(R)-isovaline homo-peptides adopt the left-handed 3(10)-helical structure

A complete series of N- and C-blocked, monodispersed homo-oligopeptides from the sterically hindered (R)-isovaline residue to the hexamer level was synthesized step by step by solution methods and fully characterized. The preferred conformation of all the oligopeptides was determined in deuteriochloroform solution by Fourier transform infrared absorption and 1H nuclear magnetic resonance. In addition, the molecular structures of tripeptide, tetrapeptide and pentapeptide were assessed in the crystal state by x-ray diffraction. The results obtained confirm the conclusions from previous studies, namely that beta-bends and 3(10)-helices are preferentially adopted by isovaline-rich peptides, a clear indication that this C alpha, alpha-disubstituted glycine tends to 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 relationship between isovaline chirality and helix screw sense is the same as that promoted by C alpha-monosubstituted glycines [(R)-amino acids give left-handed helical structures]. A comparison is also made with the conclusions extracted from published work on homo-oligopeptides from other C alpha-methylated amino acids.