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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 gem-dimethyl 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 Tm were 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 T1 were 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 MnO42- 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.

The boron–oxygen hole center (BOHC) that is formed by irradiation of boron oxides has previously been characterized extensively by continuous wave and pulsed electron paramagnetic resonance. We now report that the electron spin relaxation rates for the BOHC in irradiated high purity B 2 O 3 , practical grade B 2 O 3 , and sodium tetraborate Na 2 B 4 O 7 exhibit substantial sample dependence. Because of the low magnetic moments for the boron nuclei, the spin echo dephasing is dominated by electron–electron interaction ( T 2 ) instead of the nuclear spin diffusion that dominates dephasing for organic radicals in lattices with high proton concentrations. The higher local concentration of defects in a sample of practical grade B 2 O 3 than in a sample of reagent grade B 2 O 3 , shortens T m (spin echo dephasing) and causes extensive cross relaxation contributions to T 1 (spin lattice relaxation) at 10 K. At temperatures below about 60 K T 1 is shorter for the BOHC in B 2 O 3 than in sodium tetraborate or for the radical formed by irradiation of calcium metaborate. T 1 for the BOHC and the radical in irradiated calcium metaborate are shorter than for other irradiated solids including glycylglycine, l -alanine and the E´ center in quartz. The temperature dependence of T 1 for the BOHC in B 2 O 3 is dominated by the Raman process with a lower Debye temperature than for the radical formed by irradiation of calcium metaborate.

We report complex formation between the chloroacetamide 2,6-diazaadamantane nitroxide radical (ClA-DZD) and cucurbit[7]uril (CB-7), for which the association constant in water, Ka = 1.9 × 106 M-1, is at least one order of magnitude higher than the previously studied organic radicals. The radical is highly immobilized by CB-7, as indicated by the increase of the rotational correlation time, τrot, by a factor of 36, relative to that in the buffer solution. The X-ray structure of ClA-DZD@CB-7 shows the encapsulated DZD guest inside the undistorted CB-7 host, with the pendant group protruding outside. Upon addition of CB-7 to T4 Lysozyme (T4L) doubly spin-labeled with the iodoacetamide derivative of DZD, we observe the increase in τrot and electron spin coherence time, Tm, along with the narrowing of inter-spin distance distributions. Sensitivity of the DEER measurements at 83 K increases by a factor 4 – 9, compared to the common spin label such as MTSL, which is not affected by CB-7. Inter-spin distances of 3-nm could be reliably measured in water/glycerol up to temperatures near the glass transition/melting temperature of the matrix at 200 K, thus bringing us closer to the goal of supramolecular recognition-enabled long-distance DEER measurements at near physiological temperatures. The X-ray structure of DZD-T4L 65 at 1.12 Å resolution allows for unambiguous modeling of the DZD label (0.88 occupancy), indicating undisturbed structure and conformation of the protein.

We report complex formation between the chloroacetamide 2,6-diazaadamantane nitroxide radical (ClA-DZD) and cucurbit[7]uril (CB-7), for which the association constant in water, Ka = 1.9 × 106 M-1, is at least one order of magnitude higher than the previously studied organic radicals. The radical is highly immobilized by CB-7, as indicated by the increase of the rotational correlation time, τrot, by a factor of 36, relative to that in the buffer solution. The X-ray structure of ClA-DZD@CB-7 shows the encapsulated DZD guest inside the undistorted CB-7 host, with the pendant group protruding outside. Upon addition of CB-7 to T4 Lysozyme (T4L) doubly spin-labeled with the iodoacetamide derivative of DZD, we observe the increase in τrot and electron spin coherence time, Tm, along with the narrowing of inter-spin distance distributions. Sensitivity of the DEER measurements at 83 K increases by a factor 4 - 9, compared to the common spin label such as MTSL, which is not affected by CB-7. Inter-spin distances of 3-nm could be reliably measured in water/glycerol up to temperatures near the glass transition/melting temperature of the matrix at 200 K, thus bringing us closer to the goal of supramolecular recognition-enabled long-distance DEER measurements at near physiological temperatures. The X-ray structure of DZD-T4L 65 at 1.12 Å resolution allows for unambiguous modeling of the DZD label (0.88 occupancy), indicating undisturbed structure and conformation of the protein.We report complex formation between the chloroacetamide 2,6-diazaadamantane nitroxide radical (ClA-DZD) and cucurbit[7]uril (CB-7), for which the association constant in water, Ka = 1.9 × 106 M-1, is at least one order of magnitude higher than the previously studied organic radicals. The radical is highly immobilized by CB-7, as indicated by the increase of the rotational correlation time, τrot, by a factor of 36, relative to that in the buffer solution. The X-ray structure of ClA-DZD@CB-7 shows the encapsulated DZD guest inside the undistorted CB-7 host, with the pendant group protruding outside. Upon addition of CB-7 to T4 Lysozyme (T4L) doubly spin-labeled with the iodoacetamide derivative of DZD, we observe the increase in τrot and electron spin coherence time, Tm, along with the narrowing of inter-spin distance distributions. Sensitivity of the DEER measurements at 83 K increases by a factor 4 - 9, compared to the common spin label such as MTSL, which is not affected by CB-7. Inter-spin distances of 3-nm could be reliably measured in water/glycerol up to temperatures near the glass transition/melting temperature of the matrix at 200 K, thus bringing us closer to the goal of supramolecular recognition-enabled long-distance DEER measurements at near physiological temperatures. The X-ray structure of DZD-T4L 65 at 1.12 Å resolution allows for unambiguous modeling of the DZD label (0.88 occupancy), indicating undisturbed structure and conformation of the protein.