First step toward the quantitative identification of peptide 3(10)-helix conformation with NMR spectroscopy: NMR and X-ray diffraction structural analysis of a fully-developed 3(10)-helical peptide standard

We have synthesized by solution methods and fully characterized the N-alpha-blocked heptapeptide methylamide mBrBz-[L-Iva-L-(alpha Me)Val](2)-L-(alpha Me)Phe-L-(alpha Me)Val-L-Iva-NHMe, fully based on conformationally constrained C-alpha-methylated alpha-amino acids. An X-ray diffraction investigation of the N-alpha-benzyloxycarbonylated analogue showed that in the crystal state both independent molecules (A and B) in the asymmetric unit of the peptide adopt a fully developed, regular, right-handed 3(10)-helical structure, although molecule A would be slightly distorted at the C-terminal residue. Solution conformational analysis on the mBrBz-blocked peptide was carried out in CDCl3 by means of NMR spectroscopy. For structure determination we performed restrained molecular dynamics simulations in CDCl3 based on a search of the conformational space derived from a simulated annealing strategy. For this peptide the NMR observables can be described by a single backbone conformation, more specifically a rigid 3(10)-helix spanning the amino acid sequence from residue 1 to residue 6. The C-terminal methylamido NH group seems to be involved simultaneously in two H-bonds (with the preceding i - 3 and i - 4 carbonyl groups). Although in this peptide model there are no distinct NOE distances for discriminating 3(10)-versus alpha-helix conformation, the sum of all NMR-derived restraints clearly results in a 3(10)-helical structure. Convergence from different starting structures (including an alpha-helix) into a 3(10)-helix was observed.