Cyclopropane analogue of valine: influence of side chain orientation on peptide folding

The cyclopropane analogue of valine (1-amino-2,2-dimethylcyclopropanecarboxylic acid, c3Val) has been synthesised and incorporated into the model peptides tBuCO-l-Pro-l-c3Val-NHiPr and tBuCO-l-Pro-d-c3Val-NHiPr. In the solid state, both dipeptides accommodate a type II β-turn stabilised by an NHiPr to tBuCO hydrogen bond. Remarkably, the peptide incorporating l-c3Val also exhibits a distorted γ-turn around the cyclopropane residue, with Pro-CO and NHiPr intramolecularly hydrogen-bonded.