VARIATION IN ELECTRON PARAMAGNETIC RESONANCE SIGNALS OF PHOTOSYNTHETIC SYSTEMS WTTH THE REDOX LEVEL OF THEIR ENVIRONMENT

Abstract— The redox dependence of the light‐induced electron paramagnetic resonance signal at g=2 in R. rubrum, R. spheroides and Chromatium chromatophore particles and quantasonie particles from spinach chloroplasts has been determined qualitatively over the range —0.3 to +0.6 V and quantatively over the range +0.3 to ±0.6 V. A light‐induced EPR signal has been titrated and demonstrated to have a midpoint potential of +0.44 v at pH 7 and 20°C. Concentration, ionic strength and pH dependence for this transition in R. rubrum chromatophores is reported. In addition to the dark signal which replaces the light signal, in chromatophore material another dark signal, occurring in the seine location as the light signal, has been demonstrated to occur at high potential. Selective chemical oxidation with K₂lrCl₆ of chromatophore particles from the three bacteria resulted in the removal of some 95 per cent of the absorbance in the near infrared and left the photoactive pigments. Two light‐induced EPR signals were found in quantasome particles by their dependence upon the redox level. Of particular interest is a signal observed at quite high potential (e.g. + 0.60 V). It was demonstrated that oxygen evolution by these quantasonie particles in the presence of K³Fe(cN)₀ occurred at the same rate at +0.55 V as at +0.40 V.