44 



D. SHUGAR 



0.4 



0.8- 



04 



0.4 



0.8 

 QA 



0.8 

 04 



350 400 450 500 550 

 X(mji) 



Fig. 1. 



400 



480 

 Afnyj) 



Fig. 2. 



560 



Fig. 1. Duroquinone radical and triplet spectra: (a), QH- in liquid paraffin; (b), 

 QH- in 50% ethanol; (c), QH- in hexane; (d), Q- in 50% ethanol; (e), triplet state in 

 liquid paraffin [from N. K. Bridge and G. Porter, Proc. Roy. Soc, A244, 276 (1958), 

 which should be consulted for methods of identification of various excited states]. 



Fig. 2. Absorption spectra of 10 -6 M chlorophyll a in cyclohexanol : S, ground 

 state; (c) as soon as possible after flash excitation; (b) 900 /jsec. after flash; (a) 2600 

 jusec. after flash; T , triplet state, obtained by extrapolation to zero time after flash 

 [R. Livingston, J. Am. Chem. Soc. 77, 2179 (1955)]. 



its potential importance in examining the excited states of pyrimidine de- 

 rivatives in connection with the phenomenon of reversible photolysis (Sec- 

 tion VII, 1) and the relationship of this latter to photoreactivation (Section 



X). 



4. Free Radicals in Biological Systems 



Reid 6 suggests that it would be surprising if triplet states were not found 

 to be intermediates in many biological reactions. Actually a number of ob- 

 servers, e.g., Michaelis, 16 have postulated that electron transport in biologi- 

 cal oxidations occurs singly rather than in pairs and consequently proceeds 

 via intermediates containing an unpaired electron, i.e., free radicals. If this 

 were the case, one would expect to find a stationary concentration of such 

 free radical intermediates in actively metabolizing tissues. 



The existence of free radicals has now been demonstrated in a variety of 

 lyophilized tissues by the technique of paramagnetic resonance absorption 



16 L. Michaelis, in "The Enzymes" (J. B. Sumner and K. Myrback, eds.), Vol. II, 

 Part 1. Academic Press, New York, 1951. 



