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Helmut Beinert and Bessel Kok 



complex preparations. Although overlap was corrected on the 

 basis of the dark signals observed after illumination, this 

 correction is not entirely satisfactory as the broad signal 

 is more intense during illumination. In the last two lines 

 determinations on chromatophores from Rhodospirillum rubrum 

 are reported. Since P700 is not a constituent of these 

 organisms only the ratio of bacterio chlorophyll [determined 

 according to (13)3 to unpaired spins is given. Approximately 

 3% of this chlorophyll are thought to represent a photocon- 

 verter, P89O, similar to P7OO. On this basis a spin per P89O 

 ratio of 0.4 to 0.5 would be obtained. 



We are aware that our experiments cannot provide a final 

 decision or an identification of the narrow light induced EPR 

 signal; they could at best rule out or make appear plausible 

 certain possible interpretations. In assessing the signifi- 

 cance of the values we obtained, two principal considerations 

 are pertinent: The first is concerned with the accuracy of 

 our quantitation procedures and the second with the question 

 of whether all radicals and radical species formed in the 

 illuminated samples were, in fact, detected by EPR. 



To the first point we can say that use of the double sam- 

 ple cavity and carefully matched cells, the use of independ- 

 ently standardized standards, attention to the conditions of 

 saturation with light and microwave power and the consistency 

 of the results obtained in the determinations at both room 

 and low temperature, make it very unlikely that gross errors 

 were committed. Nevertheless, in view of the uncertainties 

 in the absolute values of EPR standards and in comparison of 

 different materials, we think that accumulation of errors 

 could have led to values which are in error by a factor of 

 2 or 3- The consistancy of the results indicates that these 

 errors, if incurred, are not random but systematic and due to 

 certain incorrect assumptions. 



The second point of concern is related to the question as 

 to what type of paramagnetic species is in fact responsible 

 for the observed signal. The simplest assumption, on which 

 our experiments here are based, is that a single free radical 

 species arises, which has a structure and environment such 

 that it can be quantitatively detected by the EPR technique. 

 However, since a one electron oxidation produces the radical, 

 it appears possible that a second radical, formed by the 

 corresponding one electron reduction (in the extreme case a 

 free electron) is simultaneously generated. There are no 



