HARRY COMMONER 361 



The Application of the ESR Technique to Biochemical 

 AND Biological Systems 



Several special problems arise in the application of the ESR tech- 

 nique to systems of interest to the biologist. Because the technique 

 is relatively new, there have as yet been relatively few efforts to 

 develop specific modifications of the basic instrument which are 

 suited to the needs of the biochemist or the biologist, rather than 

 to the needs of the physical chemist or physicist. 



Fortunately, since its inception, the program at Washington Uni- 

 versity has been designed to correlate the interests and needs of in- 

 vestigators in physics, chemistry, and biology. One of our earliest 

 decisions was to undertake the development of a series of ESR spectro- 

 meters designed specifically to accommodate biochemical and bio- 

 logical experiments. These developments, which have been carried 

 out by Professor Townsend, have been absolutely essential to the 

 experimental work which is described below. 



The crucial problem which must be solved in applying ESR spec- 

 troscopy to biochemistry and biology is the matter of sensitivity. It 

 was at first apparent on theoretical groinids, and it is now evident 

 from our own results, that the concentration of free radicals ex- 

 pected in such systems is of the order of lO-'^ to 10-^ M, and this 

 fact alone requires a spectrometer of high intrinsic sensitivity. Un- 

 fortunately, two additional factors, inherently associated with the 

 systems of interest to the biologist, happen to militate against the 

 realization of a spectrometer's full sensitivity. The first of these is 

 the ubiquitous presence of liquid water in functional biochemical 

 and biological systems. The microwaves employed in ESR spectro- 

 meters are heavily absorbed by liquid water, thereby resulting in a 

 significant loss in sensitivity. To some extent this problem can be 

 counteracted by drying the material or by examining it in the frozen 

 condition, since ice does not have a high absorbancy for microwaves. 

 However, under these conditions only static data are possible and one 

 loses a great deal of information which could be obtained from func- 

 tional systems. The first spectrometer developed at Washington Uni- 

 versity in 1950 was not sufficiently sensitive to accommodate wet 

 samples, and we were forced to study frozen-dried materials rather 

 than active systems. A second problem is due to the relatively fast 

 rates of processes of interest to the biologist. This precludes the use 

 of an ESR spectrometer in which a slow scanning speed (which' 

 averages out noise and thereby increases the effective signahnoise 

 ratio) is used to attain an enhanced sensitivity. 



When it became apparent from our earliest studies of frozen-dried 



