360 



DANIEL 1. ARNON 



FRESH PREPARATION 

 (ILL.-D) 



AGED PREPARATION 

 (ILL.-D) 



-0 2 



Light on 



40 

 min 



y^ ^^ Vit. K5 



» -.- Vlt. K3 



Control 

 DPN 



Light off 



Light off 



Fig. 8. Effect of vitamin K and other cofactors on the reduction of cytochrome 

 C2 in cell-free preparations of Chronuitiinn. The cytochromes were oxidized by 

 previous illumination (cf. Fig. 7). The reaction mixture included in a final volume 

 of 3 -o ml., dialyzed cell-free suspension (PS) containing o-o6 mg. bacteriochloro- 

 phyll, 0-02 /imole of purified cytochrome To and 0-03 /xmole of the respective 

 cofactors. Difference in optical density was measured on a Beckman DU spectro- 

 photometer with an attached photomultiplier using cuvettes with the respective 

 cofactor omitted as controls in each case (Nozaki, Ogata, and Arnon, [114]). 



LIGHT-INDUCED CHANGES IN CHLOROPHYLL 



New experimental evidence for the electron flow theory has come from 

 the recent work of Arnold and Clayton [115] who, on illuminating bacterial 

 chromatophores, observed temperature-independent (i K to 300°K) 

 reversible spectral changes in the absorption bands of bacteriochlorophyll. 

 These spectral changes in chlorophyll that are independent of temperature 

 are consistent with the proposed electron shift that would result from the 

 primary photochemical act (compare equations (2) and (2a)). 



8. Multiple sites in cyclic photophosphorylation 



As already discussed, cyclic photophosphorylation is catalyzed by 

 vitamin K and FMN and also by non-physiological factors such as 

 phenazine methosulphate. This latter fact has given rise to questions 

 whether vitamin K and FMN, or equivalent quinone and flavin consti- 

 tuents of photosynthetic tissues, are to be considered the physiological 

 catalysts of cyclic photophosphorylation or whether they are to be regarded 

 as non-specific agents in no way distinguishable from non-physiological 

 catalysts. The question was of special interest in connection with phenazine 

 methosulphate because this dye has given rates of photosynthetic phos- 

 phorylation higher than either vitamin K or FMN [cf. 74]. 



