488 



M. B. Allen and J. C. Murchio 



The photochemical activity of the complex that has been most 

 extensively studied is the formation of a free radical on 

 illumination. This radical exhibits a single line electron spin 

 resonance (ESr) signal of line width and g value similar to the 

 signal induced by iQng wave length red light in intact cells of 

 C. pyrenoidosa.^^^oj However, lonlike the signal from the intact 

 cells, which decays in a few milliseconds, that in P-672 persists 

 upon darkening the cells (2). The radical can, however, be 

 discharged by reaction with ferrocyanide or with natural electron 

 acceptors present in the soluble fraction of the cell homogenate. 

 These properties, as well as the redox potential calculated from 

 the degree of discharge of the radical in ferri -ferrocyanide 

 mixtures, are strikingly similar to those described for P-700 by 

 Kok, Beinert, and Hoch vT^o}^ j^ j^g therefore been of interest 

 to determine whether these properties are common to different 

 kinds of chlorophyll complexes, or whether the P-700 system is 

 contained in P-672. 



Action spectra for induction of the ESR signal were obtained 

 with a Varian 4500 EPR spectrometer \d.th optical transmission 

 cavity. Light from a 5OO w. projection lamp was collimated with 

 aspheric lenses and passed through narrow band Spectrolab inter- 

 ference filters into the cavity. The voltage on the light was 

 varied so that equal numbers of qvianta were supplied at all wave- 

 lengths except 400 and ^4-35 t^j where the lamp output was insuf- 

 ficient to obtain this. Potassium ferrocyanide was added to the 

 P-672 preparations to discharge the radical after illumination. 



Results of such measurements are sho■^^n2 in Figure 5^ both for 

 material containing membranes (pelleted) and for the dispersed 

 particles. The former shows a distinct peak at 695 nJ^^ as well 

 as in the blue, with smaller peaks in the central portion of the 

 spectrum. The signal in this sample increased linearly with 

 light, so that the peak in the blue is considered significant 

 even though lower light intensities were used for these points. 

 The dispersed material gives a rather featTireless curve except 

 for a small peak at 695 n^ and a large one in the blue. The 

 695 inn peak has been observed with chlorophyll concentrations 

 varying from 0.8 to 2,7 mg chlorophyll/ml . 



The curves shown in Figure 5^ like other ESR action spectra, 

 show the steady state signal level as a function of wavelen^h. 

 The interpretation of such curves is not clear, since this is 

 not an action spectrum in the usual sense of the term, in which 

 the rate of a reaction, e.g. oxygen evolution, is determined at 

 different wavelengths. In order to obtain information that might 



