Probing protein-sugar interactions

We have investigated the partial specific volumes (v) over bar(2) (ml/g), h ydration, and cosolvent interactions of rabbit muscle aldolase by equilibri um sedimentation in the analytical ultracentrifuge and by direct density in crement (partial derivative(rho)/partial derivative C-2)(mu) measurements o ver a range of sugar concentrations and temperature. In a series of sugars increasing in size, glucose, sucrose, raffinose, and alpha-cyclodextrin, (p artial derivative(rho)/partial derivative C-2)(mu) decreases linearly with the solvent density rho(o). These sugar cosolvents do not interact with the protein; however, the interaction parameter B-1 (g water/g protein) mildly increases with increasing sugar size. The experimental B-1 values are smal ler than values calculated by excluded volume (rolling ball) considerations . B-1 relates to hydration in this and in other instances studied. It decre ases with increasing temperature, leading to an increase in (v) over bar(2) due to reduced water of hydration electrostriction. The density increments (partial derivative(rho)/partial derivative C-2)(mu) however, decrease in concave up form in the case of glycerol and in concave down form for trehal ose, leading to more complex behavior in the case of carbohydrates playing a biological role as osmolytes and antifreeze agents. A critical discussion , based on the thermodynamics of multicomponent solutions, is presented.