EPR AJTO OPTICAL STUDIES ON SCENEDESMUS MUTMTS 

 Ellen C. Weaver and Norman I. Bishop 



There is more than one way to obtain information on single e- 

 lectron transfers as they are taking place in an illuminated pho- 

 tosynthetic system. One which we have been using for some time is 

 electron paramagnetic resonance (EPR) spectroscopy(l) . There are 

 two light induced resonances which are typical for the chloro- 

 plasts of higher plants and several species of algae. They can "be 

 differentiated on the basis of g- value, line shape, and the ki- 

 netics of their formation and decay. The most prominent one forms 

 and decays in less than a second; hence the designation R, for 

 "rapidly decaying". It has a g-value of 2.0025, is about eight 

 gauss wide, is unstructured and Gaussian in shape. It has been 

 shown to be dependent on the presence of chlorophyllU;2) . The 

 other resonance forms rather quickly, but persists in the absence 

 of illumination for periods up to an hour or more; hence the des- 

 ignation S, for "slowly decaying". It has a somewhat higher g- 

 value, 2.0046, is twenty gauss wide, and displays partially re- 

 solved hyperfine structure. It has been tentatively identified 

 with the semiquinone of plastoquinonev 3) . 



There are two classes of mutants which have provided further- 

 evidence on the role of the two EPR signals in photosynthesis^.^) . 

 Thesepossess all the readily identifiable wild type pigments in 

 normal quantities, and yet are unable to photosynthesize. Those 

 in one class are termed "oxygen" mutants, because, although they 

 are able to photoreduce carbon dioxide with hydrogen, have a 

 greatly reduced quinone-Hill reaction. All of these have a typi- 

 cal R signal, but display only a trace of the persistant S sig- 

 nal. This observation, together with that on manganese-deficient 

 cultures, which also lack both Hill reactivity and the S signal'^ 3 j 

 and photosynthetic bacteria which evolve no oxygen and also lack 

 the S signal, enables one to identify the broad, structured sig- 

 nal with the ability of the system to evolve oxygen in photosyn- 

 thesis, although other interpretations have been proposed^ -' . 

 They all possess the normal distribution of plastoquinones; some 

 other essential link in the electron transport chain, not direct- 

 ly observable with EPR spectroscopy, is missing. 



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