The IR absorption and NMR analysis of the preferred conformation of monodisperse Z-(Aib)"-OtBu (Z = (benzyloxy)carbonyl, OtBu = tert-butoxy, Aib = -aminoisobutyric acid; = 1-12) strongly indicates the formation of fully developed stable 310-helices for the higher (n = 8-12) homooligomers (pleionomers). This experimental study was performed in the solid state as well as in deuteriochloroform solution, in the latter case as a function of concentration, temperature, and addition of dimethyl sulfoxide and the free radical Tempo. A picture of the mode of self-association of the helical structures has also been obtained. A detailed theoretical study of Aib IV-acetyl-lV'-methyl amide by conformational energy computations indicates that the fully extended structure is less stable than the helical structures, irrespective of the actual value of the N-C"-C' valence angle.
SynopsisThe conformational preferences of linear peptides containing a,a-disubstituted a-amino acids, derived from the crystal structures of 28 compounds, are reviewed. In particular, the sensitivity of peptide conformation to the geometry of these unusual amino acids is underlined. We also consider possible future directions of research, which, we hope, will result in a complete understanding of the structures adopted by peptaibol antibiotics.
Terminally blocked, isotactic molecular and crystal structures of the Z-trimer is folded in a type I11 /?-turn conhomopeptides from the sterically de-protected trimer, hexamer, heptamer and formation, the longest homopeptides manding a-methylvaline of general for-octamer have been determined by X-ray form well-developed, regular, rightIn the crystal state. while the ed 3,,-helices. The screw sense in the helix Bz, Ac; n = 2-8) have been prepared of the pBrBz-blocked octamer has been step-by-step in solution and fully characconfirmed to be right-handed by solidterized. The conformations preferred in state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers solution (B-turn and right-handed 3,,-heis discussed.lix) have been assessed by FT-IR, 'HNMR and CD spectroscopy. The
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