A Time-resolved Spectroscopic Comparison of the Photoisomerization of Small β-Turn-forming Thioxopeptides

The monosubstituted thioxopeptide Boe-Ala-Pro-psi(SC-NH)-Aib-Ala-OMe is investigated by time-resolved UV-pump/IR-probe and IR-pump/IR-probe spectroscopy, steady-state FTIR-spectroscopy, and NMR-techniques. The compound has a high propensity to adopt a i --> i + 3 hydrogen-bonded conformation. Time-resolved infrared measurements reveal the opening of this beta-turn structure upon trans --> cis photo isomerization of the thioamide bond. Comparison is made with three protected tripeptides containing the -SC-NH-Aib- moiety with different thio-substituted residues. Very similar photo isomerization dynamics and comparable quantum efficiencies are found. Differences are seen for the thermally activated cis --> trans relaxation in the electronic ground state, where thioxopeptides with larger residues next to the thioamide moiety exhibit subsecond isomerization times. Anisotropy measurements indicate a very rigid Aib-containing core structure for all four thioxopeptides in acetonitrile solution.