SOME PHOTOCHEMICAL AND PHOTOPHYSICAL 



REACTIONS OF CHLOROPHYLL 



AND ITS RELATIVES^ 



Melvin Calvin 



DepariDienl of Chemistry and Lawrence Radiation Laboratory 

 Unix'ersity of California, Berkeley, California 



Introduction 



This is a discussion ot some of the photochemistry and photophysics 

 of porphyrins which has accumulated in the course of the last fif- 

 teen to twenty years and Avhich has some bearing on the problem 

 in ^vhich ^ve are primarily engaged, namely, the conversion of elec- 

 tromagnetic into chemical energy as it occurs in photosynthetic or- 

 ganisms. The history of the photochemical and similar properties 

 of chlorophyll and its related materials is an old one. From the very 

 beginning of the recognition of chlorophyll as a prime light-absorber 

 and converter in green plants (26) , there has been a steady flow 

 of model experiments with chlorophyll and related materials, in- 

 tended to discover in simplified systems the types of transformations 

 which conceivably might be taking place in the living organism in 

 its operations for the conversion of electromagnetic into chemical 

 energy. 



From very early times, even before the chemical structure of chloro- 

 phyll and its relatives was known (32, 76) , a number of photo- 

 chemical properties of the chlorophyll molecules in solution had 

 been observed. These can be most easily described in two classifica- 

 tions. The first is a photosensitized oxidation reaction with molecular 

 oxygen in which the chlorophyll is the photosensitizer, that is, the 

 chlorophyll absorbs the light and causes, in some way, the oxidation 

 of some other substrate with molecular oxygen; some of the pigment 

 may be destroyed, depending on the conditions of the reaction, but 

 conditions can be found in which the chlorophyll itself is relatively 

 stable and acts as a photosensitizing dyestuff which will cause the 



' The preparation of this paper was sponsored hv the VS. Atomic Energy 

 Commission. 



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