Frequency dependence of electron spin relaxation times in aqueous solution for a nitronyl nitroxide radical and perdeuterated-tempone between 250MHz and 34GHz

Abstract

Electron spin relaxation times of perdeuterated tempone (PDT) 1 and of a nitronyl nitroxide 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) 2 in aqueous solution at room temperature were measured by 2-pulse electron spin echo (T2) or 3-pulse inversion recovery (T1) in the frequency range of 250 MHz to 34 GHz. At 9 GHz values of T1 measured by long-pulse saturation recovery were in good agreement with values determined by inversion recovery. Below 9 GHz for 1 and below 1.5 GHz for 2, T1~ T2, as expected in the fast tumbling regime. At higher frequencies T2 was shorter than T1 due to incomplete motional averaging of g and A anisotropy. The frequency dependence of 1/T1 is modeled as the sum of spin rotation, modulation of g and A-anisotropy, and a thermally-activated process that has maximum contribution at about 1.5 GHz. The spin lattice relaxation times for the nitronyl nitroxide were longer than for PDT by a factor of about 2 at 34 GHz, decreasing to about a factor of 1.5 at 250 MHz. The rotational correlation times, τR are calculated to be 9 ps for 1 and about 25 ps for 2. The longer spin lattice relaxation times for 2 than for 1 at 9 and 34 GHz are due predominantly to smaller contributions from spin rotation that arise from slower tumbling. The smaller nitrogen hyperfine couplings for the nitronyl 2 than for 1 decrease the contribution to relaxation due to modulation of A anisotropy. However, at lower frequencies the slower tumbling of 2 results in a larger value of ωτR (ω is the resonance frequency) and larger values of the spectral density function, which enhances the contribution from modulation of anisotropic interactions for 2 to a greater extent than for 1.

Multifrequency Pulsed EPR and the Characterization of Molecular Dynamics

“…Studies of the frequency dependence of spin-lattice relaxation for carbon-centered trityl radicals (Owenius et al, 2005), nitrogen-centered nitroxide radicals (Biller et al, 2013; Biller et al, 2012; Hyde et al, 2004), and semiquinones (Elajaili et al, 2014) in fluid solution have demonstrated the significance of additional relaxation mechanisms. These frequency-dependent processes modulate anisotropic interactions – g anisotropy (Eq.…”

Electron spin–lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

“…These studies have now been extended to a wider range of nitroxides and solvents to investigate i) the relative importance of spin rotation and modulation of nitrogen hyperfine anisotropy when τ R = 4 to 50 ps at ambient temperature, ii) whether additional mechanisms contribute to relaxation at frequencies lower than ∼9.5 GHz, and iii) the thermally-activated process that is proposed to have maximum impact on T 1 at 1 to 2 GHz, and has been observed previously in both the fast [21] and slow tumbling regimes [22]. T 1 and T 2 were measured by pulsed methods for the nitroxides shown in Figure 1, at frequencies between 250 MHz and 34 GHz.…”

“…Recently we reported T 1 and T 2 for aqueous solutions of a nitronyl nitroxide and of PDT at five frequencies between 250 MHz and 34 GHz [21] in the fast motional regime where the tumbling correlation time, τ R , is less than about 25 ps. These studies have now been extended to a wider range of nitroxides and solvents to investigate i) the relative importance of spin rotation and modulation of nitrogen hyperfine anisotropy when τ R = 4 to 50 ps at ambient temperature, ii) whether additional mechanisms contribute to relaxation at frequencies lower than ∼9.5 GHz, and iii) the thermally-activated process that is proposed to have maximum impact on T 1 at 1 to 2 GHz, and has been observed previously in both the fast [21] and slow tumbling regimes [22].…”

Rapid Scan Electron Paramagnetic Resonance Opens New Avenues for Imaging Physiologically Important Parameters <em>In Vivo</em>

“…The nitronyl nitoxide 32 was used as another example of wide spectrum imaging, but has been used in the past for spin-trapping of NO •33,34 . Imaging of a phantom containing nitronyl is shown in Figure 5 (C, D).…”