132 



Helmut Beinert and Bessel Kok 



Weaver and Bishop (8) recently failed to observe the nar- 

 row EPR signal in a Scenedesmus mutant (#8). Spectroscopic 

 examination of this alga indeed did not reveal a light in- 

 duced bleaching of P700 (9). 



Spectroscopic determination of P700 revealed a maximum ab- 

 sorbancy change at 700 mu of 1 unit per 300-400 units of 

 total chlorophyll absorbancy (at^ — ^675 mu). Its soret band 

 (at hj>2 mu) as well as its nearly identical solubility in 

 organic solvents indicate P700 is a chlorophyll _a molecule. 

 We assume that a special binding site causes a long wave 

 change of its absorption band and underlies its function as 

 a photoconverter . This assumption assigns to P700 a molar 

 extinction coefficient similar to that of chlorophyll a 

 ( -^10^ in vivo ) . 



A second assumption: that complete bleaching occurs in 

 the photoact yields a ratio of one trapping center per 300- 

 400 sensitizing chlorophyll molecules — in good agreement with 

 measurements of the "photosynthetic unit" (10,11). 



If the oxidized form of P700, P700*, which is formed in 

 the long wavelength photoact, were identical with the free 

 radical species observed by EPR, the quantitative relation- 

 ship, between the number of spins represented by the EPR 

 signal and the amount of P700 detected optically, which we 

 observed in a preliminary experiment (1), should hold rather 

 generally for photosynthetic materials and furthermore, the 

 kinetics of appearance and disappearance of the EPR signal 

 should match that of the typical absorption band of P700, 



In the present work we undertook a direct approach at 

 quantitation of the number of spins represented by the light 

 induced EPR signal and the amount of chlorophyll and P700 

 present in a variety of materials. Knowledge of any consist- 

 ent quantitative relationships would obviously be of interest, 

 even if the EPR signal were not due to P700'^ itself or a 

 closely associated electron carrier or trap. 



Vife were aware during the course of this work that quanti- 

 tation of EPR signals is beset with many difficulties, that 

 we had to make certain assumptions, which are not readily 

 amenable to experimental verification, and that a definitive 

 identification of the component responsible for the EPR sig- 

 nal could not be expected from our approach. The EPR data 

 were evaluated on the assumption that the light induced free 



