Selective suppression of lipid resonances by lipid-soluble nitroxides in NMR spectroscopy

The ability of lipid-soluble nitroxides to suppress selectively the peaks of lipid resonances in 31P, 1H, and 13C NMR spectra was investigated in serum as part of studies aimed at using these contrast agents for magnetic resonance imaging and magnetic resonance spectroscopy in vivo. Nitroxides are especially interesting potential contrast agents because they can reversibly be converted in cells to diamagnetic hydroxylamines, with conversion rates that are dependent on the redox potential and the intracellular concentration of oxygen; the characterization of nitroxide-dependent changes in NMR spectra may therefore be a useful means to measure oxygen-dependent redox metabolism in vivo. The fatty acid analogs, doxyl stearates, suppressed the methyl resonance of choline and the methyl and methylene peaks of lipids in the 1H NMR spectra of serum samples. As a consequence, lactate peaks, which were not readily detected became clearly resolved and could be evaluated quantitatively. The 31P resonance of phosphatidylcholine in the 31P NMR spectrum was suppressed by 5-doxyl stearate and 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxy l,iodid e (Cat16). In the 13C NMR spectrum, the resonances of the methyl groups of choline and the lipids also were broadened significantly by addition of 5-doxyl stearate. Differential suppression of lipid resonances can be employed to facilitate quantitation of lactate.