Energetics of hydrogen bonding in proteins: A model compound study

Abstract

Differences in the energetics of amide-amide and amide-hydroxyl hydrogen bonds in proteins have been explored from the effect of hydroxyl groups on the structure and dissolution energetics of a series of crystalline cyclic dipeptides. The calorimetrically determined energetics are interpreted in light of the crystal structures of the studied compounds. Our results indicate that the amide-amide and amide-hydroxyl hydrogen bonds both provide considerable enthalpic stability, but that the amide-amide hydrogen bond is about twice that of the amide-hydroxyl. Additionally, the interaction of the hydroxyl group with water is seen most readily in its contributions to entropy and heat capacity changes. Surprisingly, the hydroxyl group shows weakly hydrophobic behavior in terms of these contributions. These results can be used to understand the effects of mutations on the stability of globular proteins.Keywords: calorimetry; enthalpy; heat capacity; hydrogen bonding; hydroxyl; model compounds; solvation Hydrogen bonds are ubiquitous in biological molecules and their contribution to the stability of these structures is of fundamental importance. Numerous studies have been performed in an attempt to quantitate the energetics of hydrogen bonding in proteins and other biological macromolecules. In spite of the considerable effort that has gone into such studies, there remains disagreement in the current literature regarding not only the magnitude, but even the sign of the contribution of hydrogen bonds to the thermodynamics, and especially the enthalpy, of proteins.In 1955, Schellman, analyzing the non-ideality of urea solutions, arrived at a value of -6.3 kJ mol" for the AH" of formation of an amide hydrogen bond (i.e., a hydrogen bond between NH and C=O groups from amide linkages) in aqueous solution (Schellman, 1955). However, a subsequent study by Klotz and Franzen (1962) looked at the dimerization of N-methyl-acetamide in aqueous solution and concluded that the AH" of formation of an amide hydrogen bond in water was zero. These studies indicated a AH" of hydrogen bond formation in water between -8 and -13 kJ mol", consistent with the earlier report of Schellman. Recent calorimetric studies on the unfolding of an alanine-based a-helix peptide also support a favorable AH" of amide hydrogen bond formation in water (Scholtz et al., 1991).Although there now is general agreement that the A H o of hydrogen bond formation in water can be favorable, it has been suggested recently that the unfavorable AH" of dehydrating polar groups upon transfer to the protein interior or into a proteinprotein interface might result in an overall unfavorable A H o for the burial of hydrogen bonded polar groups (Yang et al., 1992; Makhatadze & Privalov, 1993). These conclusions are based on experimental values for the AH" of transferring model compounds from vapor into water or on theoretical calculations of this process. The precision of such approaches is questionable because they generally require taking the difference between large...

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