EVIDENCE FOR GLUTAMATE SELF-CAPPING WITHIN A PEPTIDE HELIX

The thermal dependence of the carbonyl carbon chemical shift of each r esidue in a helical peptide may be analyzed in terms of a two-state he lix/coil transition. Such analyses generate values for the chemical sh ift of each residue in the helical and in the coil conformational ense mbles of the peptide. The sequence dependence of the difference in the se two values, termed the difference chemical shift, provides a descri ption of the mean distribution of helicity within the helical ensemble . In this report, we improve two aspects of the procedures used to ana lyze prior chemical shift measurements of the helical peptide acetylW( EAAAR)(3)Aamide. The new difference chemical shift values for 16 of th e 18 residues describe a very uniform central helical ensemble with fr ayed ends. However, the difference chemical shift values for the Two r emaining residues, alanines 03 and 08, are significantly diminished re lative to this uniform distribution. Each of these two alanine residue s is located i - 4 to a glutamate residue. It is suggested that the di fference chemical shifts for these two alanine residues are diminished by a self-capping interaction within the i + 4 glutamate residues. (C ) 1998 John Wiley & Sons, Inc.