Observation of the closing of individual hydrogen bonds during TFE-induced helix formation in a peptide.

Helix formation of an S-peptide analog, comprising the first 20 residues of Ribonuclease A and two additional N-terminal residues, was studied by measuring hydrogen bond (H-bond) (h3)J(NC') scalar couplings as a function of 2,2,2-trifluoroethanol (TFE) concentration. The (h3)J(NC') couplings give direct evidence for the closing of individual backbone N-H***O = C H-bonds during the TFE-induced formation of secondary structure. Whereas no (h3)J(NC') correlations could be detected without TFE, alpha-helical (i,i +4) H-bond correlations were observed for the amides of residues A5 to M15 in the presence of TFE. The analysis of individual coupling constants indicates that alpha-helix formation starts at the center of the S-peptide around residue E11 and proceeds gradually from there to both peptide ends as the TFE concentration is increased. At 60% to 90% TFE, well-formed alpha-helical H-bonds were observed for the amides hydrogens of residues K9 to Q13, whereas H-bonds of residues T5 to A8, H14, and M15 are affected by fraying. No intramolecular backbone H-bonds are present at and beyond the putative helix stop signal D16. As the (h3)J(NC') constants represent ensemble averages and the dependence of (h3)J(NC') on H-bond lengths is very steep, the size of the individual (h3)J(NC') coupling constants can be used as a measure for the population of a closed H-bond. These individual populations are in agreement with results derived from the Lifson-Roig theory for coil-to-helix transitions. The present work shows that the closing of individual H-bonds during TFE-induced helix formation can be monitored by changes in the size of H-bond scalar couplings.