160 M. D. IL^MEN 



pensions (1) and also in the chromatophores derived from the strict 

 anaerobe, Chromatium (27). Thus, it appears that in both the plant 

 and bacterial systems, sufficient separation of the photolytic products 

 can be achieved in cell-free extracts to obtain a back oxidation in 

 which useful biochemical energy can be stored. 



In this connection, it is of interest to note that Wessels (40) has 

 suggested a scheme for participation of hematin compounds in which 

 phosphorylation is the end result of the light process. He suggests that 

 vitamin K or a compound analogous to it is the natural Hill reagent 

 in the chloroplast. According to his scheme, partial reoxidation of 

 photochemically reduced vitamin K by cytochrome c (or cytochrome 

 /) generates a high-energy phosphate bond(s) which may cooperate 

 in the reduction of DPN (possibly proceeding via diaphorase). The 

 oxidative phosphorylation of the reduced vitamin K by the cyto- 

 chrome produces a tautomeric form of vitamin K in the phosphoryl- 

 ated state. The splitting of this compound could result in the 

 formation of the more stable para-quinone structure of the vitamin, 

 thus making the phosphate bond in this tautomeric structure energy- 

 rich and available for conversion of ADP to ATP. Newton and 

 Kamen have attempted to test this hypothesis of the involvement of 

 vitamin K in the anaerobic system from Chromatium. The results 

 were negative (27). Menadione, an analog of vitamin K, appears to 

 be involved in the anaerobic phosphorylation by chloroplasts, however 

 (1). It should be noted that although chloroplasts contain large 

 amounts of vitamin K, there appears to be no appreciable amount of 

 this vitamin in the anaerobic photosynthetic bacterium, Chromatium 



(27). 



In all of this discussion there is a hint that the "splittmg of water" 

 as such may not be involved in the initial light reaction, but can 

 occur instead as a result of the transfer and storage of energy during 

 dark back oxidations of the type which have been postulated. In 

 other words, single quantum events are assumed to mediate the 

 production of ATP and other high-energy compounds. The evolution 

 of oxygen is imagined to occur as a result of the gradual accumulation 

 of a store of oxidized material. In the case of the bacteria, the oxi- 

 dants are removed by reaction with exogenous H donors. 



It is necessary in this connection to bring forward one more bit of 

 speculation. The hematin compounds resemble chlorophyll and the 

 other magnesium porphyrins closely enough so that direct excitation 



