13C nuclear overhauser polarization-magic-angle spinning nuclear magnetic resonance spectroscopy in uniformly 13C-labeled solid proteins.

A recently introduced (13)C polarization technique based on the nuclear Overhauser effect in rotating solid (nuclear Overhauser polarization-magic-angle spinning, NOP-MAS) (Takegoshi, K.; Terao, T. J. Chem. Phys. 2002, 117, 1700-1707) is applied to uniformly (13)C, (15)N-labeled proteins. NOP enhancement factors per scan of 1.5 approximately 2.0 are obtained, while that by cross polarization (CP) is less than 1.0. We show that uniform enhancement of all (13)C signals by CP is difficult to attain, while it is easily achieved by NOP, thus enabling quantitative comparison of signal intensities. NOP is easy to carry out under fast MAS and works well even for somewhat mobile molecules, for which CP does not work. Moreover, in labeled protein samples containing nonlabeled additives, NOP can eliminate the latter signals. For these features, NOP is superior to CP in many uniformly (13)C labeled proteins.