Thio amino acids can be integrated into the backbone of peptides without significantly perturbing their structure. In this contribution we use ultrafast infrared and visible spectroscopy as well as state-of-the-art ab initio computations to investigate the photoisomerization of the trans form of N-methylthioacetamide (NMTAA) as a model conformational photoswitch. Following the S2 excitation of trans-NMTAA in water, the return of the molecule into the trans ground state and the formation of the cis isomer is observed on a dual time scale, with a fast component of 8−9 ps and a slow time constant of ∼250 ps. On both time scales the probability of isomerization to the cis form is found to be 30−40%, independently of excitation wavelength. Ab initio CASPT2//CASSCF photochemical reaction path calculations indicate that, in vacuo, the trans → cis isomerization event takes place on the S1 and/or T1 triplet potential energy surfaces and is controlled by very small energy barriers, in agreement with the experimentally observed picosecond time scale. Furthermore, the calculations identify one S2/S1 and four nearly isoenergetic S1/S0 conical intersection decay channels. In line with the observed isomerization probability, only one of the S1/S0 conical intersections yields the cis conformation upon S1 → S0 decay. A substantially equivalent excited-state relaxation results from four T1/S0 intersystem crossing points.
|Titolo:||A fast photoswitch for minimally perturbed peptides: investigation of the trans→cis photoisomerization of N-methylthioacetamide|
|Citazione:||Jan, H., Harald, B., Jens, B., Rolf, P., Peter, H., Robert, H., et al. (2004). A fast photoswitch for minimally perturbed peptides: investigation of the trans→cis photoisomerization of N-methylthioacetamide. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 126(28), 8823-8834.|
|Appare nelle tipologie:||1.1 Articolo in rivista|