EXAMPLES OF SENSITIZATION BY CHLOROPHYLL 509 



Meyer (1935) found, with the same substrate, the absorption of only a 

 half mole of oxygen, and obtained products with a definite antirachitic 

 activity. 



In table 18.1 are collected the main results of sensitization experi- 

 ments with chlorophyll, ethyl chlorophyllide, and leaf extracts. The table 

 is divided into three parts: (A) photodynamic effects, that is, oxidations 

 in vivo of unknown cellular constituents, recognizable by their physio- 

 logical effects; (B) autoxidations in vitro, reversible as well as irreversible; 

 and (C) oxidation-reductions. Phenomena of the last group are of 

 particular interest to us, since the function of chlorophyll in photo- 

 synthesis is to sensitize an oxidation-reduction. Unfortunately, the 

 investigations of Bohi (1929) and Allison (1930) have all the short- 

 comings of the work emanating from Baur's laboratory: in addition to 

 the "electrochemical language" in which the experiments are described, 

 with "anodic" and "cathodic" "depolarizers" substituted for reductants 

 and oxidants (cf. Chapter 4), we miss an exact photochemical technique. 

 For instance, in experiments with two-dye systems (chlorophyll as 

 sensitizer and another dye as oxidant), no attempt was made to use light 

 absorbed by one component only, although the behavior of methylene 

 blue, which is reduced by phenylhydrazine in light even in the absence 

 of chlorophyll, clearly shows that the direct photochemical reactions of 

 the "acceptor" dyes cannot be neglected. 



As a further illustration of a combined photochemical action of both components 

 of a two-dye system, we may mention that Hoist (1934, 1936, 1937) found that the 

 oxidation-reduction equilibrium: 



(18.31) methylene blue + phenylhydrazine sulfonate ^ 



leuco methylene blue + phenyl diazo sulfonate 



is shifted in one direction by light absorbed by methylene blue, and in the opposite 

 direction by hght absorbed by phenyl diazo sulfonate. 



The only quantitative investigations among all those listed in table 

 18.1 were those by Gaffron (1927, 1933) and by Ghosh and Sen-Gupta 

 (1934). Gaffron studied the autoxidation of allyl thiourea in acetone, 

 with ethyl chorophyllide as sensitizer, and found that the quantum 

 yield, y, reaches unity (in red, yellow, green, and blue light) if the sub- 

 strate concentration, [A], is at least 0.01 mole per Hter. The yield 

 drops at the lower acceptor concentrations and at the higher concentra- 

 tions of the sensitizer (Table 18.11). The quantum yield can be repre- 

 sented by the following empirical equation: 



ns'^2^ Q-004 [A] 



^^^■'^^f ^ 0.004 [A] + 0.023 [Chi] + 1 



Its theoretical impHcations will be discussed later (pages 518 et seq., and 

 546). 



