A time-resolved spectroscopic comparison of the photoisomerization of small beta-turn-forming thioxopeptides

The monosubstituted thioxopeptide Boc-Ala-Pro-psi(SCNH)-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 -SCNH-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.