Model dipeptides incorporating the trans cyclohexane analogues of phenylalanine: further evidence of the relationship between side-chain orientation and β-turn type

In order to study the influence of the side-chain orientation on the peptide backbone conformation we have synthesised the model dipeptides t-BuCO-l-Pro-(1S,2R)-c6Phe-NHMe and t-BuCO-l-Pro-(1R,2S)-c6Phe-NHMe, incorporating each enantiomer of the trans cyclohexane analogue of phenylalanine (trans-1-amino-2-phenylcyclohexanecarboxylic acid). The orientation of the aromatic side-chain determines the β-turn type accommodated by these peptides to the point that the (1S,2R)-c6Phe derivative retains the type I β-turn in the crystalline state, in contrast to the behaviour exhibited by the natural counterpart t-BuCO-l-Pro-l-Phe-NHMe.