Protein surface water behavior

On hydrophilic surfaces, such as PVA or poly(HEMA), OH-groups of the materials are incorporated in the network structure of adsorbed water molecules (see Sect. 4.4). In consequence, the absolute value of Wj(3 — Wi1 is considered to become still smaller, where - owing to the stabilization of water molecules on the hydrophilic surface - the water-removing-process (reverse reaction of Eq. (2.6)) proceeds slowly. Many experiments were carried out with water-adsorbed hydrophilic surfaces, the behavior of which was time-dependent. In a similar way, the water removal from the proteins [Eq. (2.9)] is also considered to proceed slowly. Thus, we must be careful in considering experimental results in comparison with the data in Tables 3, 4 and 5. 

The most important feature affecting the functional and organoleptic properties of a protein is its surface structure. Surface structures affect the interaction of a protein with water or other proteins. By modifying the structure of the protein, particular functional and organoleptic properties are obtained. Functional properties of a protein are physicochemical characteristics that affect the processing and behavior of protein in food systems (Kinsella, 1976). These properties are related to the appearance, taste, texture, and nutritional value of a food system. Hydrolysis is one of the most important protein structure modification processes in the food industry. Proteins are hydrolyzed to a limited extent and in a controlled manner to improve the functional properties of a foodstuff. 

Corredig, M. and Dalgleish, D.G. 1995. A differential microcalorimetric study of whey proteins and their behavior in oil water emulsions. Colloids Surface 4, 411-422. 

As already noted, we suggest that the behavior of the second virial coefficient of the apoferritin in acetate buffer is due to the adsorption of Na+ ions upon the negative sites of the protein surface, which depends on the concentration of the Na+ ions in the liquid in the vicinity of the surface. In what follows, the adsorption of acetate ions upon the positive sites or of neutral Na+—CH3COO " pairs on the neutral sites of the protein surface will be neglected and it will be assumed that only the dipoles of the ion pairs formed through the association ofNa4 to the acidic sites of the surfece polarize the neighboring water molecules. 

Two of the hydrophihcity scales in Table 2 were derived from experimental measures of the behavior of amino acids in various solvents, namely partitioning coefficients [K-D index of Kyte and Doolittle (30)] or mobility in paper chromatography [Rf index of Zimmerman et al. (31)]. By contrast, the Hp index was obtained from quantum mechanics (QM) calculations of electron densities of side chain atoms in comparison with water (32). The Hp index is correlated highly with these two established hydrophobicity scales (Table 4). Therefore, like the polarizability index, it is possible to represent fundamental chemical properties of amino acids (hydrophUicity, Hp) with parameters derived from ab initio calculations of electronic properties. However, in contrast to polarizabihty (steric effects), hydrophihcity shows significant correlation with preference for secondary structure. Thus, hydrophobic amino acids prefer fi-strands (and fi-sheet conformations) and typically are buried in protein structures, whereas hydrophilic residues are found commonly in turns (coil structure) at the protein surface. 

Several authors reported measurements of the preferential binding parameter in the system water (l)/protein (2)/PEG (3) [10—14]. It was found that for various proteins, various PEGs molecular weights, and various PEG concentrations, the protein is preferentially hydrated and the PEG is excluded from the vicinity of the protein molecule. The prevalent viewpoint which explains such a behavior is based on the steric exclusion mechanism suggested by Kauzmann and cited in Ref. [15]. According to this mechanism [12,14], the deficit of PEG and the excess of water (in comparison with the bulk concentrations) are located in the shell (volume of exclusion) between the protein surface and a sphere of radius R (see Fig. 1) [12,14]. However, Lee and Lee [10,11] suggested that the preferential exclusion of the PEG from the protein surface also involves the protein hydrophobicity and charge. 

For electrostatic models based on dielectric theory the experimental solvent dielectric constant, reflecting the contribution of electronic polarizability and dipole reorientation, is usually used throughout (e.g. for water at 25°C e=78.6). In principle however, Eqn. (4) is equally applicable to water and could be used to model how the dielectric constant of water might be perturbed at the protein surface, although the contribution of cooperative motions is particularly hard to treat accurately. In addition the accuracy of explicit water molecules, and the difficulties of obtaining dielectric behavior from MM and MC simulations have restricted this approach. 

Trypsin in aqueous solution has been studied by a simulation with the conventional periodic boundary molecular dynamics method and an NVT ensemble.312 340 A total of 4785 water molecules were included to obtain a solvation shell four to five water molecules thick in the periodic box the analysis period was 20 ps after an equilibration period of 20 ps at 285 K. The diffusion coefficient for the water, averaged over all molecules, was 3.8 X 10-5 cm2/s. This value is essentially the same as that for pure water simulated with the same SPC model,341 3.6 X 10-5 cm2/s at 300 K. However, the solvent mobility was found to be strongly dependent on the distance from the protein. This is illustrated in Fig. 47, where the mean diffusion coefficient is plotted versus the distance of water molecules from the closest protein atom in the starting configuration the diffusion coefficient at the protein surface is less than half that of the bulk result. The earlier simulations of BPTI in a van der Waals solvent showed similar, though less dramatic behavior 193 i.e., the solvent molecules in the first and second solvation layers had diffusion coefficients equal to 74% and 90% of the bulk value. A corresponding reduction in solvent mobility is observed for water surrounding small biopolymers.163 Thus it... 

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol... 

However, in spite of these similarities, the adsorbed amounts and the structure of the adsorbed mucin and collagen layers on the surfaces studied are entirely different. The behavior of these proteins is analyzed here on the hydrophobic polyethylene surface (water contact angle 0 20 95°), on the surface modified polyethy-lenes oxidized polyethylene (0h q 74°) and poly(maleic acid) grafted polyethylene ( Ho0 74°) a d on the hydrophilic mica surface ( H2 0 0°) Acidic pH = 2.75 (for collagen) and slightly alkaline pH = 7.2 (for mucin) were chosen in order to minimize the association of these proteins in solution and to make possible the analysis of their adsorbabilities in comparable conditions. 

MD simulations and experiments clearly show that the single particle motion of water molecules next to a protein surface is different than in the bulk. Here, single particle refers to measures of the average behavior of individual water molecules, as opposed to coherent behavior of collections of water molecules, which will be discussed in more detail below. The perturbation of the translational and rotational mobility of protein hydration water (defined using the 4 A distance criterion) is depicted in Fignre 16.1a and b, respectively. We will discuss the data for the native (N) state first, and snbsequently compare the native and MG states. In bulk water, after an initial rapid ( 2ps) rise corresponding to ballistic motion. 

If the protein molecule and the surface are polar, it is probable that some hydration water is retained between the surface and the adsorbed protein layer. However, if (one oO the surfaces are (is) apolar, dehydration would be a driving force for adsorption. Although the apolar residues of a globular protein in water tend to be buried in the interior of the molecule, the water accessible surface of the protein may still comprise a significant apolar fraction, even up to 40%-50% (cf. Section 13.2). In this context it should be realized that apart from the polarity of the outer shell the overall polarity of the protein could be relevant for its adsorption behavior. The overall polarity influences the protein structural stability (cf. Section 13.3) and, hence, the extent of structural perturbation upon adsorption. This, in turn, affects the adsorption affinity, as discussed in the following section. 

Exact determination of the absolute amount of adsorbed protein at the solid/water surface will require measurement of both residual protein and water after adsorption equilibrium. A positive excess adsorption thus does not mean that only protein and no water is adsorbed. It rather means that protein is adsorbed in excess (in term of the proportion in which protein and water exist in solution) of any water that is adsorbed. Negative adsorption indicates higher preference of the surface for water than for protein molecules. In [66], assuming the constant amount of water (b) bound to the adsorbent surface with zero absolute adsorption of protein the equations are given to discuss unusual behavior of protein adsorption with increase in initial protein concentrations ... 

Most adsorbed surfactant and polymer coils at the oil-water (0/W) interface show non-Newtonian rheological behavior. The surface shear viscosity Pg depends on the applied shear rate, showing shear thinning at high shear rates. Some films also show Bingham plastic behavior with a measurable yield stress. Many adsorbed polymers and proteins show viscoelastic behavior and one can measure viscous and elastic components using sinusoidally oscillating surface dilation. For example the complex dilational modulus c obtained can be split into an in-phase (the elastic component e ) and an out-of-phase (the viscous component e") components. Creep and stress relaxation methods can be applied to study viscoelasticity. 

In addition to studying the interaction with antifreeze protein with various ice surfaces we have also performed several simulations of these proteins in boxes of water. This was done for two main reasons. The first was to investigate the behavior and stability of antifreeze proteins in water and secondly to study protein interactions with water. In this section we summarize the molecular dynamics simulations of antifreeze proteins in water. 

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