Assignment of proton endor resonances of nitroxyl spin-labels in frozen solution

Spin-label nitroxyl derivatives of tetramethylpyrroline and tetramethylpyrrolidine in frozen solutions of perdeuterated methanol have been characterized by electron nucleus double resonance (ENDOR spectroscopy). With use of selectively deuterated derivatives of 2,2,5,5-tetramethylpyrroline-1-oxyl-3-carboxamide, proton ENDOR resonance features have been assigned to the vinylic proton in the five membered pyrrolinyl ring and to the methyl groups. The ENDOR resonance features were analyzed on the basis of their dependence on H0. Two pairs of resonance features were assigned to the vinylic proton and were shown to correspond to parallel and perpendicular hyperfine coupling (hfc) components. Six pairs of resonance features were ascribed to the methyl groups. The proton ENDOR spectra of the 3-carboxylic acid spin-label derivatives of tetramethylpyrroline and of tetramethylpyrrolidine compounds exhibited comparable features with nearly identical line splittings. From the observed ENDOR splittings, we have estimated the isotropic hfc component of the vinylic proton in 2,2,5,5-tetramethylpyrroline-1-oxyl-3-carboxamide to be -1.81 +/- 0.04 MHz in frozen methanol. On the basis of the anisotropic dipolar hfc components, the electron-to-vinylic proton distance is estimated as 3.78 +/- 0.01 A, in excellent agreement with that of 3.79 A calculated from X-ray defined coordinates.