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 conformat ionally constrained C-alpha-methylated alpha-amino acids. An X-ray dif fraction investigation of the N-alpha-benzyloxycarbonylated analogue s howed that in the crystal state both independent molecules (A and B) i n the asymmetric unit of the peptide adopt a fully developed, regular, right-handed 3(10)-helical structure, although molecule A would be sl ightly distorted at the C-terminal residue. Solution conformational an alysis on the mBrBz-blocked peptide was carried out in CDCl3 by means of NMR spectroscopy. For structure determination we performed restrain ed molecular dynamics simulations in CDCl3 based on a search of the co nformational space derived from a simulated annealing strategy. For th is peptide the NMR observables can be described by a single backbone c onformation, 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 m odel there are no distinct NOE distances for discriminating 3(10)-vers us alpha-helix conformation, the sum of all NMR-derived restraints cle arly results in a 3(10)-helical structure. Convergence from different starting structures (including an alpha-helix) into a 3(10)-helix was observed.