is a useful substitute for cholesterol provided that it is used in dilute mixtures with excess cholesterol: the cholesterol/spin label mole ratio in these mixtures should be greater than 100. Despite the structural similarity between the two compounds, there are still significant differences in their physico-chemical properties. These differences come to the fore when cholesterol is totally replaced by the spin-label: 3-doxyl-5a-cholestane is significantly less soluble in bile salt and mixed bile salt micelles than cholesterol and, in contrast with cholesterol, it interacts only very weakly, if at all,with phosphatidylcholine. The potential of the ESR method for detecting cholesterol crystal growth in human bile is discussed.

Phase behavior of tyloxapol/Triton X100/water mixtures

The influence of admixture of Triton X100 to the polymer tyloxapol and the phase behavior of the mixtures in contact with water was investigated by viscometry, polarization microscopy, and X-ray scattering. The viscosity of tyloxapol/Triton X100 mixtures is lower than that of pure tyloxapol. With increasing Triton X100 content, the phase behavior of the surfactant mixtures in contact with water evolves from the complex behavior of tyloxapol to the simpler one of Triton X100. In contact with water, mixtures rich in tyloxapol form hexagonal, cubic, and lamellar lyotropic liquid crystalline phases over a wide range of concentration and temperature, whereas Triton X100/water mixtures form only a hexagonal phase in a limited concentration and temperature range. The polymeric nature of tyloxapol stabilizes the liquid crystalline structures, and the size of the supramolecular structures in the water-containing surfactant systems is independent of the tyloxapol/Triton X100 mixing ratio but depends highly on water content. The ability of tyloxapol to form stable lyotropic liquid crystalline phases at body temperature, which has been proposed as a basis for the development of novel peroral sustained-release systems, is not significantly impaired by the addition of appropriate amounts of Triton X100. Admixture of Triton X100 to tyloxapol may thus provide a way to circumvent processing problems during the preparation of pharmaceutical formulations based on the highly viscous tyloxapol.

Characterization of native and drug-loaded human low density lipoproteins

Low-density lipoproteins (LDLs), the physiological vehicles for lipids, are potentially useful drug delivery devices for (hydrophobic) drugs. The physicochemical characteristics of LDL loaded with the adriamycin derivative AD 32 or the N-mustard derivative WB 4291 were compared to that of native and reconstituted LDL at different temperatures. X-ray solution scattering indicates that loading with AD 32 has no detectable effect on the particle structure at room temperature, in contrast to WB 4291. According to 19F NMR data, AD 32 molecules are located in two distinct chemical environments with restricted motional freedom of the CF3 groups in samples stored as lyophilisates. 1H NMR signals from AD 32 were not observed, while those from WB 4291 could be distinguished from those of LDL constituents. WB 4291 molecules are in an environment with a higher motional freedom than AD 32 molecules. 1H NMR data suggest a higher fluidity of the core components for the WB-loaded LDLs compared to the other LDL preparations. While the motional freedom of the phospholipid head groups seems to be temperature independent, there is an increase in the mobility of the lipid components in the core region of the LDL particles with temperature.

Solution scattering from 50S ribosomal subunit resolves inconsistency between electron microscopic models

Models of the 50S ribosomal subunit from electron microscopy on isolated particles and on ordered sheets display significantly different features. A model of the shape of the native Escherichia coli 50S subunit in solution and of its RNA-rich core at 4-nm resolution has been produced by using methods for joint interpretation of x-ray and neutron small-angle scattering data obtained by contrast variation. The good agreement between the shape of the entire 50S subunit and the electron microscopic models of isolated particles and between the RNA-rich core and the model obtained from ordered sheets leads to the conclusion that the latter, which is based on the subjective contouring of density maps, is heavily biased toward the RNA.

Structural model of the 50 S subunit of Escherichia coli ribosomes from solution scattering. II. Neutron scattering study

The X-ray and neutron contrast variation data of the 50 S ribosomal subunit of Escherichia coli in solution are interpreted in the frame of a two-phase model described by the shapes of the 50 S subunit and its RNA-rich core taking into account density fluctuations inside the RNA and the protein moiety. The shape of the envelope of the 50 S subunit and of the RNA-rich core are evaluated with a resolution of about 4 nm. The shape of the envelope is in good agreement with the models of the 50 S subunit obtained from electron microscopy on isolated particles. The shape of the RNA-rich core correlates well with the model of the entire particle determined by the image reconstruction from ordered sheets indicating that the latter model which is based on the subjective contouring of density maps is heavily biased towards the RNA.

Structural model of the 50 S subunit of Escherichia coli ribosomes from solution scattering. I. X-ray synchrotron radiation study

The application of new methods of small-angle scattering data interpretation to a contrast variation study of the 50 S ribosomal subunit of Escherichia coli in solution is described. The experimental X-ray data from contrast variation with sucrose are analysed in terms of the basic functions in real space and the scattering curves from the volume inaccessible to sucrose and from the regions inside this volume occupied mainly by RNA and by proteins are obtained. From these curves models of the shape of the 50 S subunit and its RNA-rich core are evaluated. These two shapes are positioned so that their difference, which approximates the volume occupied by the proteins, produces a scattering curve which is in good agreement with the scattering from the protein moiety.

Influence of the supramolecular structure of free lipid A on its biological activity

The three-dimensional supramolecular structures and the states of order of the acyl chains of lipid A from different Gram-negative species were investigated at 40 degrees C, high water content (80-90%), and different [lipid A]/[Mg2+] molar ratios using synchrotron radiation X-ray diffraction and Fourier-transform infrared spectroscopy. Measurements were made on free lipid A from Salmonella minnesota R595, mono- and bi-phosphoryl, as well as those from the non-enterobacterial strains Rhodobacter capsulatus 37b4, Rhodopseudomonas viridis F, and Rhodocyclus gelatinosus 29/1. Parallel to differences in their chemical primary structure, the structural polymorphisms and states of order at 37 degrees C of the non-enterobacterial lipid A were found to be different from those of enterobacterial lipid A. A clear correlation between the supramolecular structure and previously determined biological activities was found. Lipid A with a strong preference for lamellar structures (Rb. capsulatus and Rp. viridis) are endotoxically inactive and lack cytokine-inducing capacity; the compounds assuming a mixed lamellar/nonlamellar structure (monophosphoryl lipid A from S. minnesota) are of lower toxicity in vivo, but may induce cytokines in vitro; those lipid A with a strong tendency to form non-lamellar inverted structures (lipid A from S. minnesota and Rc. gelatinosus) exhibit full endotoxicity in vitro and in vivo. In contrast, anti-complementary activity is most pronounced for compounds with lamellar and least expressed for those with inverted structures. The states of order at 37 degrees C vary non-systematically, exhibiting the highest values for lipid A of S. minnesota and the lowest for that of Rc. gelatinosus. We propose to extend the term 'endotoxic conformation', which is used to describe the conformation of a single lipid A molecule required for optimal triggering of biological effects, to 'endotoxic supramolecular conformation' which denotes the particular organization of lipid A aggregates in physiological fluids causing biological activity.

Age-dependent structural changes in intact human lenses detected by synchrotron radiation X-ray scattering. Correlation with Maillard reaction protein fluorescence

To understand alterations in the spatial organization of the crystallins, the major determinant of lens transparency, the x-ray scattering by intact normal human lenses from individuals 6-82 years of age was measured using synchrotron radiation. The angular dependence of the integrated scattering intensity is consistent with short-range order within the crystallin assemblies. A significant change in the scattering patterns of the lenses occurs after 55 years of age, in parallel with an increase of the fluorescence of the urea-insoluble crystallin fraction. This correlation suggests a gradual derangement of the short-range order as a result of cross-linking of the crystallin subunits by advanced Maillard reaction products that are generated by the continuous reaction of sugars, such as glucose or fructose, with proteins.

The influence of the effectors of yeast pyruvate decarboxylase (PDC) on the conformation of the dimers and tetramers and their pH-dependent equilibrium

The influence of effectors of yeast pyruvate decarboxylase, phosphate, pyruvamide, thiamin diphosphate and Mg++, on the pH-dependent equilibrium between dimers and tetramers was studied by synchrotron radiation X-ray solution scattering. Thiamin diphosphate and phosphate shift the equilibrium to higher pH values without altering the structure of the oligomers. Pyruvamide, a substrate analogue activator, induces a significant change in the structure of the tetramer. By eliminating radiation damage by addition of dithioerythrol to the buffers, the scattering curves could be measured accurately over a large angular range. They were expanded in terms of spherical harmonics to obtain the shapes of the dimers and tetramers with higher resolution than was hitherto possible. This also allowed us to position the dimers, which are centrosymmetric at low resolution, in the tetramers which have 222 symmetry. The results indicate that addition of pyruvamide results in a less compact tetramer owing to structural changes in the dimers and to their displacements.

Structural polymorphisms of rough mutant lipopolysaccharides Rd to Ra from Salmonella minnesota

The structural polymorphisms of rough mutant lipopolysaccharides (LPS) Rd, Rc, Rb, and Ra from Salmonella minnesota (strains R4, R7, Rz, R5, R345, and R60, respectively) were investigated as a function of temperature, water content, and Mg2+ concentration. The gel to liquid crystalline (beta<==>alpha) phase transition temperature (Tc) and the state of order within each phase were measured by Fourier transform infrared spectroscopy. The amount of bound water was determined by differential scanning calorimetry and the three-dimensional structures in each phase state were characterized by synchrotron radiation X-ray diffraction. The results indicate an extremely complex dependence of the structural behavior of LPS on ambient conditions. The beta<==>alpha acyl chain melting temperatures at high water contents (95-97%), Tc = 31 to 32 degrees C for LPS Rd, 33 to 35 degrees C for LPS Rc to Rb, and 36 degrees C for LPS Ra, increase with decreasing water content and in the presence of Mg2+ cations with a concomitant broadening of the transition range. Below 30 to 50% water content, no distinct phase transitions can be observed. These effects are most pronounced for LPS with the shortest sugar chains. Below 50% water content, only lamellar structures can be observed in the temperature range 5 to 80 degrees C, independent of the Mg2+ concentration. Above 50% water concentration, for large [LPS]:[Mg2+] molar ratios the predominant structure above Tc is nonlamellar; for smaller [LPS]:[Mg2+] molar ratios a superposition of lamellar and nonlamellar structures is found. For all LPS Rd to Rb at low Mg2+ concentrations, the phase transition is connected with a change in the three-dimensional structure from lamellar or mixed lamellar/nonlamellar to a pure nonlamellar, probably cubic structure. The tendency to form non-lamellar structures decreases with the length of the core oligosaccharide. At an equimolar ratio of [LPS] and [Mg2+] a multibilayered organization is observed. Some of the nonlamellar structures are cubic phases with periodicities between 12 and 16 nm. The molecular dimensions of the single endotoxin molecules in the absence and the presence of external water are estimated from the different lamellar periodicities, including those of free lipid A and deep rough mutant LPS Re. These observations are discussed with respect to the biological importance of LPS as a potent inducer of biological effects in mammals.

Cloning and expression of Saccharomyces cerevisiae copper-metallothionein gene in Escherichia coli and characterization of the recombinant protein

The gene sequences for intact and truncated forms of copper-binding metallothionein from Saccharomyces cerevisiae were cloned and overexpressed in Escherichia coli BL21(DE3)pLysE cells. In contrast to several other genes, the intact and truncated metallothionein genes are amplified in the polymerase chain reaction when Mg2+ is replaced by Co2+. The recombinant truncated protein binds copper in vivo and in vitro. A ratio of 8 Cu/12 cysteines was determined from atomic absorption, X-ray fluorescence and amino acid analysis. Extended X-ray absorption spectroscopy indicates that all Cu is in Cu(I) form and coordinated to three S atoms.

Synchrotron radiation solution X-ray scattering study of the pH dependence of the quaternary structure of yeast pyruvate decarboxylase

The pH dependence of the quaternary structure of pyruvate decarboxylase from yeast was studied in the range 6.2 less than pH less than 8.4. There is an equilibrium with a midpoint around pH 7.5 between tetramers and dimers, and the catalytic activity of the enzyme depends on the volume fraction of tetramer. This equilibrium may provide an additional regulating mechanism besides substrate activation since accumulation of pyruvate would lead to a reduction in pH and hence an increase of the concentration of the catalytically active tetramer. Radiation damage during the X-ray scattering experiments results in a shift of this equilibrium and in the formation of octamers. These effects could be circumvented and analyzed using experimental and data processing methods which can be readily applied to other radiation-sensitive systems. The low-resolution shapes of the dimers and tetramers were determined from the scattering curves using spherical harmonics. The results indicate that a conformational change must occur in the dimers upon formation of the tetramers, in agreement with earlier circular dichroism measurements.

Phase diagram of deep rough mutant lipopolysaccharide from Salmonella minnesota R595

The structural polymorphism of deep rough mutant lipopolysaccharide--in many biological systems the most active endotoxin--from Salmonella minnesota strain R595 was investigated as function of temperature, water content, and Mg2+ concentration. Differential scanning calorimetry was used to determine the amount of bound water and the enthalpy change at the beta<==>alpha gel to liquid crystalline acyl chain melting. The onset, midtemperature Tc, and completion of the beta<==>alpha phase transition were studied with Fourier-transform infrared spectroscopy. Synchrotron radiation X-ray diffraction was used to characterize the supramolecular three-dimensional structures in each phase state. The results indicate an extremely complex dependence of the structural behavior of LPS on ambient conditions. The beta<==>alpha acyl chain melting temperature Tc lying at 30 degrees C at high water content (95%) increases with decreasing water content reaching a value of 50 degrees C at 30% water content. Concomitantly, a broadening of the transition range takes place. At still lower water content, no distinct phase transition can be observed. This behavior is even more clearly expressed in the presence of Mg2+. In the lower water concentration range (< 50%) at temperatures below 70 degrees C, only lamellar structures can be observed independent of the Mg2+ concentration. This correlates with the absence of free water. Above 50% water concentration, the supramolecular structure below 70 degrees C strongly depends on the [LPS]:[Mg2+] ratio. For large [LPS]:[Mg2+] ratios, the predominant structure is nonlamellar, for smaller [LPS]:[Mg2+] ratios there is a superposition of lamellar and nonlamellar structures. At an equimolar ratio of LPS and Mg2+ a multibilayered organization is observed. The nonlamellar structures can be assigned in various cases to structures with cubic symmetry with periodicities between 12 and 16 nm. Under all investigated conditions, a transition into the hexagonal II structure takes place between 70 and 80 degrees C. These observations are discussed in relation to the biological importance of LPS as constituent of the outer membrane of gram-negative bacteria and as potent inducer of biological effects in mammals.

Rate and mechanism of the assembly of tropomyosin with actin filaments

The rate of assembly of tropomyosin with actin filaments was measured by stopped-flow experiments. Binding of tropomyosin to actin filaments was followed by the change of the fluorescence intensity of a (dimethylamino)naphthalene label covalently linked to tropomyosin and by synchrotron radiation X-ray solution scattering. Under the experimental conditions (2 mM MgCl2, 100 mM KCl, pH 7.5, 25 degrees C) and at the protein concentrations used (2.5-24 microM actin, 0.2-3.4 microM tropomyosin) the half-life time of assembly of tropomyosin with actin filaments was found to be less than 1 s. The results were analyzed quantitatively by a model in which tropomyosin initially binds to isolated sites. Further tropomyosin molecules bind contiguously to bound tropomyosin along the actin filaments. Good agreement between the experimental and theoretical time course of assembly was obtained by assuming a fast preequilibrium between free and isolatedly bound tropomyosin.

The induction of lamellar stacking by cholesterol in lecithin-bile salt model systems and human bile studied by synchrotron X-radiation

Small angle X-ray scattering (SAXS) with synchroton radiation was used to investigate interactions among lipid particles in lecithin-bile salt model systems and in native gallbladder biles. In model systems in the absence of cholesterol, isotropic, continuous spectra were found, indicating the absence of periodic structures. In the presence of excess cholesterol, interaction in the form of lamellar stacking was detected by the appearance of discrete diffraction peaks. In the supersaturated cholesterol region of the commonly accepted phase diagram [1], where cholesterol crystals were expected, we found lamellar stacking. The high proportion of cholesterol to bile salts seems to be the common denominator of these models. The lamellar stacking was also found in native unprocessed bile. This effect of cholesterol on lipid structure has not been previously described. Lamellar stacking may contribute to cholesterol solubilization. Its influence on the kinetics of cholesterol crystallization is presently unknown.

Structural analysis of turkey tendon collagen upon removal of the inorganic phase

Calcified leg flexor tendons in which the inorganic phase content had been lowered by progressive demineralization were studied by small angle X-ray diffraction and thermogravimetry. The X-ray diffraction results agree very well with the data previously obtained on calcified turkey tendon indicating that the method used to decalcify tendons provides good correspondence with the process of calcification. Up to five thermal processes can be detected in the thermogravimetric scans: (1) water release; (2) collagen decomposition; (3 and 4) combustion of the residual organic components; (5) carbonate removal from the apatitic phase. The temperature of collagen decomposition decreases at lower inorganic phase content in agreement with the higher thermal stability of calcified collagen fibrils compared with uncalcified ones. The decrease of collagen thermal stability upon decalification is paralleled by a decrease of the structural order of the collagen fibrils as indicated by small angle X-ray diffraction data. Decalcification down to about 40% wt of inorganic phase does not significantly alter the inorganic blocks that are regularly arranged inside the gap zone of the collagen. Further removal of inorganic phase down to about 15% wt provokes a variation of the intensity distribution of the small angle meridional reflections that can be ascribed to a reduction of the mean height of the inorganic blocks. At inorganic phase contents below 15% wt the gap region is more free to contract upon air drying as a result of the reduction of the mean length of the inorganic blocks.