1518 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



Krasnovsky and Brin (1950) surveyed different oxidants for the reoxi- 

 dation of photochemically reduced chlorophyll. The following compounds, 

 all with negative oxidation potentials, were found to accelerate the return 

 of absorption in the red peak (in order of decreasing efficiency) : thionine 

 (5 X 10-'' M), quinone (1 X 10-^ M), methylene blue (5 X lO"" M), 

 phenol-indophenol (5 X 10"^ M) and dehydroascorbic acid (5 X 10'^ M). 

 (The position of the latter in the series shows that the other enumerated 

 oxidants, although they are known to react with ascorbic acid, reoxidize 

 chlorophyll directly, and not through the intermediary of ascorbic acid.) 

 Among compounds with positive potentials, not reacting with ascorbic acid 

 in the dark, the order of effectiveness in the reoxidation of reduced chloro- 

 phyll was: hematin (5 X lO"" M), NO3-(10-2 M), NO2-(10-2 M), 

 Fe+++(10-^ M), Cu++(10~^ M), and air. Among compounds with posi- 

 tive potentials, safranin T, neutral red, and Nile blue (all 5 X 10-* ilf) ac- 

 celerated reoxidation; also 5 X 10"* M riboflavin, and 1 X 10^^ or 5 X 

 10-" M DPN (^0 = +0.32 volt) (c/. above). Since xanthin (^o = +0.37 

 volt) showed no influence, the authors concluded that the normal oxidation- 

 reduction potential of the system chlorophyll-reduced chlorophyll is about 

 +0.35 volt. (However, the potentials had been measured in water, while 

 the observations of Krasnovsky and Brin were made in pyridine.) It is 

 interesting to compare these results with those obtained with chloroplast 

 suspensions; there, only oxidants with normal potentials below —0.1 

 volt were reduced with a good yield in air (c/. sections B4(d), (e) below); 

 in the absence of oxygen, the reduction of compounds with normal poten- 

 tials up to +0.1 volt could be observed; but compounds such as DPN, with 

 E'o > 0.3 volt, were reduced only to a very slight degree, so that their re- 

 duction could be ascertained only by ''trapping" with specific enzymatic 

 systems (to be described in section B4(/)). Of course, in the Hill reaction 

 of chloroplasts (as in photosynthesis) the reductant is water (£"o = —0.8 

 V.) and not ascorbic acid (^'o = -0.0 v.), making the (net) hydrogen trans- 

 fer that much more difficult. Further studies are needed to find out whether 

 a reduced form of chlorophyll, with the strong reducing power found by 

 Krasnovsky in vitro, does play a role in photosynthesis, but is somehow 

 prevented from displaying its full power in chloroplast suspensions. Per- 

 haps, two types of reversible photochemical changes of chlorophyll occur in 

 photosynthesis— one that permits it to acquire hydrogen from water (this 

 capacity is preserved in chloroplast preparations) and one that permits it to 

 transfer hydrogen to compounds with a normal potential >0.3 volt — this 

 capacity being preserved in chlorophyll solutions in pyridine. This is, 

 however, only a speculation, and one which calls for the minimum quantum 

 requirement of photosynthesis to be 8 (c/. chapter 7). An alternative is 

 that in vivo chlorophyll is able to transfer hydrogen directly from a system 



