Psi [CH2NH] backbone-modified peptides: first unequivocal observation of a C7 structure in a linear peptide

We have elucidated the X-ray diffraction structures of the psi[CH2NH] backbone-modified analogs of Z-Pro-Leu-Gly-NH2 and t-Boc-Pro-Leu-Gly-NH2 (N alpha-protected derivatives of the tripeptide amide representing the C-terminal tail of oxytocin) with the "reduced peptide bond" located at the Pro-Leu sequence. The comparative results of these pseudopeptides show that conformational properties are similar (i.e., C7 structure at the Pro), whereas the unmodified peptides diverge substantially (i.e., t-Boc-Pro-Leu-Gly-NH2 and H-Pro-Leu-Gly-NH2 each show type-II beta-bend at the Leu-Gly; and Z-Pro-Leu-Gly-NH2 shows an open folded structure). The results for t-Boc-Pro psi[CH2NH]Leu-Gly-NH2 represent the first unequivocal proof for the existence of a C7 structure in a linear peptide.