PHOTOCHEMICAL REACTIONS —
the chemical reactions happening under the influence of light; have the extreme all-biological importance.
On character biol. effect F. rubles subdivide on physiological and damaging. Carry F to the first. the river, the cornerstone of photosynthesis (see), biosynthesis of physiologically important substances — vitamins (see), pigments (see), etc., and also the reactions providing fiziol. the functions connected with obtaining information from the environment — sight (see), tropisms (see), a taxis (see). Damaging F. rubles have in the basis effect of light, especially the Uv-radiation (UV rays), on nucleic acids (see) and proteins (see). As a result death of cells, an intensive mutagenesis (see), an inactivation of enzymes, etc. can be observed.
Everyone F. the river can be divided into three stages: 1) the act of absorption of a photon, at Krom there are electronic excited atoms or molecules (see the Molecule, excited states of molecules); 2) primary photochemical processes, in to-rykh participate these electronic excited particles; 3) secondary (dark) transfomation of the most various character.
Quantitative measure of efficiency F. the river serves the photon yield representing the relation of number of the reacted molecules to number of the molecules which absorbed photons. The size of a photon yield fluctuates in extremely wide limits. E.g., photorecovery of methylene blue has the photon yield equal 1,6 - 10 ~ 4; for the ineffective processes proceeding in makromolekulyar-ny systems it can be even less. The low photon yield, as a rule, is explained by a considerable share of processes of physical deactivation of the excited molecules or quickly proceeding back reaction. Sometimes, on the contrary, the photon yield is extremely high. It takes place at nek-ry multistage processes, in particular at chain reactions (see). So, for photochlorination in a gas phase the photon yield reaches size 106. Distinguish F. river of the following types.
Photos accession, refer photodimerization and photooxygenating to Krom. Among biologically important reactions of photoaccession it is necessary to call photodimerization of the pirimidinovy bases (see) — thymine, uracil, a tsitozin, the cornerstone of effect of light on nucleinic to - you (see the Mutation). One of the major for all biosphere F. the river is formation of ozone (see), a cut it is possible to consider as photooxygenating of oxygen. The ozone which is formed in an upper atmosphere intensively absorbs pernicious for all live short-wave UF-izlu-cheniye.
Fotoperegr at both p and r about in-to and, to a cut carry a photoisomerization and phototautomerization. A special case of a photoisomerization is the photostereoisomerization; very important in biol. the relation process is cis-the trance - izosherigation the retina of la (see Retinolum) making fundamentals of chemistry of sight (see Rhodopsins). Final stages of synthesis of vitamins of group D — cholecalciferol and ergocalciferol also belong to reactions of a photoisomerization (see Calciferols).
Photchemical dissociation (photolysis). Among biol. reactions of photchemical dissociation it is necessary to mention a photolysis of polypeptides and proteins, at Krom the leading role belongs to destruction of the remains of aromatic amino acids in a polypeptide chain and to a rupture of disulfide bridges.
A phot about p an er a nose an electron and. Reactions of this type can be considered as primary oxidation-reduction process (see Redoxreactions). Absorbing light quantums (see), molecules of substance enter interaction with each other or with molecules of other substances. Among them there can be both donors, and electron sinks. Transfer of an electron on a fotovozbuzh-day molecule (photorecovery) or from the photoexcited molecule (photooxidation) results.
F about t about with e N with and and l and z and c and I. At these F. the river a light quantum is absorbed not by that molecule, in a cut there are chemical changes, and a molecule of a special photosensitizer capable in wild spirits to stimulate chemical transformation of others, the substances which are not absorbing light (see. The photosensitizing means). Reactions of a photosensitization are various and numerous (see Fotosensibilization). The major treats their number biological F. river — photosynthesis (see). The xanthopathy under the influence of Uv-radiation (suntan) can be considered as the reaction of a photosensitization inducing biosynthesis of melanin (see) too. It patol. manifestation can be considered photodermatoses (see).
See also Photochemistry.
Bibliography: See bibliogr. to St. Photochemistry. V. I. Rozengart.