HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 419 



illuminated chloroplasts. This was discovered by Dr. David Krogmann 

 who has made an extensive study of the phenomenon [48-51]. Most of his 

 work has been done with trichlorophenol indophenol, but the behaviour 

 in which we are interested is manifested by phenol indophenol itself, and 

 by many of its derivatives [51]. 



The unique behaviour of trichlorophenol indophenol may be sum- 

 marized as follows. A catalytic amount of dye supports a moderate rate of 

 photophosphorylation (50-100 ^umoles per mg. chlorophyll per hour) but 

 only in the presence of oxygen. Under these circumstances the dye causes 

 an oxvgen exchange between molecular oxygen and water at a rate com- 

 mensurate with the best photophosphorylation rates which can be induced 

 by dye. The dye is rapidlv photoreduced and less rapidly photo-oxidized, 

 and the phosphorylation accompanies the latter process. The reaction is 

 not inhibited by cvanide. In order to obtain maximum photophosphoryla- 

 tion rates, the dye must be kept in the reduced state. A catalytic amount 

 of dve is therefore emploved, with an excess of reducing agent, such as 

 ascorbate, glutathione or reduced diphosphopyridine nucleotide. With the 

 latter reductant, which does not itself cause much H.^Oo generation, the 

 best P jze ratio for oxidative photophosphorylation has been shown to be 

 about two, and the photo-oxidation of dye by O2 has been shown not to 

 involve H^O., production. The process of oxidative phosphorylation occurs 

 with washed chloroplast fragments and is relatively insensitive to reagents 

 which inhibit the Hill reaction (e.g. orthophenanthroline, chlorophenyl 

 dimethyl urea, concentrated tris buffer, etc.). 



Scheme 6 represents an attempt to show how all the above facts can be 

 accommodated to the same photophosphorylation site as that localized at 



hv 

 Chi 



R< — > dye < — > OX > hO.^ 



O., 



p,~p 



Hill Reaction : 



.A. R + oxidized dye > OR + reduced dye 



B. OX > lO. + X 



" Photo-oxidative " photophosphorylation : 



C. R + iOo + 2Pi + 2ADP >OR + 2ATP 



D. OX + reduced dye > X + oxidized dye 



Scheme 6 



