Vibrational and electronic couplings in ultraviolet resonance Raman spectra of cyclic peptides

Ultraviolet resonance Raman (UVRR) spectra are reported for a series of cyclic amides. 2-Azacyclotridecone, which has a 13-membered ring, shows a classic trans-amide UVRR spectral pattern, with comparable enhancement of the amide modes II, III and S. When the ring is diminished to eight (epsilon -caprolactam) or seven (2-azacyclooctanone) members, this pattern is replaced by a single strong band near 1497 cm(-1), characteristic of the Cz-N stretch of a cis-amide vibration (amide IIc). Further shrinkage of the ring decreases the amide IIc frequency. It is lowered over 100 cm(-1) to 1389 cm(-1) in the case of a 4-membered ring (2-azaidine), reflecting diminution of the Cz-N bond order due to ring strain and pyramidalization of the C and N atoms. At the same time the amide Ic (Cz=O stretching) frequency increases, reflecting the localization of the Cz=O double bond. Also the sensitivity to hydrogen-deuterium exchange reverses for amide Ic and IIc modes as ring size decreases. The UV absorption maximum, which is red-shifted for cis-relative to trans-amides, shifts increasingly to the blue as the ring size decreases, again reflecting localization of the pi bonding. In the case of amides with 5- and 6-membered rings (2-pyrrolidinone and delta-eloctam) multiple UVRR bands are seen in the amide IIc region, whose relative intensities are temperature-dependent. These are assigned to conformers in which different members of the ring are out of the mean plane, resulting in variable perturbations of the amide bond. The cyclic dipeptides cyclo(Gly-Gly) and cyclo(Gly-Pro) have perturbed amide IIc frequencies, reflecting the kinematic mixing of the amide coordinates into in- and out-of-phase modes. Excitation profiles reveal electronic mixing as well, with the transition dipoles adding for the in-phase and cancelling for the out-of-phase modes.