Electron Spin Relaxation of Nitroxide Spin Labels and Relaxation Processes

MTSL is the nitroxide spin label that is most commonly used in site-directed spin labeling. However, due to rotation of its gem-dimethyl groups that average anisotropic interactions, Tm becomes short above about 70 K and this makes DEER experiments difficult at these temperatures. Strategies for decreasing spin echo dephasing and electron spin lattice relaxation rates are important for design of nitroxide spin labels and molecular qubits. In searching for labels with longer Tm, new nitroxide spin labels devoid of gemdimethyl groups or with more rigid structures were synthesized at the University of Nebraska and pulsed EPR measurements were done at the University of Denver. In nitroxides without gem-dimethyl groups longer Tm values were obtained, and no methyl rotation enhancement was observed. Other strategies for lengthening Tmwere explored, including the addition of organic macromolecules that form inclusion complexes with nitroxide radicals. For example, in the presence of beta-cyclodextrin (β-CD) and cucurbit [7] uril (CB7) longer Tm and narrower distance distributions were obtained by DEER at 80 and 160 K. To gain insight into processes that govern electron spin relaxation, variable temperature measurements of Tm and T1were conducted between about 4.5 and 260 K for nitroxide radicals, 4.5 to 160 K for V(IV) vanadate complexes, and 4.2 to 60 K for manganate ion. 1/T1 was modeled with a sum of direct, Raman, and local mode processes for nitroxide radicals, V(IV) vanadate complexes, and manganate ions in rigid lattice environments. Tm was measured by two pulse spin echo and data were fit with a single or stretched exponential. Nuclear spin diffusion dominates 1/Tm between 4 and 60 K in nitroxide radicals. Methyl rotations and motional modulation of anisotropy drive 1/Tm at higher temperatures. In V(IV) complexes, methyl groups on the R3NH+ counterions dominate Tm when they are close to the V(IV) center as observed in (Et3NH)2[V(C6H4O2)3] and (n- Bu3NH)2[V(C6H4O2)3]. Beyond 10 Å, their impact becomes minimal and 1H diffusion dominates relaxation at low temperatures as seen in (n-Hex3NH)2[V(C6H4O2)3] and (n- Oct3NH)2[V(C6H4O2)3]. In MnO4 2- in glassy alkaline LiCl, Tm is dominated by 1H nuclear spin diffusion at low temperatures and driven by T1 as the temperature increases. Methods for fitting T1 inversion recovery curves were compared in nitroxide radicals, V(IV) vanadate complexes, [4Fe-4S]+ clusters, and irradiated boron oxide glasses. In the absence of very wide T1 distributions, T1 can be fit with either a sum of two exponentials or a stretched exponential. The presence of strains in [4Fe-4S]+ or other species such as defects produced from irradiation in B2O3 glasses leads to very wide distributions in T1. Unless there exists a pre-existing physical model, a model-free distribution termed Uniform-Penalty Inversion of Multiexponential Decay (UPEN) is preferred. A stretched exponential decay is appropriate for fitting Tm when it is dominated by nuclear spin diffusion or a dynamic process such as methyl rotations. Motional modulation of anisotropy drives Tm at high temperatures and Tm can be fit with a single exponential decay. The observation that longer Tm values are obtained in nitroxide radicals devoid of methyl groups and vanadate complexes with methyl groups farther away from the V(IV) center may guide future improved molecular design