Elastin hydration

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One of the key arguments for neutral site binding is the presence of (3-turns and associated conformations. This puts certain restrains on the structure of the fibrous protein. For elastin, conformations with bound calcium are likely to be inside-out with respect to hydrophobicity. Such structures are acceptable only for molecules functioning in a non-polar environment (cell-membranes) but not for a hydrated elastin fibre. Binding of calcium would stabilize a rigid inside-out conformation437. ... 

Despite different sequences and repetitive motives, all gliadins have the same secondary structure of loose spirals which are a balanced compromise between the p-spiral and poly-L-proline structure (polyproline helix II) (Parrot et al., 2002), the balance is dependent on temperature, type of solvent, and hydration level (Miles et al., 1991). Similar sequences can be found in other proteins, mainly animal proteins such as elastin and collagen, and they are responsible for particular biomechanical properties connected to reverse P-spirals or p-sheet structures (Tatham and Shewry, 2000). 

Little recent work has been reported on the water relations and sw elling properties either of native elastin or elastin that has been isolated by various procedures or treated with enzymes or hydrolytic reagents. This is to be deplored since the study of the hydration of a protein fiber is often a sensitive means of revealing the pattern of lateral bonds which lends structural stability. A comparison of the hydration of elastins derived from different animals, or different tissues of the same animal, would be particularly valuable since the methods are relatively insensitive to the preseruic... 

Baer, Hiltner, and colleagues (see Hiltner, 1979, and references cited therein) have used dynamic mechanical analysis to examine the hydration of collagen, elastin, and several polypeptides. A torsional pendulum constructed of the sample was examined for low-frequency (i.e., IHz) mechanical loss as a function of hydration and temperature. A common feature is a dispersion that is absent in the dry protein and appears at... 

The formation of spanning H-bonded water networks on the surface of biomolecules has been connected with the widely accepted view that a certain amount of hydration water is necessary for the dynamics and function of proteins. Its percolative nature had been suggested first by Careri et al. (59) on the basis of proton conductivity measurements on lysozyme this hypothesis was later supported by extensive computer simulations on the hydration of proteins like lysozyme and SNase, elastine like peptides, and DNA fragments (53). The extremely interesting... 

In one study, a model for elastin, the main protein that confers elasticity on solid structures in mammals, had its mobility investigated by examining 1H-13C and 1H dipolar couplings extracted from isotropic-anisotropic correlation experiments.29 The elastic properties of elastin are almost certainly conferred by molecular degrees of freedom, so such studies are important in understanding how this material works in Nature. The motional amplitudes determined from these experiments were found to depend upon the degree of hydration, with the mean square fluctuation angles found to be 11-18° in the dry protein and 16-21° in the 20% hydrated protein. 

Recently, we have measured the partial molar heat capacity Cp of hydrated samples of methylcellulose and elastin at temperatures ranging from 110°K to 330 K (37). For both, just as for collagen, at any temperature, the partial molar heat capacity of water appears to be independent of water content. Its values is equal to that of ice from 110°K to 150°K, where it starts to increase over that of ice and then increases virtually linearly with temperature to values close to that of liquid water at room temperature. Schematically, the results are given in... 

Volumetric data can be essential in the thermodynamic treatment of the "polymer-solvent" interaction process. The lack of them for many important fibrous proteins is due to the difficulty of measuring the density, at controlled temperature and hydration degree, for these systems. As far as elastin is concerned, it has been reported that when completely hydrated this protein has a negative and very large coefficient of thermal expansion (15), a result which has been interpreted as evidence of a hydrophobic character of the protein (16). 

Elastin, which is substantially amorphous but fibrous at all levels of investigation (starting from the largest filaments which are about 6 fim in diameter and down to about 10 nm (17,18)), is a fragile, glassy substance when dry and has a glass-to-rubber transition temperature at about 200 C (19) upon hydration or solvation with appropriate solvents, it becomes a rubbery system with the glass transition below room temperature (20). 

Calorimetric data have shown that only half of the total water sorbed by elastin (about 0.6 g water / g dry protein) is really "bound", the remaining water being freezable ( 1). The volumetric data reported in the literature (15,16) refer therefore to an essentially heterophase system, so that the negative and very large coefficient of thermal expansion of the fully hydrated protein does not appear to be suitable for the Interpretation of the thermoelastic data and calculation of the... 

Figure 2. Density and specific volume of dry and hydrated purified elastin samples as a function of temperature at water weight fractions of —) 0 (Is) 0.05 (O) 0.14 (%) 0.18 (A) 0.29...

Figure 2 shows the density of purified elastin as a function of temperature and water content. The corresponding expansion coefficient values are reported in Table I. At temperatures lower than 30°C and with the exception of the most hydrated sample, density changes only slightly with water content at higher temperatures, on the contrary, density depends strongly on the hydration level. 

Table I - Expansion coefficient and volume contraction of hydrated purified elastin...

A glass transition temperature of about 40 C for the hydrated elastin sample (w = 0.14) fits fairly well into the glass transition temperature-water content curve calorimetrically determined by Kakivaya and Hoeve (23) for hydrated elastin. 

It has to be pointed out that, in the temperature range explored in the present work, the volume expansion coefficient of hydrated elastin is positive and constant on both sides... 

Figure 3. Experimental specific volume of hydrated elastin as a function of concentration at three selected temperatures ( ) 30°C (O) 45°C (%) 60°C straight lines are the ideal additive volumes (lower 30°C middle 45°C upper 60°C ...

In conclusion, the most interesting results of the present work are first, that the volumetric temperature coefficients of elastin at low and moderate levels of hydration are positive and small (rather than negative and large, as reported for the fully hydrated but heterophase protein) and, second, that the interaction with water is accompanied, as reported previously for wool, by large volume changes, which appear to be indicative of strong interactions at the molecular level. 

The skin is an excellent barrier to microbial and parasitic infections. The most superficial layer of the skin is composed of flattened squamous cells, which are highly keratinized. Beneath this is the epidermal layer composed of cells tightly interconnected by desmosomes and other intercellular structures. These, in turn, are attached to the basement membrane composed of covalently bound or interwoven macromolecules. Between the basement membrane and a target blood vessel is an extracellular matrix rich in type I collagen, elastin and proteoglycan. Elastin and type I collagen are both interwoven fibrillar molecules, whereas the carbohydrate-rich proteoglycan behaves like a hydrated gel. For details of these macromolecular interactions, the reader is referred to reviews on the structure of skin. 

M. Lillie, J. GosUne, The effects of hydration on the dynamic mechanical properties of elastin. Biopolymers 29 (1990) 1147—1160. 

The resilience of preconditioned SELP-47K fibres, either autoclaved or chemical cross-linked, approached that of native elastin, which is 90% [60], It is worth noticing that mechanical properties of SELP-47K fibres change dramatically between the dry state and the full hydrated state (Table 12.3). This is likely due to the plasticizing effect of water, which gives the silk-like blocks conformational flexibility by decreasing their crystallinity. 

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