348 PHOTOMECHANICAL CONSIDERATIONS 



responsible for the chromatophore ESR signal and a compound which 

 has been shown to react with the chromatophore electron transfer 

 components in a photochemical reaction. 



DISCUSSION 



The data presented above indicate that the reduced dyes DPIPH2 

 and TMPD can interact with the chromatophore component responsible 

 for the light-induced ESR signal in R. nihruiyi chromatophores. More- 

 over, we have confirmed Calvin and Androes' (3) and Beinert, Kok and 

 Hoch's (11) observations that the addition of the oxidizing agent fer- 

 ricyanide to chromatophores and chloroplast preparations produces a 

 large ESR signal in the dark. This observation and the fact that both 

 dried and wet chromatophores produce a light-induced ESR signal, 

 which has kinetics similar to the chlorophyll- related optical absorption 

 changes, support the contention that the ESR signal is due to the photo- 

 oxidation of bacteriochlorophyll. 



A plausible interpretation of the foregoing results is that light 

 partially oxidizes the bacteriochlorophyll to produce free radicals 

 which, in turn, give rise to the ESR signal of the chromatophore. A 

 reduced dye, such as DPIPH2, may then couple with the oxidized 

 chlorophyll and, by donating an electron, promotes the decay of the 

 ESR signal. 



The interaction of the positive radical ions of TMPD and the free 

 radicals produced by light in the chromatophore represents a more 

 complex relationship. When the light is turned on, it appears that the 

 time constant of formation of the TMPD free radicals is faster than 

 that of the chromatophore component. Once the TMPD radicals have 

 been formed, the kinetics at different magnetic field positions indicate 

 that a decrease in the total signal occurs, probably due to the inter- 

 action of the TMPD free radicals with redox agents contained in the 

 chromatophore. The rapid changes observed for the TMPD signal 

 when the light is turned off are more difficult to explain. Knowledge 

 of the exact relationship of the two free radicals must await further 

 studies. 



SUMMARY 



1. The dye DPIPH2 promotes the decay of the light-induced ESR 

 signal from R. n(bnim chromatophores. This effect is increased with 

 increasing concentrations of dye. 



2. A more complex relationship exists upon addition of TMPD to 

 an anaerobic system. Both the formation and decay of TMPD positive 

 radical ions in the light and in the dark are more rapid than those of 



