ROUBAL; NATURE OF FREE RADICALS 



mobilized sufficiently, however, so as to be de- 

 tectable for fairly long periods of time. 



CHARACTERISTIC FORM AND APPEARANCE 



OF EPR SIGNALS OBSERVED IN 



DRIED PRODUCTS 



Although a high resolution EPR analysis can 

 usually be performed with dilute solutions of 

 soluble, low molecular weight organic radicals, 

 the same is seldom true for powdered samples, 

 and especially for powdered samples of complex 

 molecules. The requirements for resolution are: 

 magnetically dilute systems (in order to prevent 

 spin-spin interaction), long relaxation times, 

 and a low rf field. In the solid state, relaxation 

 times are shortened because of the more effective 

 coupling between spin states and the surround- 

 ing lattice — cooling the sample (perhaps to the 

 temperature of liquid nitrogen or below) will 

 often increase the relaxation time to acceptable 

 values. Related to relaxation is the line broad- 

 ening arising with molecular dipole interactions. 

 The notable example is the so-called "oxygen 

 effect" — some radicals will be far removed from 

 the magnetic influences of the molecular oxygen 

 di-radical while other free radicals in the sam- 

 ple will be near oxygen molecules. Consequently, 

 free radicals of the sample will experience a 

 variety of magnetic fields, producing a collective 

 band of resonances resulting from the distribu- 

 tion of collective magnetic fields superimposed 

 on the external instrument magnetic field. There- 

 fore, in solid state studies, radicals, because of 

 their random alignment, exhibit anisotropic 

 coupling which broadens the lines and makes 

 interpretation difficult. Spectral features which 

 can be used to characterize the radicals are the 

 measurement of the g-value, line shape, and 

 changes in these parameters upon chemical or 

 physical treatment of samples. 



Figure 1. — EPR spectra for protein essentially free of 

 lipid and for a lipid-treated protein, all exposed to air. 

 A. Freeze-dried and solvent extracted rockfish myofibril- 

 lar protein. B. Freeze-dried human serum albumin. C. 

 Crude bovine serum albumin (BSA) (upper trace). 

 Crude BSA -|- C22:6 fatty acid (2:1 by \vt) oxidized 

 in air at room temperature for 2 hr (middle trace). 

 Same material in air at room temperature at the end 

 of 4 hr (lower trace). The arrows denote the g = 2 or 

 free-spin value. 



Figure 2. — EPR lipid signals in marine protein con- 

 centrates expo.sed to air. A. Freeze-dried rockfish flesh 

 exposed to air for 20 hr at room temperature. B. Com- 

 mercially available FPC, now 2 years old, low lipid ini- 

 tially, which still exhibits a weak lipid signal. C. Freeze- 

 dried silver salmon light flesh exposed to air for 10 hr 

 at room temperature. Arrows denote g =r 2. 



EPR SPECTRA OF FISHERY PRODUCTS 



Shown in Figures 1, 2, and 3 are EPR signals 

 which are observed in lipid-]3rotein models and 

 in dry ice-frozen, freeze-dried fish tissue, all of 



which are under investigation in this laboratory. 

 As with carefully freeze-dried samples of the 

 type discussed above (liquid nitrogen frozen and 

 freeze-dried), the resonances are devoid of 

 hyperfine structure of the type normally 



373 



