Conformational restrictions of peptides via backbone modification: solution and crystal-state analysis of Boc-L-Pro-Δ^ZPhe-Gly-NH₂

Abstract

An N^α-protected model tripeptide amide containing, in the central position, an α,β-dehydrophenylalanine (Z-configurational isomer), Boc-L-Pro-Δ^Z-Phe-Gly-NH₂ (Boc, tert-butyloxycarbonyl), has been synthesized by solution methods and fully characterized. IR absorption and ¹H NMR studies provided evidence for the occurrence of a significant population of a conformer containing two consecutive, intramolecularly H-bonded (type II-III') β-bends in solution. However, an X-ray diffraction analysis clearly indicates that only the type-II β-bend structure survives in the crystal state.