BENTLEY GLASS 



871 



absorption at TOO mp. is iiureased by acUling lerrocyaniclc or iccliued 

 phena/ine metliosuHate (PMS): and the color can be l)leached again 

 by adding lenicyanide. Ferricyanide or other dyes that are electron 

 acceptors in the Hill reaction will, as might be expected, inhibit the 

 light-indnced oxidation of P 700 (the negative elTect), since they 

 produce more or less complete oxidation prior to the illumination. 

 Conversely, phena/ine methosulfate, which reduces P 700 in the dark, 

 precludes its reduction by means of light but stimulates the extent 

 of the oxidation occurring in the negative light-eftect. 



Kok and Hoch, in order to provide for a role of P 700 in jjhoto- 

 synthesis, propose a cyclic scheme (Fig. 7). They assume that the 

 natural photosynthetic oxidant X reacts with reduced P 700 in a first 

 light-induced reaction. Since preliminary measurements indicate 

 an oxidation-reduction potential (£'„) for P 700 of +0.46, and that 

 of cytochrome / is +0.365, the oxidized P 700 could be the agent re- 

 sponsible for the fast oxidation of cytochrome / by light. This first 

 phase may be short-circuited by ferricyanide and is equivalent to the 

 Hill reaction. When all the P 700 is oxidized, the reaction stops. 

 In the second phase of the cycle, the oxidized P 700 becomes reduced, 

 again in a reaction induced by the absorption of light, but this time 

 chieHy by the accessory pigment, while oxygen is evolved. Strikingly 

 suggestive of this possibility is the fact that, although the light- 

 induced oxidation of P 700 induced by the absorption of light by 

 chlorophyll a is quite stable as long as the particulate chloroplast- 

 protein complex remains inact, the reduction of P 700 (mediated 



Light 

 Chlorophyll a and carotenoid 



X + P 



;d •*- 



Pox * ^" 



\ 



Spontaneous 



— PMS--"^ /^DCMU 



Mnar\ 



§6tC>1^^ 



Darksteps 



Chlorophyll a ' 



Accessory pigment 



Light 



Fig. 7. Schematic summary of discussed observations. 



