Abstract
The nuclear spin-lattice relaxation rate 1/T(1) depends on the correlation time τ(c) of the molecule bearing the nuclear spin, and can therefore probe changes of τ(c) upon binding of a rapidly moving small ligand to a more slowly moving larger protein. In practice however, the dependence is such that only a small difference in relaxation rate is obtained at high field. Here we present a scheme in which nuclear spins are first hyperpolarized using DNP, and then allowed to relax at low magnetic field in presence of a target protein, which generates a large T(1) contrast. The sample is subsequently transferred into a conventional nuclear magnetic resonance probe (NMR), where the effect of the low-field relaxation is read out using high-field liquid-state NMR. Using only 14 μM of a (13)C-labeled reporter ligand, we observe protein binding reliably for protein concentrations as low as 2 μM in a single scan. The scheme is expanded to a label-free ligand via a competitive binding experiment in which the label-free ligand displaces the (13)C-labeled reporter ligand.