Synthesis and conformation of an analog of the helix-loop-helix domain of the Id1 protein containing the O-acyl iso-prolyl-seryl switch motif

Abstract

Synthetic peptides reproducing the helix-loop-helix (HLH) domains of the Id proteins fold into highly stable helix bundles upon self-association. Recently, we have shown that the replacement of the dipeptide Val-Ser at the loop-helix-2 junction with the corresponding O-acyl iso-dipeptide leads to a completely unfolded state that only refolds after intramolecular O --> N acyl migration. Herein, we report on an Id HLH analog based on the substitution of the Pro-Ser motif at the helix-1-loop junction with the corresponding O-acyl iso-dipeptide. This analog has been successfully synthesized by solid-phase Fmoc chemistry upon suppression of DKP formation. No secondary structure could be detected for the O-acyl iso-peptide before its conversion into the native form by O --> N acyl shift. These results show that the loop-helix junctions are determinant for the folded/unfolded state of the Id HLH domain. Further, despite the high risk of DKP formation, peptides containing O-acyl iso-Pro-Ser/Thr units are synthetically accessible by Fmoc chemistry.