Site specific polarization transfer from a hyperpolarized ligand of dihydrofolate reductase

Abstract

Protein–ligand interaction is often characterized using polarization transfer by the intermolecular nuclear Overhauser effect (NOE). For such NOE experiments, hyperpolarization of nuclear spins presents the opportunity to increase the spin magnetization, which is transferred, by several orders of magnitude. Here, folic acid, a ligand of dihydrofolate reductase (DHFR), was hyperpolarized on ¹H spins using dissolution dynamic nuclear polarization (D-DNP). Mixing hyperpolarized ligand with protein resulted in observable increases in protein ¹H signal predominantly in the methyl group region of the spectra. Using ¹³C single quantum selection in a series of one-dimensional spectra, the carbon chemical shift ranges of the corresponding methyl groups can be elucidated. Signals observed in these hyperpolarized spectra could be confirmed using 3D isotope filtered NOESY spectra, although the hyperpolarized spectra were obtained in single scans. By further correlating the signal intensities observed in the D-DNP experiments with the occurrence of short distances in the crystal structure of the protein–ligand complex, the observed methyl proton signals could be matched to the chemical shifts of six amino acids in the active site of DHFR-folic acid binary complex. These data demonstrate that ¹³C chemical shift selection of protein resonances, combined with the intrinsic selectivity towards magnetization originating from the initially hyperpolarized spins, can be used for site specific characterization of protein–ligand interactions.