242 Walter Gordy 



work, found resonances in certain amino acids. Their results did not agree 

 with ours, except with those for glycine. 



Our group has now obtained electron spin resonances of scores of biological 

 substances which have been subjected to ionizing radiation. These include 

 amino acids (4), peptides (5), fatty acids (6), nucleic acids (7), various proteins 

 (4, 8), enzymes (8), homiones (9), and vitamins (9). Some of these results we 

 think we understand, at least partially; others we do not pretend to understand. 

 This does not discourage us, however. Some twenty to thirty years were required 

 for obtaining reasonably definitive interpretations of x-ray diffraction patterns 

 of a few of the simpler proteins. Nevertheless, it must have been apparent 

 from the first that these patterns contained a wealth of information which 

 would eventually be decoded by the persistent scientist. In electron spin 

 resonance we now have a direct method for studying radiation damage which is 

 comparable to the x-ray diffraction method for the study of structures. It is, 

 in fact, a specific for such studies, for it 'sees' not the normal biological matter 

 but the radicals, or broken pieces of molecules torn apart by ionizing radiations. 



Descriptions of microwave spectrometers for observation of electron 

 magnetic resonances are available (10, 11). Such spectrometers can now be 

 obtained commercially. Descriptions of theoretical methods and applications 

 to chemical and biochemical problems are given in recent publications (11, 12, 

 13, 14, 15, 16). 



In the observation of electron magnetic resonance the sample to be investi- 

 gated is placed in a microwave cavity at a point where the magnetic component 

 of the microwave radiation is strongest. The cavity containing the sample is 

 so placed in a d.c. magnetic field that the lines of the d.c. field lie perpendicular 

 to the magnetic component of the microwave radiation. When the d.c. field is 

 adjusted to the proper strength for resonance, microwave radiation will be 

 absorbed. The value of the field for resonance is: 



Numerically, 



H (gauss) = 0.7 HSi' (Mc/sec)/^ (2) 



where g is the spectroscopic splitting factor for the paramagnetic species. 

 It is found that for practically all organic free radicals, including those produced 

 in solids by ionizing radiation, the g value is very close, within a fraction of 

 a per cent, to the g factor for the free electron spin, 2.0023. This comes about 

 because possible orbital moments are largely averaged out by the motion of the 

 unpaired electron, or by the spreading out over a number of atoms (delocali- 

 zation) of its molecular orbital. The persistent observation of a ^ factor near 

 that of the free electron spin has led to the designation of this resonance as 

 electron spin resonance. 



In the vector model, the electron spin vector would precess about the direction 

 of the applied field H. Quantum mechanically there are only two stable orien- 

 tations for this precessing vector, which represents an average or the 'expectation 

 value' for the electron spin momentum. These correspond to the two observable 

 components, +| and — |, of the electron spin vector along a fixed direction 

 in space. Because of the interaction of the magnetic moment of the spinning 



