Thermodynamics of Hydration of Ternary Mixture of Protein, Lipid and Carbohydrate in the Presence and Absence of Neutral Salts

Abstract

Extents of binding of water (n 1 ) to a ternary mixture of protein, lipid and carbohydrate (BSA, cholesterol and cellulose in 1:1:1 weight ratio) have been determined in the absence and presence of some inorganic electrolytes and urea using isopiestic vapour pressure technique at a fixed temperature (37°C). The shapes of the water vapour adsorption isotherms for the ternary mixture in the range of water activity (a 1 ) from zero to unity are in qualitative agreement with type III BET isotherm. At water activity (a 1 ) aproaching unity, maximum number of moles of water bound (Δn 0 1 ) per kg of mixture has been evaluated by extrapolation method. At a given value of a 1 , n 1 for the ternary mixture is found to be significantly less than their ideal value obtained from the additivity rule due to the interaction present between protein, lipid and carbohydrate in the sample system. Standard free energy changes (ΔG 0 ) and enthalpy changes (ΔH 0 ) for the mixture have been evaluated from the experimental data. From an examination of these thermodynamic parameters, hydration of the ternary mixture has been found to be a spontaneous process controlled both by enthalpy and entropy effects. The excess hydration of the ternary mixture in the presence of some solutes (NaCl, LiCl, KCl, KBr, KI, Na 2 SO 4 , CaCl 2 and urea) has been estimated from isopiestic experiments and using the integrated form of the Gibbs adsorption equation, standard free energy changes (ΔG 0 ) of excess hydration have been evaluated. The absolute values of water and salt binding to the ternary mixture, have also been obtained from thermodynamic considerations.