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β-Turn Preferences Induced by 2,3-Methanophenylalanine Chirality

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posted on 05.09.1998, 00:00 by A. I. Jiménez, C. Cativiela, A. Aubry, M. Marraud
The model dipeptides RCO-l-Pro-c3Phe-NHMe, where c3Phe stands for each of the four cyclopropane analogues of phenylalanine (2,3-methanophenylalanine), have been studied in solution by using 1H NMR and FT-IR spectroscopy and in the solid state by using X-ray diffraction. The conformational behavior of these compounds has been compared to that of the analogous Ac3c and l- or d-Phe-containing dipeptides. When associated to proline, the cyclopropane residues in the so-called i + 2 position exhibit a marked tendency to β-folding in solution, even in DMSO. The type II β-turn is generally favored, but the βI/βII-turn ratio depends both on the experimental conditions (solvent, solution, or solid state) and on the chirality of the c3Phe moiety, which rigidly fixes the orientation of the phenyl ring with respect to the peptide backbone. The (2S,3S)c3Phe residue, analogous to l-Phe with the χ1 angle constrained to about 0°, is the most propitious to βI-folding in the i + 2 position of a putative β-turn.