title: Free radicals in primary photobiological processes.
abstract: Formation of a semiquinone free radical derived from chlorophyll in the reaction of photoreduction has been discovered by A. A. Krasnovsky, Sr. in 1953. This review consider the results obtained in the author's laboratory, concerning the participation of free radicals in photochemical reactions under UV-irradiation of aromatic amino acids, proteins, and lipids, as well as in the reactions of chemiluminescence (CL) in the protein and chlorophyll-containing systems. Free radicals are the very first products of photochemical reactions in all systems studied. The back reactions of radicals are accompanied with photon emission. From the point of view of the molecular energetics, the radiativeless electronic transition in molecules is the most probable event, the transition triplet level is less probable, and the transition to the singlet excited level is virtually impossible. This may explain the low quantum yield of CL, similarity of CL and phosphorescence (rather than fluorescence) spectrum of the reaction products, low quantum yield of CL, and its high temperature coefficient.
authors.affiliation: Russian State Medical University, Moscow. avlad@iki.rssi.ru
authors.forename: Y A
authors.initials: YA
authors.lastname: Vladimirov
mesh_terms: D002734:Chlorophyll / Q000378:metabolism*; D005609:Free Radicals / Q000378:metabolism; D008163:Luminescent Measurements; D008956:Models, Chemical; D010777:Photochemistry; D010788:Photosynthesis; D010944:Plants / Q000378:metabolism* / Q000528:radiation effects; D014466:Ultraviolet Rays
chemical_list: D005609:Free Radicals; D002734:Chlorophyll