title: A fast photoswitch for minimally perturbed peptides: investigation of the trans-->cis photoisomerization of N-methylthioacetamide.
abstract: 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 approximately 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.
authors.affiliation: Physikalisch-Chemisches Institut, Winterthurerstrasse 190, Universität Zürich, CH-8057 Zürich, Switzerland.
authors.forename: Jan
authors.initials: J
authors.lastname: Helbing
authors.forename: Harald
authors.initials: H
authors.lastname: Bregy
authors.forename: Jens
authors.initials: J
authors.lastname: Bredenbeck
authors.forename: Rolf
authors.initials: R
authors.lastname: Pfister
authors.forename: Peter
authors.initials: P
authors.lastname: Hamm
authors.forename: Robert
authors.initials: R
authors.lastname: Huber
authors.forename: Josef
authors.initials: J
authors.lastname: Wachtveitl
authors.forename: Luca
authors.initials: L
authors.lastname: De Vico
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci
mesh_terms: D007700:Kinetics; D008958:Models, Molecular; D018716:Molecular Mimicry; D010455:Peptides / Q000737:chemistry*; D010777:Photochemistry; D011485:Protein Binding; D011487:Protein Conformation; D013055:Spectrophotometry, Infrared / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013237:Stereoisomerism; D013853:Thioacetamide / Q000031:analogs & derivatives / Q000737:chemistry*
chemical_list: C492098:N-methylthioacetamide; D010455:Peptides; D013853:Thioacetamide