1576 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



These results, together with the earlier data of Holt and French (tables 

 35. VIII and 35. IX), and of Macdowall (table 35. IX A), establish rather 

 clearly that, as far as electrode-active redox systems of the quinone- 

 hydroqiiinone type are concerned, illuminated chloroplast suspensions can 

 displace, by the transfer of hydrogen from water, the equilibrium of all of 

 them whose normal redox potentials (at pll 6.5) are under —0.1 volt; if 

 air is excluded (and the back reaction thus slowed down, because only the 

 photochemically produced O2, or its precursors, are available for reoxida- 

 tion), the displacement of the equilibrium in light can be recognized poten- 

 tiometrically for systems with normal potentials up to +0.1 volt. 



The question whether free radicals are formed as intermediates in Hill 

 reaction with quinones or quinonoid dyes was taken up by Uri (1952) and 

 Wessels (1954) by inquiring whether this reaction has an effect on polymeri- 

 zation of methyl aery late (Uri) or acrylonitrile (Wessels), or on the oxida- 

 tion of benzene to phenol (Wessels), As mentioned in part A, Uri was able 

 to confirm, by means of the polymerization test, the formation of free radi- 

 cals in the photochemical reaction of dissolved chlorophyll with ascorbic 

 acid; but neither he nor Wessels could obtain similar evidence for the 

 presence of radicals in the quinone or dye reduction sensitized by chloro- 

 plast suspensions. 



(/) Respiration Intermediates and Other Cellidar Materials 



The behavior in the Hill reaction of the compounds known to occur as 

 intermediate oxidation-reduction catalysts in respiration is of particular 

 interest, since these compounds could conceivably serve as links connecting 

 the photochemical apparatus to an enzymatic system capable of reducing 

 carbon dioxide in the dark. Among these intermediates, the most inter- 

 esting ones are the well known "coenzymes" I and II (dipyridine nucleo- 

 tide, DPN, and tripyridine nucleotide, TPN), and the more recently dis- 

 covered "coenzyme A" ("thioctic" or "hpoic" acid). These catalysts 

 have such high reduction potentials (—0.3 volt, cf. page 222) that their 

 successful photochemical reduction would constitute a close approach — 

 as far as energy utilization is concerned — to the reduction of carbon dioxide 

 itself. More specifically, of the two steps in the reduction of carbon dioxide 

 to the carbohydrate level (we imagine CO2 to be first incorporated in a 

 carboxyl, RH + CO2 ^ RCOOH), the first one, the reduction of the car- 

 boxyl group to a carbonyl group, requires a normal potential of around 0.5 

 volt; but the second one, the reduction of a carbonyl group to a hydroxyl 

 group, needs only about 0.25 volt {cf. table 9. IV). Reduced pyridine 

 nucleotides are capable of bringing about the second step by themselves, 

 but not the first reduction step. The first one becomes possible if the car- 

 boxyl is "activated" before reduction by conversion into a phosphate ester 



