Solvent-dependent conformational behaviour of model tetrapeptides containing a bicyclic proline mimetic

Two model tetrapeptides containing bicyclic analogues of either D‐ or L‐proline were synthesised and their conformational properties were studied by NMR in different solvent systems and by molecular modelling techniques. Compound 1, with the bicyclic D‐proline mimetic in the i+1 position, generated a unique trans isomer, and the peptide showed a well organised structure, in accordance with the tendency of D‐proline to act as a good turn inducer with respect to its enantiomer. Peptide 2 displayed structures equilibrating from type I,II to type VI β‐turns, thus confirming the hypothesised relationship between the chirality of BGS/Bgs and proline enantiomers on nucleating compact turns. Moreover, such behaviour suggested a tool for peptidomimetic design of reverse turn peptides containing BGS/Bgs bicyclic proline mimetics, as the choice of chirality might influence the generation either of compact γ‐ and β‐turns or of flexible equilibrating reverse turn structures. The effect of solvent on conformational behaviour was also studied.