Nitroxides relaxation times

Nitroxide relaxation times include (1) the electron spin lattice or longitudinal relaxation time, Tj (2) the electron spin-spin or transverse relaxation time, T2 (3) the... 

In the second step, the longitudinal relaxation trace in the presence of Dy(iii) has to be divided by the reference trace, measured without Dy(m) label, or in a sample where paramagnetic Dy(m) is substituted by a diamagnetic La(iii) or Lu(iii). One can assume that the distances from nitroxide radicals to Dy(iii) centres and the orientations of Dy(iii) g-tensor eigenframes do not correlate with the distribution of non-perturbed nitroxide relaxation times in the absence of Dy(m). Under this assumption, the trace obtained by the mentioned division contains only the RE-induced contributions to the relaxation of nitroxide radicals. 

Zi(Air, x) and 7)(N2, x) are spin-lattice relaxation times of nitroxides in samples equilibrated with atmospheric air and nitrogen, respectively. Note that W(x) is normalized to the sample equilibrated with the atmospheric air. W(x) is proportional to the product of the local translational diffusion coefficient D(x) and the local concentration C(x) of oxygen at a depth x in the membrane, which is in equilibrium with the atmospheric air ... 

Collision of nitroxides with fast-relaxing radicals, such as oxygen and metal ion complexes, causes spin exchange that effectively shortens the spin-lattice relaxation time T of the nitroxide (Hyde and Subczynski, 1989). This effect can be measured either by continuous wave (CW) power saturation techniques or by saturation recovery methods. Collision frequency is directly proportional to the accessibility of the paramagnetic reagent to the nitroxide radical and is defined as... 

Kusumi et al. [42] published Tj values of several nitroxide membrane probes dissolved in liposomes of dimyristoylphosphatidylcholine as a function of 1/T (see Fig. 3). The dotted lines indicate the pre- and main phase transition temperatures. The relaxation times change at the phase transition temperature, and therefore reflect an aspect of membrane fluidity. It is notable that they change by only about a factor of 3 over a very wide range of conditions. 

Hyde et al. [236] have reviewed the spin-probe-spin-label method, which was first described by Leigh [60] to determine the distance between a spin probe (a paramagnetic metal ion) and a spin label (usually a nitroxide). This theory is also applicable to two interacting metal ions one a fast relaxing probe and the other a slower relaxing label . The experimental observable is usually the decreased intensity of the slower relaxing label, C. If C and the relaxation time, T, of the spin probe are known, then the distance, r, between probes may be obtained from the relationship... 

The effect of nitroxide spin-labeled lipids on 13C spin-lattice relaxation times of lecithin vesicles has been discussed (Levine et al.,... 

Measuring in frozen solution is desired in order to avoid the averaging out of the dipole-dipole interactions and, in particular, the strong decrease in transverse relaxation time T2 that is induced even by moderate spin label d3mamics. Additionally, the proper choice of temperature is important in pulsed EPR to optimize relaxation rates. T = 50 K is ideal for DEER at nitroxides in aqueous solution, so liquid helium cooling is advantageous [85]. 

Secondary stmcture can be obtained by studying the accessibility of the nitroxide label to paramagnetic colliders. The collision rate (more precisely the Heisenberg exchange frequency) with the spin label influences the relaxation time of the latter which can be measured and be used to estimate the local concentration of a paramagnetic quencher near a nitroxide spin label. 

One of the major problems facing distance determination by pulsed EPR on spin labeled membrane proteins is the short relaxation time (generally around 1 ps). Solvent deuteration is routinely used (10-20 vol% deuterated glycerol as cryopro-tectant for membrane-embedded or detergent-solubilized samples, or deuterated buffer) to slow down the nitroxide relaxation, thus extending the range of distance measurement and sensitivity. However, the problems coimected to the too fast relaxation time called for a systematic analysis of all factors modifying the... 

BSisoB + Aiso)//i. The width of the lines in this fast tumbling case is defined by (nT2), where T2 is the transversal relaxation time of the electron spin. For nitroxide radicals and magnetic field values of about 0.3 T this is valid for rotational correlation times faster than 10 ps. 

The self-diffusion coefficients of toluene in polystyrene gels are approximately the same as in solutions of the same volume fraction lymer, according to pulsed field gradient NMR experiments (2fl). Toluene in a 10% cross-linked polystyrene swollen to 0.55 volume fraction polymer has a self-diffusion coefficient about 0.08 times that of bulk liquid toluene. Rates of rotational diffusion (molecular Brownian motion) determined from NMR spin-lattice relaxation times of toluene in 2% cross-linked ((polystytyl)methyl)tri-/t-butylphosphonium ion phase transfer catalysts arc reduced by factors of 3 to 20 compai with bulk liquid toluene (21). Rates of rotational diffusion of a soluble nitroxide in polystyrene gels, determined from ESR linewidths, decrease as the degree of swelling of the polymer decreases (321. 

Consequently, CW ESR lineshapes reflect the spin-spin relaxation time Tj of the paramagnetic species. Since the HF anisotropy of nitroxides is of the order of 100 MHz,... 

To investigate the dynamics of the interface, the usefulness of nitroxide spin probes [82] and the H relaxation times T is considered. 

One technique, Overhauser Dynamic Nuclear Polarization (ODNP), is based on the well-known chemical shift of water in NMR spectra. Ordinarily, the liquid water signal intensity is low however, intensity can be magnified 1000-fold by addition of a nitroxide spin label such as TEMPO. Precession of the unpaired electron in TEMPO at the Larmor frequency results in Nuclear Overhauser-mediated polarization of the protons in water. These get polarized within 15 A of the spin labels and then relax with a relaxation time determined by the local diffusivity, i.e. in bulk water, the diffusivity is high and so relaxation is rapid by contrast, in hydration layers, relaxation takes 10-fold longer than in bulk water. Next, the trick is to covalently tether spin labels to surfaces of interest and measure how relaxation rates in hydration layers change as adhesive proteins approach and locally dehydrate the surfaces. 

Figure 9.3 Mip-mediated surface water displacement from polystyrene (PS). PS beads were labeled with tethered nitroxide spin labels. Upon excitation during electron spin resonance, polarization was transferred to the surrounding water molecules within 15 A. Polarized water molecules present relaxation correlation times (t in Table inset) that reflect the local environment. Longer times denote more confinement and lower diffusivily. Only one adhesive protein, mfp-3s, is capable of increasing the surface water relaxation time, presumably by adsorbing directly onto the PS surface. Mfp-3s is notable for having a high hydrophobicity (inset table), as indicated by the negative hydropathy value, which denotes a normalized per average free energy of transfer from water to the non-polar solvent Data from ref. 22.

Na-nitroxide interaction Proton spin-spin, spin-lattice relaxation times 78Goll... 

Li Relaxation times have been used to study complex formation with nitroxides in the aqueous phase, and the interaction of Li with Mn. Dynamic NMR spectroscopy has been used to study equihbria of chelated lithium phenolates. The formation of a stable betaine Hthium salt adduct has... 

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