A γ-turn induced by a highly constrained cyclopropane analogue of phenylalanine (c3diPhe) in the solid state

In order to evaluate the possible influence of the side chain orientation on the backbone conformation we have synthesized the model dipeptides tBuCO-l-Pro-c3diPhe-NHiPr, where c3diPhe represents (2S,3S)- and (2R,3R)-1-amino-2,3-diphenylcyclopropanecarboxylic acid, two cyclopropane analogues of phenylalanine. In the solid state, the (2S,3S)c3diPhe-containing compound adopts a classical βII-turn disposition. In contrast, the dipeptide incorporating the (2R,3R) enantiomer exhibits an open βII-turn structure that lacks the usual i+3 to i hydrogen bond, together with a γ-turn centred at the c3diPhe residue.