On the temperature coefficient of the solubility of some glycyl peptides in water-ethanol mixtures.

The solubility of glycine, diglycine, and triglycine was investigated in water-ethanol mixtures (20% and 50% ethanol) as a function of the temperature. The transfer free energy, @go,,,, for the peptide group was evaluated and the separate enthalpy and entropy components of the transfer process were determined. In 20% ethanol solutions, the hypothesis of additivity of the contribution of various molecular groups to the transfer parameters was verified. Values obtained as the difference triglycine-diglycine in 20% ethanol at 25” were AF&‘,,, = +120 cal per mole, AS&, = 1.7 e.u., AHEoN, = -390 cal per mole. These values appear to be in excellent agreement with previous results obtained for the coil --j helix transformation of poly-L-ornithine in 20% ethanol solutions. It has been confirmed that helical stabilization in mixed solvents is an entropic effect, arising from the negative value of the transfer entropy, AS&,, (from water to water-organic solvent mixtures).