C 13 nuclear Overhauser polarization nuclear magnetic resonance in rotating solids: Replacement of cross polarization in uniformly C13 labeled molecules with methyl groups

Abstract

A new C13 polarization technique in solids is presented on the basis of a recently proposed C13–C13 recoupling sequence [13C–1H dipolar-assisted rotational resonance (DARR), K. Takegoshi, S. Nakamura, and T. Terao, Chem. Phys. Lett. 344, 631 (2001)] operative under fast magic angle spinning (MAS), in which a rf field is applied to H1H with a rotary resonance condition but none to C13. The H1 irradiation in DARR saturates H1 signals, leading to the C13 signal enhancement due to the nuclear Overhauser effect for fast rotating methyl groups, if any. If we use a uniformly C13 labeled sample, C13–C13 polarization transfer enhanced by DARR successively distributes the enhanced methyl carbon polarization to the other C13 spins, leading to uniform enhancement for all C13 spins even under very fast MAS. In uniformly C13 labeled rotating samples, the enhancement factor in cross polarization (CP) is about 2.4, while in the present nuclear Overhauser polarization (NOP), it is 3.0 in the fast rotation limit of the methyl groups. While the CP enhancement becomes smaller for molecules with short T1ρ of H1 or C13, NOP would work well for such mobile molecules, and also NOP enables us to acquire a signal with a short repetition time even if H1 T1 is long. Further, NOP has the advantage of quantitativeness, and is very easy to carry out, being insensitive to the adjustment of rf field intensity and requiring only very low rf power. These features are demonstrated for uniformly C13, N15-labeled L-threonine and uniformly C13, N15-labeled glycylisoleucine. NOP-MAS is also applied for a naturally abundant C13 sample.