The Photochemistry of Chlorophyll 

 in Vitro 



ROBERT LIMNGSTON, Institute of Technology, University of Min- 

 nesota, Minneapolis, Minnesota 



It is the primary act, the capture of light energy for chemical pur- 

 poses, which makes photosynthesis the most important and the most 

 fascinating of biological processes. Even if every enzyme, with its 

 attendant substrate, product, and coenzyme, which enters into the 

 biochemical cycles of the living plant were known, we might still be 

 completely ignorant of the primary process which distinguishes photo- 

 synthesis from other biological processes. There is no known straight- 

 forward way by which the nature of the primary act can be deter- 

 mined. In fact, there are very few photochemical reactions of complex 

 molecules in solution whose primary act has been unequivocally de- 

 termined. In view of the difficulty of the problem, it appears worth 

 while to investigate many different photochemical and spectroscopic 

 properties of chlorophyll in solution, in synthetic and natural aggre- 

 gates of chlorophyll molecules, and in the intact cell. 



The study of dilute solutions of chlorophyll is certainly insufficient 

 to establish the nature of the primary act of photosynthesis. How- 

 ever, knowledge gained from such study should enable the student 

 of photosynthesis to reject many otherwise plausible interpretations of 

 the primary act. This discussion is limited to recent developments 

 of the photochemistry of chlorophyll solutions. Irreversible photo- 

 chemical reactions of chlorophyll and reactions photosensitized by 

 chlorophyll have been excluded. Although the irreversible reactions 

 are of intrinsic interest to organic chemistry, they have no direct re- 

 lation to normal photosynthesis. An adequate discussion of photo- 

 sensitized reactions would require more space than is warranted by 

 their importance. Furthermore, such reactions, occurring in dilute 

 homogeneous solutions, cannot serve as reasonable models of photo- 

 synthesis. 



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