BARRY COMMONER 359 



type of biraclical is represented by the triplet state. The g value of 

 an aromatic Tree radical may vary between 2.002 and 2.007 (14) . 

 A particular free radical is characterized, but not uniquely, by a 

 given ^ value, so that in practice it is often difficult to distinguish 

 between ESR signals due to different free radicals solely by this means. 

 Tlie width and shape of the ESR signal obtained from organic free 

 radicals is affected by a relatively large number of factors and varies 

 markedly from one free radical to the next. It is also affected by 

 the structural organization of the substance (12) . As already indi- 

 cated, an organic free radical may yield an ESR signal which is 

 characterized by a precise pattern of hyperfine splitting if the un- 

 paired electron is localized near nuclei which themselves possess 

 magnetic moments. From quantitative determinations of the g value, 

 line width, and the spacing and intensities of hyperfine peaks ex- 

 hibited by an ESR signal, one can extract considerable information 

 about the molecular structure and kinetic behavior of a free radical. 

 The methods and principles employed in such an analysis of ESR 

 signals are described in a recent summary by Weissman (13) . 



Despite the considerable interest in the possibility of ESR studies 

 of triplets, which is in part engendered by their supposed importance 

 in biological systems, this has proved to be one of the most difficult 

 matters in ESR technique. The difficulties arise from the complex 

 interactions which are possible between two unpaired electrons that 

 are present in the same molecule. In general, if the two electrons are 

 unpaired and separated by only 4 A or less, their interaction is 

 sufficiently strong to cause a broadening of the ESR signal so extreme 

 as to result in undetectability. This effect may be overcome if the 

 free radical is in the form of a single crystal and is observed in a 

 specific orientation. Hutchinson has observed a light-induced triplet 

 in these special conditions (8) . Triplet molecules that are randomly 

 distributed in space do not exhibit a detectable ESR signal at g r= 2. 

 There is some very recent evidence (11) that in random glassy solu- 

 tions triplet molecvdes may give rise to a characteristic signal at 

 g := 4 (i.e., a signal which occurs at a magnetic field strength 

 which is 1/2 of that required to induce a signal at g- == 2) . In 

 general, there is no expectation of detecting an ESR signal due 

 to triplet molecules in biological systems at the ordinary g value 

 of 2, unless they occur in a macroscopically large anisotropic struc- 

 ture — a very unlikely circumstance. No ESR signal that can be attri- 

 buted to a triplet state has been observed, as yet, in any biochemical 

 or biological system. 



