Linear oligopeptides — effect of lengthening of the main chain by one tetrahedral carbon atom in the -Aib- l-Ala- sequence: a solid-state conformational analysis of segments of polypeptide antibiotics

Abstract

The infrared absorption and X-ray diffraction conformational analysis of t-Boc-Aib- l-Ala-Aib-OMe have shown the presence of the type-1 4→1 intramolecularly H-bonded peptide conformation (β-bend) in the solid state. Lengthening of the chain by one tetrahedral carbon atom, as in t-Boc-Aib-βAla-Aib-OMe, has a disruptive effect on this folded structure. It has also been found that two water molecules co-crystallize with each molecule of the l-Ala containing tripeptide. These results are discussed in comparison with those, previously reported, obtained in chloroform solution.