Conformational studies of cyclic tetrapeptides

The conformations of chlamydocin and cyclo (Ala-Aib-Phe-D-Pro)(Ala4-chlamy-docin) in chloroform have been investigated by nuclear magnetic resonance, infrared and circular dichroism spectroscopy. The data obtained from these experiments establish an all transoid, bis γ-turn conformation for both compounds in chloroform with the following torsional angles (± 20d): Ala44-chlamydocin: Aib, O+ 60d, Ψ– 50d; ω+ 160d; Phe O– 120d, Ψ+ 120d, ω– 160d; D-Pro O+ 60d, O– 55d, ω+ 160d; Ala O– 110d, Ψ+ 110d, ω– 160d. Chlamydocin adopts a closely related conformation in neat chloroform. Nuclear Overhauser Effect (NOE) data are utilized to assign amide bond geometries in the cyclic tetrapeptide ring system.