Determination of intermediates is of necessity to establish the mechanism of elementary chemical events of enzymatic reactions. Photochemical methods are used frequently to model intermediates. The photochemical reactions in molecular systems are determined by the complex processes of conversion of light energy into other types of energy. Modern theoretical and experimental approaches allow detection of these main processes and selective determination of detailed properties of high-energy products generated in these processes (excited states, free radicals, or solvated electrons). Many of these products are intermediates of dark chemical reactions. For studying reaction mechanisms, model photochemical systems are preferable to radiolytic systems because they allow direct stabilization of the functional reactive intermediates without concurrent generation of highly active products of water radiolysis, the initiators of formation of nonspecific half-products.Redox enzyme cofactors-adenine, NADH, and flavin-are chemical redox agents in many biological processes. Their photochemical properties (namely photoionization associated with electron release) make them a convenient object to study the mechanism of redox processes and, in particular, to determine potential biological electron donors or acceptors (for instance, nucleotides). Photointeraction of biological cofactors with nucleotides may clarify the details of many important biological processes (e.g., the photodynamic effect; the group of reactions associated with hydrogen transfer, including DNA synthesis, some mutations, and, possibly, ATP synthesis), in which free radicals of these cofactors serve as coenzymes or substrates of reactions.It is known that free radicals of the substrates of ATP synthesis-inorganic phosphate and ADP-are generated by photoionization within their absorption bands ( λ < 200 nm and λ ~ 260 nm, respectively). However, these free radicals may be detected along with hydrogen atoms during irradiation in the absorption bands of biological cofactors at λ > 260 nm and λ > 320 nm. Generation of free radicals in this case results, presumably, from stepwise light and dark interactions. In this photochemical system, we detected phosphate radicals with hydrogen atoms selectively with the use of EPR. These radicals were photoinduced by adenine ( λ max = 260 nm) in frozen diluted aqueous solutions at physiological pH values [1]. For example, we studied a model of ATP photosynthesis in which ATP synthesis was detected during the formation of a flavin-photosensitized free radical of ADP ( λ > 320 nm) [2]. Hydrogen atoms found in these systems presumably serve as an indicator of an important channel of electron consumption which leads to the formation of an important intermediate of photochemical conversions.In this work, the conditions of occurrence of hydrogen atoms were determined by EPR and nanosecond photolysis in studied the following photosystems: Ade + P i (240 nm < λ < 300 nm), Fl + ADP + P i ( λ > 320 nm), and NADH + P i ( λ > 320 nm). This...
