Volumetric characterization of interactions of glycine betaine with protein groups

We report the partial molar volumes and adiabatic compressibilities of N-acetyl amino acid amides and oligoglycines at glycine betaine (GB) concentrations ranging from 0 to 4 M. We use these results to evaluate the volumetric contributions of amino acid side chains and the glycyl unit (-CH(2)CONH-) as a function of GB concentration. We analyze the resulting GB dependences within the framework of a statistical thermodynamic model and evaluate the equilibrium constant for the reaction in which a GB molecule binds each of the functionalities under study replacing four water molecules. We calculate the free energy of the transfer of functional groups from water to concentrated GB solutions, ΔG(tr), as the sum of a change in the free energy of cavity formation, ΔΔG(C), and the differential free energy of solute-solvent interactions, ΔΔG(I), in a concentrated GB solution and water. Our results suggest that the transfer free energy, ΔG(tr), results from a fine balance between the large ΔΔG(C) and ΔΔG(I) contributions. The range of the magnitudes and the shape of the GB dependence of ΔG(tr) depend on the identity of a specific solute group. The interplay between ΔΔG(C) and ΔΔG(I) results in pronounced maxima in the GB dependences of ΔG(tr) for the Val, Leu, Ile, Trp, Tyr, and Gln side chains as well as the glycyl unit. This observation is in qualitative agreement with the experimental maxima in the T(M)-versus-GB concentration plots reported for ribonuclease A and lysozyme.