3(10)-HELICES IN PEPTIDES AND PROTEINS AS STUDIED BY MODIFIED ZIMM-BRAGG THEORY

Recent experimental data suggest that the amount of 3(10) helical conf ormation in peptides and proteins might be larger than previously expe cted (Millhauser, G. L. Biochemistry 1995, 34, 3873-3877). This led us to explore a principal assumption of Zimm-Bragg theory of the helix-c oil transition, that only one helical state can occur in polypeptides. In the present work we modify Zimm-Bragg theory to include the 3(10) helix as a competing helical state. Incorporation of the second helica l state does not significantly change the nature of the helix-coil tra nsition, preserving good agreement between theory and the large amount of relevant experimental data. The analysis of the model indicates th at 3(10) helices should be on average shorter than alpha-helices. Also shorter polypeptides are predicted to have a significant ratio of 3(1 0) helical to alpha-helical hydrogen bonds. Moreover, as the total num ber of hydrogen bonds in the polypeptide decreases, the probability fo r a particular hydrogen bond to be in the 3(10) state rather than the alpha-helical state increases. The present analysis provides somewhat unexpected support for the recent proposal of the 3(10) helix as a the rmodynamic intermediate in alpha-helix folding.