| Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties such as spin lattice relaxation times, spin packet linewidths, and nuclear hyperfine splitting. Initial measurements of relaxation times for six low molecular weight nitroxides at X-band stimulated further measurement at frequencies between 250 MHz and 34 GHz. The impact of tumbling was studied with perdeuterated 2,2,6,6-tetramethyl-4-piperidinyl-1-oxyl (PDT) in five solvents with viscosities resulting in tumbling correlation times, tauR, between 4 and 50 ps. A set of three 14N/ 15N pairs of nitroxides in water was selected such that tau R varied between 9 and 19 ps. To test the impact of structure on relaxation, three additional nitroxides with tauR between 10 and 26 ps were studied.;In the fast tumbling regime 1/T2 ∼ 1/T1 and relaxation is dominated by spin rotation, modulation of A-anisotropy and a thermally activated process. The contribution to 1/T1 from spin rotation is independent of frequency and decreases as tauR increases. The modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as tauR increases, dominating at low frequencies for tau R∼ 15 ps. The modulation of g anisotropy is significant only at 34 GHz. Inclusion of a thermally activated process was required to account for the observation that for most of the radicals, 1/T1 was smaller at 250 MHz than at 1-2 GHz. The thermally activated process likely arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.;A phantom of three 4 mm tubes containing different 15N, 2H-substituted nitroxides was constructed for use at 250 MHz. Projections for 2D spectral-spatial images were obtained by continuous wave (CW) and rapid scan (RS) EPR using a bimodal cross-loop resonator. Relative to CW projections obtained for the same data acquisition time (5 min), RS projections had significantly improved image quality. All experiments were facilitated by advancements in resonator design and testing, which are also described. |
