1488 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



apparently under the influence of minor impurities. For example, the life- 

 time of the bleached state in methanol solution was found by McBrady 

 and Livingston to be of the order of 1 sec, in approximate agreement with 

 the observations of Porret and Rabinowitch (1937), but about 100 times 

 shorter than had been found in the earlier measurements of Livingston 

 (1941). The life-time of the bleached state could be extended considerably 

 by adding to methanol a small amount of carbon tetrachloride. Contrary 

 to expectation, the rate of the dark back reaction (as well as the position of 

 the photostationary state) was found to be insensitive to temperature 

 changes, between 7.5° C. and 25° C. 



Chlorophyll h showed a slightly stronger reversible bleaching (and a 

 somewhat slower irreversible bleaching) than chlorophyll a. 



Addition of 0.5 mole/liter allylthiourea, or 50% isoamylamine — sub- 

 stances whose autoxidation is sensitized by chlorophyll — as well as the pres- 

 ence of other redudants, such as phenylhydrazine or hydroquinone, had no 



4 - 



0> 



O 





2 - 



- 



50 100 150 



TIME, seconds 



200 



250 



Fig. 35.1. Reversible photobleaching of chlorophyll in oxygen-free solutions 

 (after Livingston 1949): (•) dark; (O) illuminated. Ac = decrease in concentra- 

 tions (mole/hter) of the red-absorbing component (assuming the reaction product 

 does not absorb red hght at all). 



depressing effect on reversible bleaching (cf. p. 437). However, allyl- 

 thiourea counteracted the enhancing effect the presence of carbon tetrachlo- 

 ride had on bleaching, thus indicating that this enhancement must have 

 been due to the presence of a reducible impurity in the ("reagent grade") 

 carbon tetrachloride used. In contrast to earlier experiments, no enhance- 

 ment of bleaching was found by McBrady and Livingston in the pres- 

 ence of 10~® mole/liter /orwz'c acid; on the other hand, bleaching was en- 

 hanced by the addition of 10 ~^ mole/liter oxalic acid (threefold increase of 



