162 LIGHT AND LIFE 



(3 X 10~- M) . The solution turned green and a free-radical signal 

 (g = 2.006, signal width = 14 gauss) appeared after the solution 

 stood at room temperature for 20 minutes. On the other hand, di- 

 hydrothioctyl dehydrogenase (approximately 10—^ M) showed no 

 signal alone or with 6 X 10-^ M DPNH. The concentrated enzyme 

 turned green on reckiction because of the combined effect of the ab- 

 sorption in the region above 500 nifx and the residual flavin color. 

 Since there is no evidence for unpairing of electrons, the 535 m^ 

 peak in the reduced flavoprotein may be due to a charge transfer 

 complex. If free radicals do exist (as represented tentatively in In- 

 termediate II) , there is sufficiently strong interaction between them 

 that the ESR spectrum does not detect it. These results may be the 

 first demonstration of a charge transfer complex in a biological 

 system, although further work is necessary to establish the nature of 

 the T (SH) 2— FMN complex. 



This work was supported in part by a grant-in-aid from the Ameri- 

 can Heart Association, Inc. The able assistance of Miss Patricia Knell 

 in the preparation of the enzyme is acknowledged with appreciation. 



REFERENCES 



1. Beineit, H., J. Amer. Chern. Soc, 78, 5323 (1956). 



2. Beinert, H., /. Biol. Chem., 225, 465 (1957). 



3. Ehrenberg, A., and Ludwig, G. D., Science, 127, 1177 (1958). 



4. Isenberg, I., and Szent-Gyorgyi, A., Proc. Nat. Acad. Sci. U. .S'., 44, 857 (1958). 



5. Isenberg, I., and Szent-Gyorgyi, A., Proc. Nat. Acad. Sci. V. S., 45, 1229 (1959). 



6. Massey, v., Biochiin. et Biophys. Acta, 37, 314 (1960). 



7. Massey, V., and Gibson, Q. H., Federation Proc, 19, 27 (1960). 



8. Savage, N., Biochem. ./., 67, 146 (1957). 



9. Searis, R. L., Federation Proc, 19, 36 (1960). 



10. Searis, R. L., and Sanadi, D. R., Proc. Nat. Acad. Sci. U. S., 45, 697 (1959). 



11. Searis, R. L., and .Sanadi, D. R., Biochem. Biophys. Res. Comm., 2, 189 (1960). 



12. Searis, R. L., and Sanadi, D. R., ./. Biol. Chem., 235, PC32 (1960). 



13. .Searis, R. L.. and Sanadi, D. R., ./. Biol. Chem., 235, 2485 (1960). 



14. Straub, F., Biochem. J., 33, 787 (19.39). 



15. Veeger, G., and Massey, V., Biorhiiii. et Biophys. Acta, .37, IHl (1960). 



