Elimination of self-association as the source of the thermodynamic nonideality in aqueous proline solutions

The effect of high concentrations of proline on the diffusion coefficient of water has been examined to assess the extent to which the resulting thermodynamic nonideality could be explained on the statistical-mechanical basis of excluded volume. In fact, such a space-filling role not only accounts for the proline concentration-dependence of the diffusion coefficient of water but it also accounts for the nonideality of proline in freezing point depression and isopiestic measurements. These findings refute the conclusion (Schobert, B. and Tschesche, H. (1978) Biochim. Biophys. Acta 541, 270-277) that the stabilization of enzyme structure by high concentrations of proline stems from self-association of the imino acid via intermolecular hydrogen bonding; and thereby support the concept that the protective effect of proline on enzyme stability must reside mainly in its action as an inert, space-filling solute.