EPILOGUE 1989 



i« incorporated in PGA {e.g., Ri = R. + H.O -^ RiH.RoOH), and the 

 organic hydroxyl group then (jxitled photochemicaily, with or without the 

 intermediate formation of an organic peroxide? Are compounds of the 

 cytochrome type inxohcd in this part of the reaction secjuence? Or carote- 

 noids, with their loiown capacity to form peroxides? 



Little progress has occurred, since 1940, also in the understanding of 

 the chemical nature of the photochemical process proper. Is chlorophyll 

 a chemical participant in it? It is most likely that this process is a light- 

 induced oxidation-reduction against the gradient of chemical potential with 

 chlorophyll as a possible intermediate acceptor of hydrogen atoms or elec- 

 trons. The alternative suggestion that this process involves, not an (intra- 

 or intermolecular) H-transfer, but the dissociation of an S — S bond, leading 

 to the formation of a biradical, made by Calvin and co-workers (p. 1098) 

 does not seem convincing. Is there something to the suggestion, made 

 recently by Wessels. that vitamin K is the oxidant in this reaction? May 

 lipoic acid serve in this capacity? Or cytochromes — c, or/ — (as suggested 

 by Hill) — or are the last two taking part only in the back reaction between 

 the primary oxidation and reduction products, leading to the production of 

 high-energy phosphates {cf. below)? 



Whether chlorophyll a (or bacteriochlorophyll "890") plays the role 

 of the H-donor in the primary photochemical process (or that of the 

 H-acceptor in this process, or of both), remains a matter of speculation 

 (encouraged by observations of reversible photochemical oxidation, and 

 reversible photochemical reduction of chlorophyll in vitro, described in 

 Chapters 18 and 35). 



The reversible changes of the absorption spectrum of photosynthesizing 

 cells have at first indicated only transformations of components other than 

 chlorophyll {cf. Chapter 37C, section 7e). Since then, however, Coleman 

 and Holt* have observed a reversible bleaching of chlorophyll in vivo at 

 680 m/i together with the appearance of a band at 515 m/x (observed already 

 by Duysens). This finding seems to indicate a reversible photoreduction 

 of chlorophyll in vivo analogous to that studied by Krasnovsky et al. in 

 vitro. 



The study of chlorophyll reactions and the investigation of the spec- 

 troscopic properties of this compound, suggest that the cyclopentanone ring, 

 with its keto-enol isomerism, may be the part of the chlorophyll molecule 

 most directly involved in the photocatalytic action ; the presence of the 

 magnesium atom in the center of the molecule appears to influence the 

 properties of this group in a significant way. 



Since four H-atom transfers are needed to reduce one molecule of carbon 

 dioxide to the carbohydrate level (or to liberate one molecule of oxygen 



* Coleman, J. W., Holt, A. S., and Rabinowitch, E., Science (in press). 



