ROUBAL; NATURE OF FREE RADICALS 



of trapping and charge migration, the data are 

 consistent with the scheme shown below: 



POLYPEPTIDE CHAINS 



A« 



D •••• C — N — H 



= C— N— H 



D •••• C N-— H 







D .... c — N— H 

 



C— N— H 

 11 I 







0=C— N — H •- A* 



— C — N— H •- A" 



0=C— N— H 



and Fomin, 1965). In the present work, for in- 

 stance, when hydroquinone or various hydro- 

 quinone derivatives with free hydroxy! groups 

 are incorporated into proteins coated with thin 

 films of unsaturated lipid, an enhanced central 

 resonance is obtained which is identical to that 

 obtained for oxidizing tissue alone. For the 

 protein-lipid hydroquinone systems, the author 

 has shown that central resonance consists chiefly 

 of trapped semiquinone radical ions. The g = 2 

 resonance in "lipid-free" proteins may very 

 well indicate prior lipid oxidation but now at 

 a point in time at which lipid signal has decayed. 

 The g = 2 retention could then be explained 

 because of resonance stability or other stabiliz- 

 ing factors for this species. For instance, some 

 commercial preparations of bovine serum albu- 

 min exhibit an EPR signal somewhat displaced 

 from the usual g = 2 signal while others do not. 

 Likewise some of the samples that are E PR- 

 active exhibit fluorescence quite characteristic 

 of malonaldehyde-amino acid interaction (for- 

 mation of iminopropene derivatives) . The var- 

 ious data taken collectively suggest the follow- 

 ing mechanism: 



C — N— H -• 



II I 







0=C — N— H •• 



Scheme 1. — Charge-transfer between donor (D) and 

 a free radical acceptor (A) in biological systems. Al- 

 though the actual pathway is not known with certainty, 

 available data are in accord with the idea that hydrogen 

 bonding of the type shown may play a role in the transfer 

 of charge. 



where D is a cellular electron donor material 

 acting in the role of an antioxidant and A is a 

 peroxy radical acceptor. To completely sub- 

 stantiate that such a mechanism does exist would 

 be very exciting indeed for this would be the 

 first instance in which the role of an antioxidant 

 at the molecular level could be designated as a 

 strong D-*A interaction. 



The data are also in accord with the idea put 

 forth sometime ago by some Russian investi- 

 gators, that the g = 2 signal in tissue is semi- 

 quinone in nature (Chetverikov, Blyumenfel'd, 



Hydroperoxide 



r,l 



Decay 



Nonradicals 



D- — * Slow Decay 

 where d is shown here as hydroquinone 



Resonance 

 'structures 



Scheme 2. — Interaction between radicals (R-, R-') and 

 cellular antioxidants (D). 



375 



