Free energy, heat, and entropy changes accompanying the sorption of water vapor by proteins

Abstract

AbstractRecently published measurements of moisture sorption vs. relative humidity for a series of proteins have previously been analyzed in a way which permits calculation of the integral free energy of sorption for any final pressure of water vapor. The present paper is an extension of these calculations to additional proteins from the list reported by Bull; integral and differential free energies and integral and differential heats and entropies of sorption have been calculated as functions of the amounts of water sorbed onsilk, wool, egg albumin (unlyophilized and heat coagulated), salmine, collagen, gelatin and lactoglobulin (crystalline and lyophilized). All calculations are referred to saturated water vapor as the standard state. Some of the proteins show positive net differential entropies of sorption at low vapor pressures. It is postulated that the partial molal entropy of sorption is made up of two terms, a negative entropy of sorption proper and a positive entropy of solution. Partial rearrangements of the protein chains at the beginning of the sorption process would effectively be an incipient solution which would give rise to a positive partial molal entropy of sorption if the second term is greater than the first. This speculation has support in the fact that the differential entropy of salmine, which dissolves at a relative vapor pressure of about 0.7, begins to show positive values at a relative vapor pressure of 0.35.