Thermodynamic functions of biopolymer hydration. I. Their determination by vapor pressure studies, discussed in an analysis of the primary hydration process

Abstract

SynopsisThe primary hydration process of native biopolymers is analyzed in a brief review of the literature, pertaining to various aspects of biopolymer-water systems. Based on this analysis, a hydration model is proposed that implies that the solution conformation of native biopolymers is stable at and above a critical degree of hydration (hp, = 0.060.1 g HzO/g polymer). This water content corresponds to the fraction of strongly bound water, and amounts to -20% of the primary hydration sphere. In order to test this model, detailed sorption-desorption scanning experiments were performed on a globular protein (a-chymotrypsin). The results obtained are consistent with the proposed hydration model. They show that under certain experimental conditions, sorption isotherms can be obtained that do not exhibit hysteresis. These data represent equilibrium conditions and are thus accessible to thermodynamic treatment. Valid thermodynamic functions, pertinent to the interaction of water with biopolymers in their solution state, can be obtained from these sorption experiments.

Water Binding in Legume Seeds

“…The application of Clausius-Clapeyron theory to determine the strength of water sorption has inherent weaknesses because of (a) uncertainties of water content determinations at very low The parameters K, K', c, and k' estimated by least squares fit for legume tissues (Table II) correspond well with parameters estimated for proteins and DNA. For example, K' ranges from 0.024 g H20/g dry weight for egg albumin to 0.114 g/g for lysozyme, K ranges from 7 (dimensionless) for skin collagen to 200 for DNA, c ranges from 0.028 g H20/g dry weight for zein to 0.183 g/g for collagen, k' ranges from 0.0062/g H20/g dw for keratin to 0.124 g/g for DNA (3,6,14). Our values of K, calculated from isotherms of axes tissue at 5°C, are higher than those reported above.…”

“…For proteins, k values are usually less than 1.0, though Luscher-Mattli and Ruegg (14) did report a few instances where calculated values of k exceeded 1. A water activity greater than 1 is clearly unlikely, so these values may indicate a limitation in the curve fitting routine.…”

“…Knowledge of structural changes induced by the addition of water to dry macromolecules is important not only for our understanding ofenzyme activation at different water activities, but also for our understanding of water as the 'structural cement' of biomolecules. Although large structural changes are not observed with the desiccation of phospholipid bilayers in seeds (15,23) or proteins from erythrocyte membranes (22), there are indications of minor changes in bilayer (23) and protein (14,21) Perhaps an important distinction is not so much that a con- (14). It has been suggested that denaturation (10,13) and enzyme activity (1,21) at different the final stage of 'wetting' of proteins is the hydration of hydrowater contents.…”

“…An alternative treatment of isotherm data is the comparison ofsorption at two temperatures via the Clausius-Clapeyron equation (2,14,22,26), where it can be shown that AH= R x (T, X T2) X ln(aw,/aw2), (T2 -TI) (2) with AH being the differential enthalpy (in J/mol water) given a constant water content, awl and aw2 are the relative vapor pressures at the lower and higher temperatures, T, and T2. R is the ideal gas constant (8.3143 J/degree/mol).…”

“…R is the ideal gas constant (8.3143 J/degree/mol). Calculated enthalpies reflect properties of the polymer surface-water interaction and will therefore reflect the strength of water binding as well as any conformational changes or swelling effects (14).…”