ELECTRON SPIN RESONANCE STUDIES 

 OF PHOTOSYNTHETIC SYSTEMS^ 



Barry Commoner 



The Henry Shaw School of Botany 



and 



The Adolphus Busch III Laboratory of Molecular Biology 



Washington University 

 St. Louis, Missouri 



Introduction 



There are good theoretical grounds for expecting free radicals 

 to occur as intermediates in photobiological processes, and reference 

 to almost any recent discussion of the mechanism of photosynthesis 

 reveals the considerable frequency with which hypothetical free radi- 

 cal intermediates are proposed. These expectations result from the 

 necessity of inicoupling paired electrons during one or more phases 

 of the photochemical or oxidation-reduction processes that partici- 

 pate in photosynthesis. We may regard an organic molecule which 

 contains one or more unpaired electrons as a free radical. 



Until about ten years ago investigations of possible free radical 

 intermediates in dilute conditions such as those found in most bio- 

 chemical and biological processes were hampered by the lack of 

 suitably sensitive detection methods. The discovery by Zavoisky in 

 1945 of electron spin resonance (ESR) absorption led to the possi- 

 bility of constructing spectrometers which were at least potentially 

 capable of detecting exceedingly low concentrations of impaired elec- 

 trons, including those associated with free radicals. 



Begiiniing in 1919 a group of investigators in the departments of 

 physics, chemistry, and botany at Washington University developed a 

 concerted effort to apply the new technique to a variety of relevant 

 problems including, among others, an analysis of the mechanism 

 of photosynthesis and other photobiological processes. The present 

 paper is a brief review of the development of this ^\•ork as it re- 



' Tliis work was supported in part by research grant C-'i!>83 from the National 

 Cancer Institute, United States I'ulilic Health Sersice, and by Contract Nonr- 

 816(10) (NR 101-227) between the Office of Naval Research. Department of the 

 Navy, and Washington I'nixcrsily. 



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