58 



PROBLEMS IN PHOTOSYNTHESIS 



The pigment used is crystalline ethyl chlorophyllicie in 0.1% acetone solu- 

 tion prepared by the method of Willstatter and Stoll (37). The concentra- 

 tion of pigment is thus C^ = 1.54 X lO^'' mole/liter. The acceptor is al- 

 lylthiourea, which is not oxidized in the dark in either the absence or presence 

 of pigment.* The acceptor concentration is C„ = 75 X 10^^ mole/liter. 

 The amount of radiation E absorbed in 300 sec is determined and the chem- 

 ical action U calculated from Xq^. The average values obtained from ex- 

 periments at various wave-lengths are depicted in Table 6, -q being expressed 

 in )ul/cal. Table 7 clearly shows that rj is independent of the radiation in- 

 tensity. 



Numerical example: 



Yellow lio-ht (5780 A) 



E = 0.0574 cal 



xoo = 25.5 yul 



t] = U/E = 25.5/0.0574 = 444 Ml/cal 



According to Einstein's law, one molecule Oo must be taken up for each 

 quantum absorbed. This is in fact so, as may be seen from Table 8. The 

 theoretical value of the quantum yield is 



1 mole O2 



if = 



1 mole quanta 



or 



22400 X 10-3 



^ = 



photochemical equivalent 



TABLE 8 



The validity of Einstein's law 



^il/cal 



The calculated value ^ is identical with the experimentally obtained value of 

 77, so that Einstein's law is valid for a relatively large spectral zone in the 

 photooxidation of ethyl chlorophyllide solutions. 



The concentration of the acceptor exerts a great influence on the yield. 

 As can be seen from Table 9, the law holds good only for high values of C,,. 



The results of similar experiments with hematoporphyrin and allylthiourea 

 in acetone also obeyed Einstein's law. The most important fact arising from 



* The importance of sulfur compounds, especially of allylthiourea, in photochemical processes 

 used in photography was recognized by Sheppard (16). 



