ABSTRACT Amino acid side chains can enhance peptide group hydrogen bond strength in protein structures by obstructing the competing hydrogen bond to solvent in the unfolded state. Available data indicate that the steric blocking effect contributes an average of 0.5 kJ per residue to protein hydrogen bond strength and accounts for the intrinsic p-sheet propensities of the amino acids. In available data for helical models, the contribution to a-helix propensities is obscured especially by large context-dependent effects. These issues are all related by a common side chain-depen-dent steric clash which disfavors peptide to wa-ter H-bond formation, peptide to catalyst com-plexation in hydrogen exchange reactions (Bai et al., Proteins 1R75-86, 19931, and peptide to peptide H-bonding in the helical main chain conformation (Creamer and Rose, Proc. Natl. Acad. Sci. U.S.A. 8959375941, 1992) but not in p-strands. o 1994 Wiley-Liss, Inc. Key words: protein structure prediction, pro-tein stability, hydrogen bond, p-sheet, amino acid propensity, steric effect, hydrogen exchang
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