LOW-TEMPERATURE PHOTOOXIDATION OF CYTOCHROME-C IN REACTION CENTER COMPLEXES FROM RHODOPSEUDOMONAS-VIRIDIS

Low-temperature photooxidation of the high-potential haem c380 (c-559) and low-potential haem c20 (c-552) has been studied in isolated RC complexes from Rps. viridis. At appropriately low redox potential, complete irreversible oxidation of c20 ensues following flash-excitation at 77 K with t 1 2 = 20 μs. Photooxidation of c380, which can be observed at high Eh, is more rapid (t 1 2 < 6 μs) and fully reversible; however, only about 20% of the haem is oxidized at 77-140 K. Full photooxidizability of c380 is attained as the temperature is raised in the range 140-200 K. Studies of the charge recombination kinetics indicate the low temperature-induced decrease of c380 photooxidizability to originate from the effect of freezing on the equilibrium constant of electron distribution between the haem and the pigment: {A figure is presented}Keq decreasing from about 100 at 293 K to about 0.25 below 140 K. A two-step mechanism of c380 photooxidation above 200 K is suggested where initial virtually isopotential temperature-independent electron dislocation from c380 to P is followed by nuclear medium reorganization which stabilizes the final c+380 P state: {A figure is presented} Freezing is likely to prevent reorganization of the medium, thus allowing for observation of the initial electron transfer step at low temperatures. Photooxidation of the low-potential haem(s) may occur via the equilibrium fraction of the c+380 P state with unrelaxed nuclear environment. In sum, the results of low-temperature experiments are fully consistent with the linear sequence of haems {A figure is presented} and there is no need to postulate parallel electron transfer pathways involved in photooxidation of high- and low-potential haems. © 1990.