Heteronuclear Cross-Relaxation Effects in the NMR Spectroscopy of Hyperpolarized Targets

Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as N-15 and C-13. This report explores certain features arising in solution-state H-1 NMR upon polarizing low- nuclear species. Following solid-state hyperpolarization of both C-13 and H-1, solution-phase H-1 NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring C-13 nuclei appeared larger than the typically dominant C-12-bonded H-1 resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from (CH)-C-13-H-1 polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the C-13-bound H-1 NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity H-1 NMR traces was explored.