Structural analysis of β-turn mimics containing a substituted 6-aminocaproic acid linker

A series of β-turn models have been prepared consisting of the dipeptide Ala-Gly cyclized with all stereoisomers of 6-amino-3,5-dimethylcaproic acid and 6-amino-3-methylcaproic acid, as were related peptides based on Gly-Gly and Ala-Ala. The requisite linkers were made using routes featuring stereoselective ring-expansion reactions and the syntheses completed using standard methodology. A preliminary examination of these compounds has been carried out using NMR spectroscopy, circular dichroism, and, in several cases, X-ray crystallography. These studies indicate that, depending on linker stereochemistry, different proportions of type II and type I turns were observed in solution. Both type I and type II turns were observed in the solid state.