Paramagnetic species shift agents

The transport of substances between the inside and outside of red cells can be monitored using NMR if the resonances from the two environments have different chemical shifts or intensities. Also, resonances outside the cell can be selectively broadened by the addition of paramagnetic species that do not cross the red cell membrane. Those used include the ferric complex of desferrioxamine, dysprosium-DTPA and the copper-cyclohexanediaminetetraacetic acid complex. To measure the rate of influx of a compound into red cells, the compound is added with the paramagnetic agent to a red cell suspension and the intensity of the resonance from the intracellular component is monitored as a function of time. This approach has been used to study the transport of glycerol, alanine lactate, choline and glycylglycine. The time scale that can be addressed covers the range of ms to hours. 

Rates of nuclear relaxations can be altered by the deliberate addition to the solutions used for NMR spectroscopy of species containing paramagnetic nuclei. The usual relaxation agent employed with substrates dissolved in organic solvents is the chelate chromium tris(acetylacetonate) (Cr(acac)3), [Me C6 CH C(6)Me]3Cr, i.e. chromium(III)-2,4-pentane-dionate. It acts to reduce values of via dipole-dipole interactions, leaving chemical shift values unaltered. Cr(acac)3, when used at concentrations of about 0.05 M, substantially reduces values of T, particularly those for quaternary nuclei simultaneously it removes the nuclear Overhauser enhancement from proton-decoupled and other resonances. Use of this reagent could therefore considerably shorten the times required for quantitative NMR measurements. 

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