Volumetric Characterization of Interactions of Protein Groups with Glycine Betaine

Abstract

We report the volumetric properties of N-acetyl amino acid amides with unionizable side chains and oligoglycines in binary solutions of water and glycine betaine, a protective cosolvent. We analyze these data within the framework of a statistical thermodynamic formalism to determine the association constants for the reaction in which glycine betaine binds to the peptide backbone and amino acid side chains replacing four water molecules. The association constants are linked to the free energy of transfer of functional groups from water to a glycine betaine solution, Gtr. Transfer free energy, Gtr is the sum of a change in the free energy of cavity formation, GC, and the differential free energy of solute-solvent interactions, GI, in a concentrated glycine betaine solution and water. We compare our results with similar data on volumetric measurements in water-urea binary mixtures. We find little differences in GI for protein functional groups in urea and glycine betaine solutions. Comparative analysis reveals that solute-cosolvent interactions are favorable for the glycyl unit and the majority of amino acid side chains in both glycine betaine and urea solutions. These results are consistent with the picture in which the free energy of cavity formation, GC, is the decisive factor determining the differential stabilizing/destabilizing action of urea and glycine betaine.