Helix propensities are identical in proteins and peptides

Our understanding of the factors stabilizing alpha-helical structure has been greatly enhanced by the study of model alpha-helical peptides. However, the relationship of these results to the folding of helices in intact proteins is not well characterized. Helix propensities measured in model peptides are not in good agreement with those from proteins. In order to address these questions, we have measured helix propensities in the alpha-helix of ribonuclease T-1 and a helical peptide of identical sequence. We have previously demonstrated excellent agreement between peptide and protein for the nonpolar amino acids [Myers, J. K., Pace, C. N., and Scholtz, J. M. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 2833-2837]. Most other amino acids also show good agreement, although certain polar amino acids are exceptions. Helix propensities measured in the ribonuclease T-1 peptide/protein are compared with those measured in other systems. Reasonable agreement is found between most systems; however, our propensities differ substantially from those measured in several model peptide systems. Alanine-based peptides overestimate the propensity differences by a factor of 2, and host/guest experiments underestimate them by a factor of 2-3.