Peptide helices as rigid molecular rulers: A conformational study of isotactic homo-peptides from C-alpha-methyl,C-alpha-isopropylglycine, [L-(alphaMe)Val]

Terminally blocked, isotactic homopeptides from the sterically demanding alpha-methylvaline of general formula Y-[L-(alpha Me)Val](n)-OtBu (Y = Z, pBr-Bz, Ac; n = 2-8) have been prepared step-by-step in solution and fully characterized. The conformations preferred in solution (beta-turn and right-handed 3(10)-helix) have been assessed by FT-IR, H-1 NMR and CD spectroscopy. The molecular and crystal structures of the Z-protected trimer, hexamer, heptamer and octamer have been determined by X-ray diffraction. In the crystal state, while the trimer is folded in a type III beta-turn conformation, the longest homopeptides form well-developed, regular, right-handed 3(10)-helices. The screw sense in the helix of the pBrBz-blocked octamer has been confirmed to he right-handed by solid-state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers is discussed.