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Heteronuclear Cross-Relaxation under Solid-State Dynamic Nuclear Polarization.

We report on the spontaneous polarization transfer from dynamically hyperpolarized 1H to 13C during magic-angle spinning dynamic nuclear polarization (DNP) at temperatures around 100 K. The transfer is mediated by 1H-13C cross-relaxation within methyl groups due to reorientation dynamics, and results in an inverted 13C NMR signal of enhanced amplitude. Further spreading of transferred polarization can then occur via 13C-13C spin-diffusion. The resulting process is equal to the nuclear Overhauser effect (NOE) where typically continuous saturation of 1H by radio frequency irradiation is employed. Here, hyperpolarization by irradiation with microwaves in the presence of typical bis-nitroxide polarizing agents is utilized for steady-state displacement of 1H polarization from thermal equilibrium and perpetual spin-lattice relaxation. An effective 13C enhancement factor of up to -15 has been measured. Presence of Gd(III) furthermore amplifies the effect likely by accelerated relaxation of 1H. We provide experimental evidence for the proposed mechanism and show that DNP-induced cross-relaxation is a robust feature within proteins and single amino acids and discuss potential applications.