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

Abstract

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-(1 S,2 R)-c 6Phe-NHMe and t-BuCO- l-Pro-(1 R,2 S)-c 6Phe-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 (1 S,2 R)-c 6Phe 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.