title: Nitroxide-doped liposomes containing entrapped oxidant: an approach to the "reduction problem" of nitroxides as MRI contrast agents.
abstract: The purpose of this investigation is to test the feasibility of a nitroxide regeneration system involving liposomes as an approach toward solving the "reduction problem" when nitroxides are used as contrast enhancing agents in MRI applications. It is shown that the inclusion of an entrapped oxidant (K3Fe(CN)6) in the aqueous compartment of nitroxide-doped liposomes causes a 4-5-fold increase in the duration of the nitroxide ESR signal in the presence of the external reductant sodium ascorbate. Confirmation was obtained by monitoring the concentration of the internalized Fe(CN)6(3-) ion versus time by visible spectroscopy at 410 nm. Trans bilayer (flip-flop) motion of the long chain nitroxide ester is the likely mechanism of this nitroxide regeneration system.
mesh_terms: D001205:Ascorbic Acid; D003287:Contrast Media; D003497:Cyclic N-Oxides; D004578:Electron Spin Resonance Spectroscopy; D005295:Ferrocyanides; D008081:Liposomes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D010084:Oxidation-Reduction; D010713:Phosphatidylcholines; D013113:Spin Labels; D013997:Time Factors
chemical_list: D003287:Contrast Media; D003497:Cyclic N-Oxides; D005295:Ferrocyanides; D008081:Liposomes; D010713:Phosphatidylcholines; D013113:Spin Labels; C015527:4-amino-TEMPO; C031835:potassium ferrocyanide; D001205:Ascorbic Acid