(R)-ISOVALINE HOMO-PEPTIDES ADOPT THE LEFT-HANDED 3(10)-HELICAL STRUCTURE

A complete series of N- and C-blocked, monodispersed homo-oligopeptide s from the sterically hindered (R)-isovaline residue to the hexamer le vel was synthesized step by step by solution methods and fully charact erized The preferred conformation of all the oligopeptides was determi ned in deuteriochloroform solution by Fourier transform infrared absor ption and H-1 nuclear magnetic resonance. In addition, the molecular s tructures of tripeptide, tetrapeptide and pentapeptide were assessed i n the crystal state by x-ray diffraction. The results obtained confirm the conclusions from previous studies, namely that beta-bends and 3(1 0)-helices are preferentially adopted by isovaline-rich peptides, a cl ear indication that this C-alpha,C-alpha-disubstituted glycine tends t o induce folded structures much more extensively than its unmethylated parent compound alpha-aminobutyric acid. Furthermore, in this work we were able to demonstrate unambiguously for the first time that the re lationship between isovaline chirality and helix screw sense is the sa me as that promoted by C-alpha-monosubstituted glycines [(R)-amino aci ds give left-handed helical structures] A comparison is also made with the conclusions extracted from published work on homo-oligopeptides f rom other C-alpha-methylated amino acids.