Rotational correlation time temperature effects

FIGURE 10.4 Effective rotational correlation time of 16-SASL in DMPC membranes plotted as a function of mole fraction of zeaxanthin at different temperatures (x2B (O) and x2C ( )). (From Subczynski, W.K. et al., Biochim. Biophys. Acta, 1105, 97, 1992. With permission.)... 

On the basis of the study of the solvent, temperature, and pressure effects, we show how the NMR rotational correlation times T2k for a heavy water molecule in neat liquid and organic solvents are cotrelated with the strength of solute-solvent interactions, in particular, H bonds. At room temperature (30 C), the correlation time is 2.1 ps in the random H-bond network in heavy water, whereas it is as small as 0.1 ps in such an apolar, hydrophobic solvent as carbon tetrachlmi because of the absence of the H bonds between water molecules. Pressure distorts H bonds and accelerates the orientational motion of water molecules in neat liquid. I%m evidence is collected for the limitations of the Stdces-Einstein-Debye (SED) law in solution. 

Above 100 K, motional effects on spectrum become pronounced with increasing temperature and, above 230 K, the spectra consist of essentially an isotropic and equally spaced hyperfine triplet, but with different relative intensities. The line shape simulations were carried out by adopting a Brownian rotational diffusion model in order to evaluate the associated (average) rotational correlation time, and its degree of anisotropy, = zpy, /... 

The modification of electrodes with PVC membranes has found applicability in ion selective electrode work [99] (so-called "coated wire electrode ). The molecular motion of species within such electrodes has been investigated by Compton and Waller [100]. Using a range of derivatives of the nitroxide spin probe TEMPO, they were able to show how the rotational correlational time was dependent upon the molecular volume of the probe and, by use of variable-temperature apparatus, how this varied with temperature. The effect of various plasticizers upon the molecular motion within the PVC membrane was investigated, rotational correlational times being dependent upon the nature of the plasticizer and the loading level. The effect of loading level upon the correlation time was shown to correlate with data obtained by Compton Maxwell [101] for the response times of K+ ion selective electrodes based upon PVC modified electrodes. 

Marinovic et al. have examined the effect of temperature on the ESR signals of TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-l-yl)oxyl) spin probe embedded NR. The temperature dependence of ESR spectra is due to change in rotational motion of the nitroxide radicals, characterized by the rotational correlation time (tr). " The representative ESR spectra of spin probed NR measured over a wide range of temperature are given in Figure 25.4(a). The separation of the outer maxima (2A f) is maximum for the spectrum of immobilized spin probe molecules at —120 °C and is slightly decreased with the increase in temperature. The shape of the spectral lines are separated when temperature approaches the glass transition temperature (Tg) (—20 °C) and above it, referred as Ts mT (the temperature at which the separation between the lAzz attains 5 mT). 

Variable temperature studies for Br gave relaxation changes corresponding to the viscosity changes. The conclusion was, as in Ref. [51], that relaxation can be referred entirely to the ion-solvent interactions and, furthermore, that the more effective relaxation observed as the methanol content increases is due to the longer rotational correlation time of methanol compared to water. In other studies of halide ion quadrupole relaxation in mixed aqueous-alcohol solvent, as described in Section 5.1.5, the increase in relaxation rate on alcohol addition has been interpreted in an entirely different way. The data of Hall et al, [221] have recently been interpreted in terms of selective ion solvation by Neggia et al, [222] and the difference between alkali and halide ion relaxation in the presence of nonpolar groups was emphasized (cf. Section 5.1.5). 

Measurement of interaction with protein. ) Measurement of rotational correlation time in H2O, dodecane, and solutions of sodium dodecyl sulfate and other surfactants. Measurement of rotational correlation times in starch solution at several temperatures. Measurement of rotational correlation times. ) Measurement in phosphatidyl choline vesicle membranes. Several related radicals also used. ) Measurement of effect of temperature on line broadening. Rotational correlation time measured in solutions of sodium dodecyl sulfate. ) Measurement of hemimicelle formation of surfactant SDS adsorbed onto alumina. ) Measurements in phase V liquid crystal solvents 7 other related radicals measured. 86Szal, 83Brol ), 87Bag2 )... 

The ESR difference spectra of the motionally restricted lipid component (cf. Fig. 3.2) lie within the slow-motion regime of spin label spectroscopy, indicating that the effective rotational correlation times are 10 ns. Experiments on the temperature dependence in rod outer segment disc membranes show changes which are diagnostic for motion on this timescale (Watts et al., 1981 1982). Thus the mobility of the motionally restricted lipid is significantly reduced relative to that of the fluid lipids ( ns), but... 

Lowering the temperature has a similar effect on the deuterium spectra as does increased loadings. In Figure 3, spectra for benzene-d6/(Na)X at 0.7 molecules/supercage over the temperature range 298 to 133 K are shown. It is observed that both benzene species are detected simultaneously between 228 and 188 K. Below this temperature the oriented benzene species becomes the predominant form. A similar situation occurs for polycrystalline benzene-dg in which two quadrupole patterns, one static and the other motionally narrowed due to C rotation, are observed to coexist at temperatures between 110 and 130 K (7). This behavior has been attributed to sample imperfections (8) which give rise to a narrow distribution in correlation times for reorientation about the hexad axis. For benzene in (Na)X and (Cs,Na)X such imperfections may result from the ion/benzene interaction, and a nonuniform distribution of benzene molecules and ions within the zeolite. These factors may also be responsible for producing the individual species. However, from the NMR spectra it is not possible to... 

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