Structures of peptides from α-amino acids methylated at the α-carbon,

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  • This is part 279 of the Linear Oligopeptide series. For part 278, see Ref. 1.

  • Abbreviations: Abu, α-aminobutyric acid; Aib, α-aminoisobuiyric acid or Cα,α-dimethylglycine or Cα-methylalanine; MeAib, α-methylaminoisobutyric acid; Hib, α-hydroxyisobutyric acid; Iva, isovaline or Cα-melhyl-α-aminobutyric acid; (αMe) Val, Cα-methylvaline; (αMe) Leu, Cα-methylleucine; (αMe) Phe, Cα-methylphenylalanine; Deg, diethylglycine; Ac, acetyl; mClAc, monochloroacetyl; pBrBz, para-bromobenzoyl; t-Boc, tert-butyloxycarbonyl; Z, benzyloxycarbonyl; NHMe, methylamino; NHiPr, isopropylamino; NHBzl, benzylamino; OMe, methoxy; OtBu, tert-butoxy; OBzl, benzyloxy; Oxl, 5(4H)-oxazolone. As for the Cα-methylated α-amino acids discussed in this paper, the L configuration corresponds to the S configuration if Iva, (αMe) Val, (αMe) Leu, and (αMe) Phe are considered as formally derived from Abu, Val, Leu, and Phe, respectively, by Cα-methylation.

Abstract

The structural preferences of peptides (and depsipeptides) from the achiral MeAib and Hib residues, and the chiral Iva, (αMe) Val, (αMe) Leu, and (αMe) Phe residues, as determined by conformational energy computations, x-ray diffraction analyses, and 1H-nmr and spectroscopic studies, are reviewed and compared with literature data on Aib-containing peptides. The results obtained indicate that helical structures are preferentially adopted by peptides rich in these α-amino acids methylated at the α-carbon. Intriguing experimental findings on the impact of the chirality of Iva, (αMe) Val, and (αMe) Phe residues on helix screw sense are illustrated. © 1993 John Wiley & Sons, Inc.

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