PRIMARY QUANTUM CONVERSION: EPR EVIDENCE 337 



tion of the quantum conversion process as shown in Fig. 2, This 

 model provides a temperature-independent mechanismfor the quantum 

 conversion act. The steps are as follows: 1) Absorption of light by 

 the active pigment system (s), resulting in formation of an exciton; 2) 

 migration of the exciton to the site of electron transfer; and 3) pro- 

 duction of a plus radical ion (hole) in a matrix of identical molecules 

 and a reduced acceptor at low oxidation potential. 



Chromatophores, the primary pigment- containing fragments of 

 bacterial photosynthetic systems, are considered to represent the 

 smallest unit that retains photosynthetic activity as defined by the 

 ability to do photophosphorylation. The photoinduced EPR absorption 

 line shape and the optical absorption spectra of chromatophores are 

 the same as those of the parent whole cell. In Rhodospirillum rubrum 

 chromatophores the EPR signals display better reproducibility than 

 those evoked in the whole cells, and these were used as the sample 

 for the experiments to be discussed. The chromatophores were pre- 

 pared in the manner described by Androes, et al. (11). 



REDOX EXPERIMENT 



The measurement of the redox potential for oxidation of the active 

 pigment is obtained by introducing an external redox couple into the 

 system and attempting to vary the oxidation level of components in the 

 electron transport chain in a controlled fashion (12). Changes in redox 

 potential, using ferricyanide, induced dark EPR signals identical to 

 those produced upon illumination. A complementarity between the 

 chemically produced and light-induced signals was observed, as shown 

 in Fig. 3. The oxidation potential for production of chemically pro- 

 duced signal up to one half of its maximal value corresponds to that 

 for reduction of the light-induced signal to one half of its maximal 

 value and occurs at an oxidation potential of ~+0.46v. Recent evidence, 

 obtained by optical absorption changes, which suggests BChl as the 

 species initiating the electron-transfer process of photosynthesis is 

 presented by Clayton (13). A value of~+0.46v for the oxidation poten- 

 tial is obtained by observing changes in the optical absorption spectra 

 of BChl in R. nibrum resulting from a redox titration with the same 

 couple (14). 



In the application of EPR to physical systems the three observables 

 which usually provide the most information about the physical environ- 

 ment of the electron are the g value, the line width and the appearance 

 of fine and /or hyperfine structure. These parameters have been meas- 

 ured in photosynthetic systems, but unfortunately have not yielded 

 evidence for precise identification of the site of the EPR signal. Until 

 precise identification of the species producing the light-induced EPR 



