237. Electron-Nuclear Double Resonance Study of Flavodoxin from

Summary. The proton electron-nuclear double rcsonancc (ENDOR) of the semiquinono of Peptostreptococcus elsdenii flavodoxin has been studied a t- 155". Studies with flavodoxin deriva-tives in which the natural coenzyme has been replaced by C(8)-CD3--, C(9)-D-, C(6,9)-D2-, and iso-FMN2) have made it possible to unequivocally assign the ENDOR lincs to C(8)-CH, and C(6)-H. Introduction.- Flavoprotein radicals exhibit poorly resolved electron spin resonance (ESR.) spectra due to anisotropic broadening [l]. Recently it has been shown that electron-nuclear double resonance (ENDOR) spectra of frozen solutions of free and protein-bound flavin radicals yield valuable information about the sub-molecular structure of the isoalloxazine moiety r2-41. Data obtained with the anioni