Rotational correlation time radicals

Method involves measuring the change in rotational correlation time for a free radical probe (e.g., nitroxide spin probes) introduced into a sample being studied. The temperature associated with a decrease in the rotational correlation time of a spin probe is assigned as Tg... 

The 2,2,6,6-tetramethylpiperidinoxyl radical (TEMPO) was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor.18 The steric hindrance of the NO bond in TEMPO makes it a highly stable radical species, resistant to air and moisture. Paramagnetic TEMPO radicals can be employed as powerful spin probes for elucidating the structure and dynamics of both synthetic and biopolymers (e.g., proteins and DNA) by ESR spectroscopy.19 Unlike solid-phase 1H-NMR where magic angle spinning is required in order to reduce the anisotropic effects in the solid-phase environment, solid-phase ESR spectroscopy can be conducted without specialized equipment. Thus, we conducted comparative ESR studies of various polymers with persistent radical labels, and we also determined rotational correlation times as a function of... 

The line shape of the ESR spectrum of a nitroxide free radical in x-band operation varie with t e rotational motion of the nitroxide over the range 10 to ]0 sec in rotational correlation time. By saturation transfer measurements even slower motions may be studied 23) The rotational correlation time for a nitroxide in a dilute solution of labelled random coil... 

Some systems cannot be well described by translational motion, so instead they require a model based on rotational diffusion. The commonly used model is one where the nuclei and radical form a bound complex, then this complex rotates to modulate dipolar coupling.74 Here, the overall correlation time consists of the rotational correlation time of the solvent complex, xT, and the exchange rate of molecules in and out of the complex, tm, where 1 /tc 1 /tr I 1 /tm. The form of this spectral density function is simpler4,25 ... 

ESR Measurement. The behavior of a stable free radical added to a medium reflects an average state of molecular rotation of the medium. Oil-soluble free radicals are used commonly to probe the rotation of oil molecules. The mobility of any oil-soluble probe is represented by its rotational correlation time (rc) as calculated from the following equation (28) ... 

An ESR study using an oil-soluble free radical probe to observe rotational correlation time, tc, of oil see Table VII), and pulsed NMR measurements indicate that the molecules of the emulsifying surfactants surround an oil particle to form a ternary oil-surfactant-water complex see Figure 6). 

If the motion of an acrylic polymer radical about the Cp bond is hindered, changing the temperature should lead to changes in the TREPR spectrum. This is indeed observed for all acrylic polymers we have examined to date. Simulation of the complete temperature dependence of TREPR spectra of acrylic polymer main-chain radicals should allow information regarding the conformational motion of the polymer in solution to be extracted, such as rotational correlation times, spin-lattice relaxation times (Ti), and activation energies for conformational transitions. 

FIGURE 14.12 Arrhenius plot for rotational correlation time data from TREPR spectra of polymeric radical 2a. Squares are the experimental data, solid line is the linear fit, with... 

Here, each Vj. term is due to each of the dipole-dipole interaction between two components radicals, the anisotropic Zeeman of the two radicals, and their anisotropic HFC terms, T, is each rotational correlation time, and co is given by gfigB h. 

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. 

If the radical motion slows down a gradual transition from a highly resolved three line spectrum of a liquid to the broad powder spectrum of a solid is observable (Fig. 5). For rotational correlation times Xc in the range of 0.01-1 ns, the Lorenzian form of individual spectral lines can be described by the width parameter being proportional to the ensemble average of the squared resonance frequency fluctuations Am. Any dynamical process which causes the resonance frequency to change will therefore contribute to the line width. Approximating that anisotropy can be described by axial tensors [52], the modulation of the... 

Spin probes I and II are studied in the system CTAB-hexanol-water [3,21]. Spin probe I, with its nitroxyl radical quite distant from the polar headgroup, probes the nonpolar region of the interface whereas spin probe II, with the nitroxyl radical close to the polar headgroup, explores the polar region of the interface. From the heights (/i) and the linewidths (A//) of the different multiplets (due to the electron-nitrogen hyperfine coupling), one can compute the rotational correlation times (t ) of these spin labels [84] from the relationship... 

Freed et al. [3] evaluated the relaxation rate of polymer chains by the analysis of the anisotropic spectra of nitroxide spin labels. The main triplet spectrum was due to hyperfine coupling caused by the nitrogen nucleus. It narrowed with an increase in mobility of the radicals because of motional averaging of the anisotropic hyperfine interaction. The rotational correlation time of nitroxide spin labels can be estimated using the procedure of Freed et al. by taking into account the anisotropic rotational motion. The ESR line width, AHmsi of the spectrum can be expressed as follows ... 

A value of lipid microviscosity was estimated by a rotational correlation time (t) of 16-DSA calculated from the EPR spectra obtained using formula for fast motion range (6 x 1(E" < T < 3 x 10" s) of the nitroxyl radicals ... 

The spin probes 5- and 16-DSA are localized in the different regions of membrane lipids 5-DSA in surface lipids at depth of 8A° and hydrophobic nitroxyl radical 16-DSA penetrates in lipid bilayer to the depth more than 20A° [14, 16], A motion of 16-DSA can be characterized by rotation correlation time (x) by which a microviscosity value is estimated and anisotropic rotation of 5-DSA by order parameter (S) [15,16],... 

The spectra of the radicals and magnetic nanoparticles were recorded at room temperature using Bruker EMX 8/2.7 X-band spectrometer at a microwave power of 5 mW, modulation frequency 100 kHz and amplitude 1 G. The first derivative of the resonance absorption curve was detected. The samples were placed into the cavity of the spectrometer in a quartz flat cell. Magnesium oxide powder containing Mn ions was used as an external standard in ESR experiments. Average amount of spin labels on protein macromolecules reached 1 per 4-5 albumin macromolecules and 1 per 2-3 thrombin macromolecules. Rotational correlation times of labels were evaluated as well as a fraction of labels with slow motion (x > 1 ns). 

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