Probing Protein-Sugar Interactions

AbstractWe have investigated the partial specific volumes v¯2 (ml/g), hydration, and cosolvent interactions of rabbit muscle aldolase by equilibrium sedimentation in the analytical ultracentrifuge and by direct density increment (∂ρ/∂c2)μ measurements over a range of sugar concentrations and temperature. In a series of sugars increasing in size, glucose, sucrose, raffinose, and α-cyclodextrin, (∂ρ/∂c2)μ decreases linearly with the solvent density ρ0. These sugar cosolvents do not interact with the protein; however, the interaction parameter B1 (g water/g protein) mildly increases with increasing sugar size. The experimental B1 values are smaller than values calculated by excluded volume (rolling ball) considerations. B1 relates to hydration in this and in other instances studied. It decreases with increasing temperature, leading to an increase in v¯2 due to reduced water of hydration electrostriction. The density increments (∂ρ/∂c2)μ, however, decrease in concave up form in the case of glycerol and in concave down form for trehalose, 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