title: Time-resolved infrared spectroscopy of thiopeptide isomerization and hydrogen-bond breaking.
abstract: The photoisomerization of the protected tetrathioxopeptide Boc-Ala-Gly(=S)-Ala-Aib-OMe was followed using time-resolved infrared spectroscopy in the amide I region in combination with isotope labeling. In acetonitrile at room temperature, approximately half of the molecules are found in a loop conformation, restrained by an intramolecular hydrogen bond, while the other half adopts more extended conformations. UV-excitation of the thioxopeptide unit immediately weakens the intramolecular hydrogen bond. After the molecules have relaxed to the electronic ground state with a 130 ps time-constant, a delayed re-formation of the intramolecular hydrogen bond is observed for molecules returning to the initial trans conformation of the thioamide bond, while the loop structure is permanently broken when the molecules isomerize to the cis conformation.
authors.affiliation: Physikalisch-Chemisches Institut, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Valentina
authors.initials: V
authors.lastname: Cervetto
authors.forename: Rolf
authors.initials: R
authors.lastname: Pfister
authors.forename: Jan
authors.initials: J
authors.lastname: Helbing
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D000577:Amides / Q000737:chemistry; D000595:Amino Acid Sequence; D006860:Hydrogen Bonding; D007536:Isomerism; D007553:Isotope Labeling; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry*; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D000097:Acetonitriles; D000577:Amides; D009842:Oligopeptides; D013438:Sulfhydryl Compounds; C032159:acetonitrile