Some Biochemical Processes Induced by Radiation as Studied by Pulse Radiolysis

The main types of biochemically important electron transfer substances can be made to undergo reactions under irradiation which in some ways mimic those which occur in nature. In the case of NAD⁺, pulse radiolysis shows that electrons add to the pyridine ring forming an NAD· radical with a characteristic absorption in the visible range which is capable of transferring its electron to oxygen. Using simpler pyridinyl compounds it has been found that, as well as the absorptions in the visible range, the radicals exhibit absorptions in the infrared range which correlate well with transitions which may be deduced from the known charge-transfer bands of the parent compound. The NAD· radical can be made by one-electron oxidation of NADH as well as by reduction of NAD⁺, and the reaction with oxygen enables NADH to be converted into NAD⁺ by an unambiguous free radical route. Cytochrome-c can be reduced either by hydrated electrons or CO₂^?. In solution buffered to neutral pH, the reduction gives rise immediately to normal reduced cytochrome-c, but in mildly alkaline solution, an unstable reduced form appears whose conformation changes over a fraction of a second to that of normal reduced cytochrome-c. As well as the hydrated electron and CO₂^?, the superoxide anion radical O₂^? has also been found to reduce cytochrome-c. This reaction is very slow (k ～ 10⁵ M^?1 sec^?1).