Application of ex situ dynamic nuclear polarization in studying small molecules.

Dynamic nuclear polarization (DNP) has become an attractive technique to boost the sensitivity of NMR experiments. In the case of ex situ polarizations two-dimensional (2D) spectra are limited by the short lifetime of the polarization after dissolution and sample transfer to a high field NMR magnet. This limitation can be overcome by various approaches. Here we show how the use of (13)C-labelled acetyl tags can help to obtain 2D-HMQC spectra for many small molecules, owing to a nuclear Overhauser enhancement between (13)C spins originating from the long-lived carbonyl carbon, which extends the lifetimes of other (13)C spins with shorter longitudinal relaxation times. We also show an alternative approach of using an optimized polarization matrix.

Optimizing the polarization matrix for ex situ dynamic nuclear polarization.

Although recent advances in dynamic nuclear polarization techniques have boosted the otherwise low sensitivity of NMR spectroscopy, the efficiency of the hyperpolarization process depends on the composition of the polarization matrix, in particular on the contact between the radical and the target molecule and the capability of the matrix to transfer polarization through spin diffusion. A concept for optimal matrix design is presented, applied to obtain two-dimensional heterocorrelated spectra of small drug-like molecules in 1-2 min after 90 min of hyperpolarization.

Optimizing the signal enhancement in cryogenic ex situ DNP-NMR spectroscopy.

Dynamic nuclear polarization (DNP) is often carried out at low temperatures to utilize the high spin polarization of the radical electrons at these temperatures. We have observed that in the absence of microwave irradiation and radical a transient negative polarization is observed for the methyl signals of acetone and DMSO. We suggest that this polarization arises from rotational tunneling levels in this system. For samples dissolved in these solvents, we see a diminished polarization with a microwave frequency of omegae - omegaN compared to omegae + omegaN, and we relate the two effects due to constructive/destructive interference with proton spin diffusion and spin symmetry diffusion mediating the intermolecular transfer of polarization. Perdeuteration of the solvents acts to nullify this effect. Awareness of this additional factor in the temperature jump DNP experiment allows improved experiment design and polarization of samples which have otherwise proven virtually impossible.