Proton NMR relaxation studies on several nitroxyl-water systems

7Li relaxation time measurements at very low magnetic field by 1H dynamic nuclear polarization

Dynamic nuclear polarization (DNP) of water protons was used to measure the relaxation time of lithium at very low magnetic field as a demonstration of the use of DNP for nuclei less abundant than water protons. Lithium (Li+) was chosen because it is an efficient treatment for manic-depressive illness, with an unknown action mechanism. After having recalled the theoretical basis of a three-spin system comprising two nuclei-the water proton of the solvent, the dissolved Li+ ion and the free electron of a free radical-we have developed a transient solution in order to optimize potential biological applications of Li DNP. The three-spin model has allowed computation of all the parameters of the system-the longitudinal relaxation rate per unit of free radical concentration, the dipolar and scalar part of the coupling between the nuclei and the electron. and the maximum signal enhancement achievable for both proton and lithium spins. All these measurements have been obtained solely through the detection of the proton resonance.

Effects of nitroxides on the magnetic field and temperature dependence of 1/T1 of solvent water protons

We report a study of the longitudinal NMRD profiles (proton longitudinal relaxation rates as a function of field strength) over a broad range of magnetic field (0.01 to 50 MHz proton Larmor frequency) and temperature (-9.6 to 37 degrees C) for aqueous solutions of (i) a fatty acid-nitroxide/albumin complex and (ii) 10 low molecular weight nitroxides. Analysis of the NMRD profile for the fatty acid-nitroxide/albumin complex provides a lower bound estimate for the rotational correlation time of the complex, which permits the calculation of an upper bound on the inner sphere contribution to relaxation of the uncomplexed nitroxides. Inner sphere processes, ostensibly due to water molecules hydrogen bonded to the nitroxide moiety, dominate the relaxation effects of the slowly rotating macromolecular nitroxide/albumin complex. By extrapolation, the contribution of these inner sphere processes are negligible for rapidly tumbling nitroxides free in solution, which affect solvent proton relaxation almost entirely through outer sphere processes (i.e., translational diffusion). A comparison of the relaxation data for aqueous solutions of the uncomplexed nitroxides with the theory of outer sphere relaxation of J.H. Freed (J. Chem. Phys. 68, 4034 (1978] yields values for the distance of closest approach of the water and nitroxide molecules, as well as for their relative diffusion constants, at five different temperatures. Our results indicate that the rather modest relaxivities of aqueous solutions of nitroxides increase substantially with increased solvent viscosity and with protein binding, supporting the potential utility of nitroxides for enhancement of contrast in nuclear magnetic resonance images.

Effects of the operating magnetic field on potential NMR contrast agents

Paramagnetic metal ions have shown promise as contrast agents for nuclear magnetic resonance (NMR) imaging. Their ability depends upon modification of the relaxation times (T1 and T2) through dipolar interactions. These interactions cause the effectiveness of the agents to be sensitive to the operating magnetic field. Studies are presented of the operating field dependence (frequency dispersion) of two metal-chelate complexes, Gd+3-ethylenediaminetetraacetate (EDTA) and Mn+2-EDTA, in a physiologically balanced electrolyte solution. Inversion recovery experiments were performed on two concentrations of each metal-chelate complex at five resonant frequencies. The frequency dispersion curves were similar in appearance for those of the corresponding aqueous solutions. The Mn+2 complex showed no unusual concentration effects. The Gd+3 complex showed an unexpected concentration dependence in the dispersion behavior. This is attributed to a difference in the dipolar correlation time between the two solutions. With its unique correlation time in electrolyte solutions, predictions of relaxation rate changes in studies in vivo may be easier for the Mn+2-EDTA complex.