Dynamic Nuclear Polarization: New Methodology and Applications - NMR of Proteins and Small Biomolecules - page 228

Advances with para-hydrogen induced polarization (PHIP) also open up new fields
of applications for portable low-field DNP NMR. Gloggler et al . [ 53 ] report the
possibility of tracing drugs down to the micromolar regime. Selectively polarized
nicotine quantities similar to those found in one cigarette and morphine extracted
from an opium solution were detectable after polarization with para-hydrogen in
single-scan 1 H NMR experiments. Moreover, they demonstrated the possibility to
enhance selectively and detect the 1 H-signal of drug molecules with PHIP in proton
rich standard solutions that would otherwise mask the 1 H NMR signal of the drug.
NMR spectroscopy is a well known and versatile technique for the study of
molecular interactions, even when these interactions are relatively weak. Signal
enhancement by several orders of magnitude through DNP NMR alleviates several
practical limitations of NMR-based interaction studies (Fig. 8 )[ 54 ]. 13 C DNP NMR
can be applied for ligand binding studies at natural isotopic abundance of 13 C[ 54 ].
Resultant screens are easy to interpret and can be performed at 13 C concentrations
below micromolar concentration. Of importance, the ligand-detected molecular
interaction can be assessed and quantified with enzymatic assays that employ
hyperpolarized substrates at varying enzyme inhibitor concentrations. Hence, the
physical labeling of nuclear spins by hyperpolarization has offered fast novel
in vitro experiments to be performed with low material requirement and without
the tedious need for synthetic modifications of target or ligand.
This increase in sensitivity by ex situ DNP-NMR has triggered new research
avenues, particularly concerning the in vivo monitoring of metabolism and disease
by NMR spectroscopy. So far such gains have mainly materialized for experiments
that focus on nonprotonated, low-gamma nuclei, targets favored by their relatively
long T 1 relaxation times, which enable them to withstand the transfer from the
Fig. 8 Comparison of 13 C NMR spectra of a ligand mixture of salicylate and ascorbate recorded
on a state-of the art NMR spectrometer (18.7 T) with cryogenic probe for signal detection (a) and
recorded with DNP-NMR on a 9.4 T spectrometer (b). Ligands were tenfold concentrated for
conventional NMR (a) to allow signal detection after 7,000 scans (20 h). Reproduced with
permission from [ 54 ]