Evidence for glutamate self-capping within a peptide helix

The thermal dependence of the carbonyl carbon chemical shift of each residue in a helical peptide may be analyzed in terms of a two-state helix/coil transition. Such analyses generate values for the chemical shift of each residue in the helical and in the coil conformational ensembles of the peptide. The sequence dependence of the difference in these two values, termed the difference chemical shift, provides a description of the mean distribution of helicity within the helical ensemble. In this report, we improve two aspects of the procedures used to analyze prior chemical shift measurements of the helical peptide acetylW (EAAAR)3Aamide. The new difference chemical shift values for 16 of the 18 residues describe a very uniform central helical ensemble with frayed ends. However, the difference chemical shift values for the two remaining residues, alanines 03 and 08, are significantly diminished relative to this uniform distribution. Each of these two alanine residues is located i-4 to a glutamate residue. It is suggested that the difference chemical shifts for these two alanine residues are diminished by a self-capping interaction within the i + 4 glutamate residues.