The Preferred Solid-state Conformation of (alpha-me)trp Peptides

The two Z-L-Ala-DL-(alpha Me)Trp-NH2 diastereomeric dipeptides were synthesized from (Z-L-Ala)(2)O and H-DL-(alpha Me)Trp-NH2. The latter racemate, prepared by phase-transfer catalyzed alkylation of the N-alpha-benzylidene derivative of alanine amide followed by acidic hydrolysis of the resulting Schiff base, was characterized by X-ray diffraction. The molecular and crystal structure of Z-L-Ala-L-(alpha Me)Trp-NH2, separated From its diastereomer by silica-gel column chromatography, was determined by X-ray diffraction analysis, Both independent molecules in the asymmetric unit of the dipeptide adopt a type-II beta-bend conformation. However, only the more regularly folded conformation of molecule B is stabilized by a 1<--4 C=O...H-N intramolecular H bond. The present results indicate that: (i) the C-alpha-methylated (alpha Me)Trp residue is a strong beta-bend and helix former, and (ii) the relationship between (alpha Me)Trp chirality and helix screw sense tends to be opposite to that of protein amino acids, The implications for the use of the (alpha Me)Trp residue in designing conformationally restricted analogs of bioactive peptides are briefly discussed. (C) Munksgaard 1995.