358 LIGHT AND LIFE 



In most ESR spectrometers, the sample is placed within a cavity 

 designed to establish a standing wave of microwave radiation. The 

 cavity itself lies between the poles of a variable magnet. To detect 

 an ESR signal, the absorption of microwave energy is monitored as 

 the strength of the external magnetic field {H) is gradually varied. 

 At a value of H which is given by the above relationship and by the 

 particular value of g characteristic of the unpaired electrons present 

 in the sample, an absorption of microwave energy will be noted. Since 

 most spectrometers impose a modulation of variable amplitude of 

 the external field, the ESR signal is usually received as the derivative 

 of the absorption curve, i.e., as the rate of change of the microAvave 

 absorption, with respect to field strength. By integrating such a 

 signal, an actual absorption curve can be obtained. The relationship 

 between these two types of presentation of the ESR signal is shown 

 in Fig. 2. The microwave frequency employed in most present ESR 

 spectrometers is 9000 megacycles/sec; when g is about 2, an unpaired 

 electron induces a resonance absorption at an external magnetic 

 field of about 3300 gauss. 



Various types of components that are encountered in biological 

 systems may possess unpaired electrons capable of yielding an ESR 

 signal. These are summarized briefly below. 



a. Paramagnetic Ions 



A number of metals of the transition series contain unpaired 

 electrons in their normal ground state and these will give rise to 

 an ESR signal. Typically these signals are complex, and the hyper- 

 fine structure exhibits spacing which is characteristic and is usually 

 very much wider than the hyperfine splitting in organic free radi- 

 cals. Manganous ion is an interesting example Avhich is observed 

 fairly often in biological material. This gives rise to an ESR signal 

 consisting of six hyperfine peaks each separated by 96 gauss. Con- 

 centrations of manganous ion as low as 10-^ M are easily detectable 

 in a suitable spectrometer. We have detected ESR signals due to 

 manganous ions in living bacteria, algae, and the green gland of the 

 crayfish, which contains an unaccountably high manganous ion 

 concentration. 



/;. Free Radicals 



In the context of this paper wc shall regard a free radical as a 

 covalent-bonded molecule which contains an unpaired electron. A 

 biradical is a molecule which contains two unpaired electrons. One 



