title: Lifetimes and reactivities of some 1,2-didehydroazepines commonly used in photoaffinity labeling experiments in aqueous solutions.
abstract: The reactive 1,2-didehydroazepine (cyclic ketenimine) intermediates produced upon photolysis of phenyl azide, 3-hydroxyphenyl azide, 3-methoxyphenyl azide, and 3-nitrophenyl azide in water and in HEPES buffer were studied by laser flash photolysis techniques with UV-vis detection of the transient intermediates. The lifetimes of the 1,2-didehydroazepines were obtained along with the absolute rate constants of their reactions with typical amino acids, nucleosides, and other simple reagents present in a biochemical milieu. The nitro substituent greatly accelerates the bimolecular reactions of the cyclic ketenimines, and the 3-methoxy group greatly decelerates the absolute reactivity of 1,2-didehydroazepines. The intermediate produced by photolysis of 3-hydroxyphenyl azide is much more reactive than the intermediate produced by photolysis of 3-methoxyphenyl azide. We propose that the hydroxyl-substituted 1,2-didehydoazepines rapidly (<10 micros) tautomerize in water to form azepinones and much more rapidly than the corresponding 3-methoxy-substituted cyclic ketenimines undergo hydrolysis. Azepinones react more rapidly with nucleophiles than do methoxy-substituted 1,2-didehydroazepines and are the active species present upon the photolysis of 3-hydroxyphenyl azide in aqueous solution.
authors.affiliation: Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA.
authors.forename: Mary S
authors.initials: MS
authors.lastname: Rizk
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Shi
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Platz
mesh_terms: D001386:Azides / Q000737:chemistry*; D019920:Photoaffinity Labels / Q000737:chemistry*; D010782:Photolysis; D012996:Solutions; D013056:Spectrophotometry, Ultraviolet; D014867:Water
chemical_list: D001386:Azides; D019920:Photoaffinity Labels; D012996:Solutions; D014867:Water; C046309:3-nitrophenyl azide; C014747:phenylazide