EFFECT OF CYANIDE ON FLASH YIELD 1455 



the photosensitive complex, such as {X-Chl-HZ|). In chapter 32, we also 

 discussed the hypothesis of the photo synthetic unit, which attempted to 

 avoid these contradictions by assuming that several thousand chlorophyll 

 molecules are connected with one "photosensitive center" (e. g., one mole- 

 cule of the intermediate oxidant, X) by an efficient mechanism of energy 

 conduction. 



3. Effect of Cyanide on Photosynthesis in Flashing Light 



Further support for the Franck-Herzfeld explanation of the flash yield 

 was derived by Weller and Franck (1941) from experiments on cyanide 

 inhibition in flashing light, which we mentioned before in chapter 12 

 (page 307), but must consider in more detail here. Emerson and Arnold 

 (1932) noticed that the presence of cyanide (1.14 X 10"^ M HCN) de- 

 creased p™'"''- in light with short dark intervals (e. g.,td = 0.035 seconds, at 

 13° C.) in the same proportion as the rate in continuous light (namely, by 

 50%), but that the influence of the inhibitor became weaker when the 

 dark intervals were lengthened (only 12% inhibition was found at ta = 

 0.106 second), and disappeared entirely at ta = 0.212 second. 



Brilliant and Krupnikova (19520 measured the effect of cyanide on the oxygen pro- 

 duction by filamentous algae in continuous and flashing Hght. With flashes of 5.9 msec, 

 and dark intervals of 11 msec, 5 X 10 ~^ mole/1. KCN caused an inhibition by 31- 

 76% (in different species); 5 X 10 ~* mole/1. KCN had no effect. The results in con- 

 tinuous light were similar. With dark periods of 85 msec, oxygen liberation not only 

 was not inhibited, but even increased (by 2 to 90% in 5 X 10 "^ mole/1. KCN, and by 

 up to 61% in 5 X 10"* mole/1. KCN). 



Emerson and Arnold first explained this result by "substrate limita- 

 tion." They assumed that the rate-limiting reaction, which supplies new 

 reduction substrate, A-C02, for the photochemical reaction (or frees the 

 acceptor from the intermediate photoproducts and thus makes it available 

 for "recharging"), is slowed down by cyanide, but, if given more time, still 

 can provide the required amount of the complexes A-C02. This implies 

 that the material on which the cyanide-poisoned catalyst is working is 

 stable, and therefore can wait until the small residual amount of the catalyst 

 left over in the presence of the inhibitor takes care of it. 



In the Franck-Herzfeld theory, this explanation has to be modified, 

 since the reaction that limits the flash yield is, in this theory, the trans- 

 formation of unstable intermediates. This material cannot wait for the 

 second or third "round" of catalyst action (w^hich would enable a small 

 residue of active catalyst to complete the task that usually requires a much 

 larger amount). The alternative to the Emerson-Arnold explanation, 



