The effects of phenazine dyes and N,N,N',N'-tetramethyl-p-phenylenediamine upon light-induced absorbance changes and photophosphorylation in Rhodospirillum rubrum extracts.

The effects of N-methylphenazonium methosulfate (PMS) have been compared with three other phenazine dyes and N, N, N’, N’-tetramethyl-&phenylenediamine (TMPD). All accelerate the recovery of the initial absorbance changes in Rhodospirillum rubrum extracts induced by a lO+ec flash, although only N-ethylphenazonium ethosulfate and TMPD stimulate photophosphorylation to the same extent as PMS. N-Ethyl-l, 2-benzophenazonium perchlorate and N-ethyl1,2,6, ‘74ibenzophenazonium chloride are less active. Absorbance changes with the dyes give evidence for the presence of at least three steps. The first is the light reaction, occurring at the instant of a flash ( <25 ,usec), generating an oxidant and a reductant. This is represented by the initial flash-induced absorbance change, which is unaffected by the presence of dyes. The second step, a slower reaction (complete in approximately 100 psec with 0.5 mM reduced PMS), is the reduction of the photo-oxidant at the expense of reduced dye. This is evidenced by the marked stimulation of recovery of the initial flash-induced absorbance change by reduced dye, coupled with the appearance of oxidized dye (TMPD). The kinetics is monophasic first order, in contrast to the slow polyphasic kinetics of recovery in absence of dye. The initial rate of recovery of absorbance change is directly proportional to reduced dye concentration. The oxidation of TMPD induced by a single saturating lo-psec flash is directly proportional to extract concentration and represents one-thirtieth of the bacteriochlorophyll content. Double flash experiments show that the initial flash-induced absorbance changes signal saturation of photochemical reaction centers. Maximal utilization of a second flash for dye photo-oxidation requires complete recovery of the initial absorbance changes induced by the first flash. The third step is the reduction of oxidized dye at the ex-