Stabilization of \gamma-Turn Conformations in Peptides by Disulfide Bridging

The ideal \gamma-turn conformation in peptides is stabilized by the formation of two intramolecular hydrogen bonds. These are between the NH of residue i and the C = O of residue i + 2 (1 \rightarrow 3, $C_{11}$) and the C = O of residue i and the NH of residue i + 2 (3 \rightarrow 1, $C_7$). While this reverse-turn structural feature has been observed in proteins, unambiguous characterization of this conformation has yet to be realized in small peptides. Several examples of a single 3 \rightarrow 1 $(C_7)$ hydrogen bond have been reported in crystal structures of cyclic peptides and inferred from spectroscopic studies in apolar solvents. We wish to describe the spectroscopic characterization of a \gamma-turn conformation in a protected tripeptide, stabilized by formation of a disulfide crosslink.