318 LIGHT AND LIFE 



oxidation of a good many substrates. Recent examples of this type 

 of reaction are the studies of G. Schenck in Germany (57) , in which 

 he has studied the photosensitized oxidation of a whole variety of 

 materials. One of the very early and more quantitative studies was 

 that of Gaffron (29) , in which he used chlorophyll as a photosensiti- 

 zer for the oxidation of allyl thiourea and, in fact, studied it thoroughly 

 enough so that it could be used as an actinometer — that is, as a 

 means of measuring actual light intensity in a beam, particularly in 

 the red. The quantum yield for this reaction, that is, the nimiber of 

 allylthiourea molecules oxidized per quantum absorbed by chlorophyll, 

 is approximately one. 



The other type of photochemical reaction in solution which chloro- 

 phyll is known to sensitize is a hydrogen transfer from some reducing 

 agent to some oxidized substance. The classic example is the reduc- 

 tion of an azo dye, such as methyl red, by a reducing agent (hydro- 

 gen donor) such as ascorbic acid, and chlorophyll has long been known 

 to sensitize the transfer of hydrogen from the reducing agent to the 

 acceptor. 



Both these types of cases, for the most part, are photosensitized 

 reactions in which the thermodynamics favors the reaction itself, and 

 the light largely serves the function of overcoming activation energy 

 for the reaction. In general, then, there is not, in any of these 

 reactions, a conversion of electromagnetic into chemical energy. 



Photochemistry of Chlorophyll Models 



Relationships between Chlorophyll, BacteriochlorophyU, 

 and Protochlorophyll 



In 1937, I first became acquainted with the chemistry of porphyrins 

 and recognized the relationship of porphin to chlorin. In order to 

 see the type of reasoning involved, I think it is best to look at the 

 structural formulas of the principal energy-capturing molecules in 

 the photosynthesizing organism. Fig. 1 shows the structural formula 

 of chlorophyll as we now believe it to be, and you will notice that 

 it is a porphyrin with an isocyclic ring and an "extra" pair of 

 hydrogens on one of the pyrrole rings, making the chlorin a di- 

 hydroporphyrin. In the last few years, Linstead and his coworkers 

 have proved that these two hydrogens are trans to each other (50) . 



It is interesting to see the relationship of the chlorophyll mole- 

 cule to two others, one of which is bacteriochlorophyll (the light- 

 capturing pigment in the photosynthetic bacteria) , and that relation- 



