DYNAMIC NUCLEAR-POLARIZATION WITH NITROXIDES DISSOLVED IN BIOLOGICAL-FLUIDS

The most widely used free radicals for dynamic nuclear polarization (DNP) experiments or related Overhauser imaging are nitroxides. The DNP parameters in biological fluids were measured in order to provide guidelines for the design of new nitroxides, adapted to the biological applications of DNP. Eighteen nitroxides were studied at a concentration of 1 mM. Extrapolation at complete electron paramagnetic resonance saturation and proton longitudinal-relaxation-time measurements enable calculation of the coupling factor between nitroxide free electrons and water protons. In deoxygenated phosphate-buffered solutions, the NMR signal enhancement by DNP ranged from -36.3 to -6.7, and the coupling factor ranged from 0.31 to 0.03, Nitroxides with a long side chain yield poor enhancement, although their relaxivity is far greater than that of nitroxides with small chains. In a I mM albumin solution, the loss in enhancement factor is mainly caused by the fact that proton relaxation occurs via interactions, not only with the dissolved free radicals but also with the albumin macromolecules. In serum, the enhancement factor is lower than that in an albumin solution, because of the higher protein concentration in serum. In red-blood-cell suspensions, the enhancement factor was further decreased, Two effects contribute to this decrease: first, the increased viscosity due to the presence of red blood cells, and second, the susceptibility effects of the paramagnetism of deoxyhemoglobin. The high sensitivity to oxygen of DNP in phosphate-buffered solution is also greatly reduced when nitroxides are dissolved in blood, In red-blood-cell solutions there is competition between the removal of O-2, which favors proton relaxation by the free radical, and the increase of deoxyhemoglobin, which provides the water protons with another sink of relaxation, On the basis of this study, the properties of nitroxides for use in biological application must fulfill four criteria; i.e., they must have an EPR linewidth narrower than 100 mG, a relaxivity greater than 2.5 (mM s)(-1), a good stability with a half-life time in biological fluids exceeding 1 hour, and low toxicity, (C) 1995 Academic Press, Inc.