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, Δ G tr, as the sum of a change in the free energy of cavity formation, ΔΔ G C, and the differential free energy of solute–solvent interactions, ΔΔ G I, in a concentrated GB solution and water. Our results suggest that the transfer free energy, Δ G tr, results from a fine balance between the large ΔΔ G C and ΔΔ G I contributions. The range of the magnitudes and the shape of the GB dependence of Δ G tr depend on the identity of a specific solute group. The interplay between ΔΔ G C and ΔΔ G I results in pronounced maxima in the GB dependences of Δ G 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.