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Dynamic nuclear polarization (DNP) is a method for enhancing nuclear magnetic resonance (NMR) signals that has many potential applications in chemistry and medicine. Traditionally, DNP signal enhancement is achieved through the use of exogenous radicals mixed in a solution with the molecules of interest. Here we show that proton DNP signal enhancements can be obtained for solid samples without the use of solvent and exogenous radicals. Radicals are generated primarily on the surface of a solid sample using electrical discharges. These radicals are found suitable for DNP. They are stable under moderate vacuum conditions, yet readily annihilate upon compound dissolution or air exposure. This feature makes them attractive for use in medical applications, where the current variety of radicals used for DNP faces regulatory problems. In addition, this solvent-free method may be found useful for analytical NMR of solid samples which cannot tolerate solvents, such as certain pharmaceutical products. [ABSTRACT FROM AUTHOR]

The chemisorption and breakdown of methionine (Met) adsorbed on Pd/γ-Al 2 O 3 catalysts were investigated by solid-state NMR. 13 C-enriched Met (ca. 0.4 mg) impregnated onto γ-Al 2 O 3 or Pd/γ-Al 2 O 3 gives NMR spectra with characteristic features of binding to γ-Al 2 O 3 , to Pd nanoparticles, and oxidative or reductive breakdown of Met. The SCH 3 groups of Met showed characteristic changes in chemical shift on γ-Al 2 O 3 (13 ppm) vs. Pd (19 ppm), providing strong evidence for preferential binding to Pd, while the NC carbon generates a small resonance at 96 ppm assigned to a distinct nonprotonated species bound to O or Pd. Additionally, NMR shows that the SCH 3 groups of Met are mobile on γ-Al 2 O 3 but immobilized by binding to Pd particles; on small Pd particles (ca. 4 nm), the NCH groups undergo large-amplitude motions. In a reducing environment, Met breaks down by C–S bond cleavage followed by formation of C 2 –C 4 organic acids. The SCH 3 signal shifts to 22 ppm, which is likely the signature of the principal species responsible for strong catalyst inhibition. These experiments demonstrate that solid-state magic-angle spinning NMR of 13 C-enriched Met can be a sensitive probe to investigate catalyst surfaces and characterize catalyst inhibition both before reaction and postmortem. [ABSTRACT FROM AUTHOR]