1524 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



that no photochemically supplied H will be wasted, BH2 must react with C 

 within one second to maintain the ratio [BH2]/[B] at the required level. 

 This puts a limit on the admissible activation energy of the reaction BH2 + 

 C -»- B + CH2. If this reaction is endothermal, the minimum value of the 

 activation energy is the heat of reaction, A//; and if the normal potential of 

 the system C/CH2 is significantly more negative than that of the system 

 B/BH2, the reaction BH + C -* B + CH is likely to be endothermal by 

 a similar amount. Furthermore, if this reaction has no activation energy in 

 excess of Ai/, the back reaction, CH2 + B ->- C + BH2 will be extremely 

 rapid, and there will be little chance for any CH2 formed to escape it by 

 stabilizing processes. The net result of all these considerations is that only 

 very limited help can be expected, in a photochemical reaction against the 

 gradient of chemical potential, from the concentration term in the free 

 energy relationship: 



E = E,+°^ log R?il 



n [Ox] 



What is needed for the success of the photochemical reaction is the capacity 

 of the primarily light-activated molecule to utilize its energy for direct 

 reduction of a compound with a potential negative enough for all subsequent 

 steps in the reaction sequence to go "downhill" (on the AH scale). In 

 more concrete terms, if there is a primary photochemical oxidant, B, its 

 reduced form must be capable of carrying on the reduction of whatever has 

 to be ultimately reduced — be it CO2, RCOOH, or various Hill oxidants— 

 by exothermal reactions. (These reactions may be made exothermal by 

 cooperation of several BH2 molecules, i. e., by an "energy dismutation" 

 mechanism — for example, with the aid of high-energy phosphates.) 



Uri (1952) noted that the yield of polymerization of methyl methacrylate, photo- 

 sensitized by chlorophyll (cf. above, section 1) increased enormously upon addition of 

 ascorbic acid or urea, indicating a great increase in the photostationary concentration of 

 free radicals. Reducing salts (ferrous sulfate, ferrocyanide) inhibited rather than stimu- 

 lated polymerization. 



Baur and Niggli (1943) in the last of the papers of the series reported in chapter 4 

 (section A4(fe)) had claimed that chlorophyll, dissolved in geraniol or phytol, and emul- 

 sified in a solution of methylene blue in dry glycerol, could sensitize the reduction of cir- 

 culating carbon dioxide to formaldehyde and liberate oxygen (from glycerol?). Lin- 

 stead, Braude and Timmons (1950) were unable to confirm these results. 



Gurevich (1948) described still another chlorophyll-sensitized oxidation-reduction: 

 the reduction of o-dinitrohenzene by phenylhydrazine. (For his experiment on the use of 

 o-dinitrobenzene as oxidant in Hill reaction, cj. part B, Sect. 4(g).) He used ethanolic 

 extracts from nettle leaves. 5 cc. of "dark-green but transparent" extract were mixed 

 with 0.5 cc. of phenylhydrazine (10% free base in ethanol) and 0.5 cc. o-dinitrobenzene. 

 One half of this solution was illuminated, at 15-30°, 15 cm. from a 300-watt lamp, (H2O 

 filter) for 30 min.; the other half kept in darkness. The formation of o-nitrophenyl- 

 hydroxylamine was proved by addition of concentrated ammonia, leading to dark-violet 



