Experimental and theoretical aspects of hydration isotherms for biomolecules

Abstract

A resonating quartz crystal microbalance technique has been used to obtain room temperature water sorption isotherms for cytochrome-c, DNA, lecithin, lysozyme and serum albumin. The results compare favourably with earlier work using more conventional techniques. A completely general formula describing the sorption isotherms is derived. With the assumption of identical and non-interacting primary sorption sites, this general formula gives exact values for the monolayer site capacity and the thermodynamic activities of all the hydration states. However, it is shown that at least one of these assumptions is not valid for the materials studied here, accordingly only limited information can be derived using this theory. This restricted usefulness also applies to other sorption theories described in the literature, most of which are based on purely modelistic or kinetic considerations. The significance of a sorption activity parameter having values greater or less than unity is discussed for the materials examined.