liAlilU COMMONER 357 



hues to ])lioi()syniIicsis, aiul it suniniarizes the present status of our 

 knowledge al)()ut the participation ol Tree racHcals and similar rom- 

 j:)onents in photosynthesis. 



The ESR phenomenon depends on an inherent property oi the 

 electron — the magnetic moment arising from its spin, and ESR data 

 are capable of describing, at least directly, only this property of the 

 electron. The basic effect arises when an impaired electron is placed 

 in an external magnetic field. The electron's magnetic moment may 

 assume one of two orientations to the external field, one of whicli 

 represents a higher energy state than the other. The separation be- 

 tween these energy levels is a function of the strength of the external 

 field [H) , so that one may impose on the system an energy gap 

 of any chosen value by selecting the appropriate external magnetic 

 field. The energy levels imposed on the system by the external mag- 

 netic field may residt in a capacity for the resonance absorption of 

 incident radiation of a particular frequency (v) , according to the 

 relationship: 



vh = g^H, 



where h is Planck's constant, ^ is a constant (the Bohr magneton) 

 and the value of g describes the interaction between the inherent 

 magnetic moment due to the electron's spin and the external magnetic 

 field. For the free electron the value of g is 2.0023, but the g value 

 exhibited by a localized unpaired electron may depart from this 

 figure as a result of the interaction between the magnetic moment 

 due to the spin and that due to the electron's orbital angular momen- 

 tum. The magnetic moment of an unpaired electron also interacts 

 with the magnetic moments of atomic nuclei with which it is as- 

 sociated. As a result, the single energy gap characteristic of an 

 isolated electron becomes split, so that a series of evenly spaced 

 absorptions occur. The most important nuclei normally encountered 

 in biological material that possess magnetic moments and are there- 

 fore capable of inducing sucli hyperfine structure in the ESR signal 

 are H^ and N^*. C'^ h^g a magnetic moment, but C^- does not. The 

 magnetic moments of H^ and N^-^ are different from those of the 

 normal isotopes. The hyperfine structure is often distinctively re- 

 lated to the molecular structure of the component and may be em- 

 ployed to describe the locus of the electron in the molecule. In this 

 situation isotope substitution experiments can be carried out which 

 yield detailed information regarding the location of the unpaired 

 electron in the molecule. 



