The molecular and crystal structures of one derivative and three homopeptides (from the di- to the tetrapeptide level) of the chiral, C(alpha,alpha)-disubstituted glycine C(alpha)-methyl, C(alpha)-benzylglycine [(alphaMe) Phe ], have been determined by x-ray diffraction. The derivative is mClAc-D-(alphaMe)Phe-OH, and the peptides are pBrBz-[D-(alphaMe)Phe]2-NHMe, pBrBz-[D-(alphaMe)Phe]3-OH hemihydrate, and pBrBz-[D-(alphaMe)Phe]4-OtBu sesquihydrate. All (alphaMe) Phe residues prefer phi,psi, torsion angles in the helical region of the conformational map. The dipeptide methylamide and the tripeptide carboxylic acid adopt a beta-turn conformation with a 1 <-- 4 C=0 ... H-N intramolecular H bond. The structure of the tripeptide carboxylic acid is further stabilized by a 1 <-- 4 C = O ... H-O intramolecular H bond, forming an ''oxy-analogue'' of a beta-turn. The tetrapeptide ester is folded in a regular (incipient) 3(10)-helix. In general, the relationship between (alphaMe)Phe chirality and helix screw sense is opposite to that exhibited by protein amino acids. A comparison is made with the conclusions extracted from published work on homopeptides from other C(alpha)-methylated alpha-amino acids.
Linear Oligopeptides .274. Structural Versatility of Peptides From C-alpha,alpha-disubstituted Glycines - Crystal-state Conformational-analysis of Homopeptides From C-alpha-methyl, C-alpha-benzylglycine [(alpha-me)phe]n
FORMAGGIO, FERNANDO;TONIOLO, CLAUDIO;
1993
Abstract
The molecular and crystal structures of one derivative and three homopeptides (from the di- to the tetrapeptide level) of the chiral, C(alpha,alpha)-disubstituted glycine C(alpha)-methyl, C(alpha)-benzylglycine [(alphaMe) Phe ], have been determined by x-ray diffraction. The derivative is mClAc-D-(alphaMe)Phe-OH, and the peptides are pBrBz-[D-(alphaMe)Phe]2-NHMe, pBrBz-[D-(alphaMe)Phe]3-OH hemihydrate, and pBrBz-[D-(alphaMe)Phe]4-OtBu sesquihydrate. All (alphaMe) Phe residues prefer phi,psi, torsion angles in the helical region of the conformational map. The dipeptide methylamide and the tripeptide carboxylic acid adopt a beta-turn conformation with a 1 <-- 4 C=0 ... H-N intramolecular H bond. The structure of the tripeptide carboxylic acid is further stabilized by a 1 <-- 4 C = O ... H-O intramolecular H bond, forming an ''oxy-analogue'' of a beta-turn. The tetrapeptide ester is folded in a regular (incipient) 3(10)-helix. In general, the relationship between (alphaMe)Phe chirality and helix screw sense is opposite to that exhibited by protein amino acids. A comparison is made with the conclusions extracted from published work on homopeptides from other C(alpha)-methylated alpha-amino acids.
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