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Helmut Beinert and Bessel Kok 



indications we know of from the behavior of the observed EPR 

 signal that it may represent two different species. This 

 would still not exclude such a possibility, for which one 

 could see some support in the high ratios of free spins 

 produced per molecule of P700 (cf. Table l). 



A serious objection to the interpretation of our experi- 

 mental results could arise from the possibility that we might 

 not be detecting more than a fraction of the radicals actu- 

 ally produced. Several explanations could be given for this. 

 We may be dealing with "lifetime" broadening, i.e., short 

 relaxation time of the unpaired electron; an exchange inter- 

 action may broaden the line; or, in case a free electron 

 were generated, it could be trapped at non-equivalent sites 

 and therefore, experience varying local magnetic influences, 

 which would lead to line broadening. Such arguments cannot 

 at present, be refuted on experimental grounds. If they were 

 valid our values would set the lower limit of radical concen- 

 tration. 



We may then conclude from our data in the light of these 

 considerations that the number of unpaired electrons induced 

 by light in the photosynthetic material studied is either 

 closely similar or bigger, certainly not smaller than the 

 amount of P700 present. It is of interest to note that other 

 components of the photosynthetic system in plants, have been 

 reported to occur at a concentration of the same range as 

 P700 and the light induced EPR signal studied here. However, 

 the metal constituents of two of these: cytochrome f and 

 plastocyanin are certainly not responsible for this signal. 



ACKNOWLEDGEMENTS 



We are indebted* to Dr. J. Heise and Mr. R. W. Treharne 

 of the Kettering Laboratories, Yellow Springs, Ohio, for 

 their kind collaboration in the experiments with the double 

 sample cavity and to Mrs. I. Harris for technical assistance. 



This work was supported by the USPHS through research 

 grants (GM-05073 and GM-O6762) and a research career program 

 award (GM-K6-I8, kk2) from the Division of General Medical 

 Sciences to H. Beinert. 



