Dynamic nuclear polarization studies of redox-sensitive nitroxyl spin probes in liposomal solution

Overhauser-enhanced magnetic resonance imaging (OMRI) studies of a membrane-permeable nitroxyl spin probe, 2H-enriched 3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (MC-PROXYL), used in simultaneous molecular imaging is reported. Phantom imaging was performed with liposomal solutions of MC-PROXYL at varying spin probe and liposome concentrations using a field-cycle mode, custom-built OMRI scanner. Dynamic nuclear polarization (DNP) spectra of the liposomal solution of the spin probe, measured at 14.529 mT using a 5 mT sweep of the electron paramagnetic resonance (EPR) irradiation field showed splitting of the low and high filed hyperfine lines. Spectral measurements using D2O and a spin broadening agent, K3Fe(CN)6 confirmed that these peaks originated from water molecules in two different environments, compartmentalized with liposomes. The nuclear Overhauser enhancement measured at different EPR irradiation times and power levels showed reduction in water nuclear magnetic resonance (NMR) signal enhancement in liposomal membrane due to the reduction in the coupling constant, ρ. This study illustrates that OMRI can be used to differentiate between the intra- and extra- membrane water by loading the liposome vesicles with a lipid-permeable nitroxyl spin probe.