Rotational correlation time mobility

PGSE-NMR provides direct information on the translational mobility of a liquid medium capable of swelling a given CFP. The self-diffusion coefficient of the swelling agent is found to be related to the nanoporosity of the matrix as determined from ISEC and to the rotational correlation time of a suitable paramagnetic probe (ESR) [22]. 

The mobility of tyrosine in Leu3 enkephalin was examined by Lakowicz and Maliwal/17 ) who used oxygen quenching to measure lifetime-resolved steady-state anisotropies of a series of tyrosine-containing peptides. They measured a phase lifetime of 1.4 ns (30-MHz modulation frequency) without quenching, and they obtained apparent rotational correlation times of 0.18 ns and 0.33 ns, for Tyr1 and the peptide. Their data analysis assumed a simple model in which the decays of the anisotropy due to the overall motion of the peptide and the independent motion of the aromatic residue are single exponentials and these motions are independent of each other. 

In addition, Nicolle et al. (Ill) have shown that the mobility of the embedded Gd-complex can be described by two rotational correlation times using the Lipari-Szabo analysis, i.e., a correlation time describing the overall rotational mobility of the micelle and a correlation time for the internal mobility of the Gd-complex inside the micellar structure. 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

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) ... 

Functional groups attached to solvent-swollen polymer chains exhibit free rotational motion as indicated by electron spin resonance rotational correlation times 132-134) These studies using nitroxide spin labels covalently bound to polystyrene matrices indicated that the mobility of the substituent is a function of the cross-link density and degree of swelling. The rotational correlation time of nitroxide within 2% cross-linked beads was about 100 times shorter in dichloromethane or benzene than in ethanol, and 2-3 times longer than nitroxide bound to non-cross-linked polystyrene. The latter observation shows that the heterogeneous reaction involving 2% cross-linked polystyrene is 2-3 times slower than the same reaction in solution. 

A spectroscopic technique that probes membrane fluidity can either directly measure mobility and order parameters for membrane constituents (NMR) or use probes (ESR, fluorescence). Some fluorescent and ESR probes are shown in Fig. 4. The connection between the rotational correlation time of a membrane embedded probe and the membrane fluidity can be illustrated using the example of a simple isotropic liquid, in which fluidity is merely a reciprocal viscosity ri and the rotational correlation time Xc for a molecule with a hydrodynamic volume V is given by the well-known Debye-Stokes-Einstein relation Xc = r VlkT, where k is the Boltzmann constant and T is the... 

Relaxation is strongly dependent on molecular motions. The overall random molecular tumbling, which is expressed in the rotational correlation time Xc, governs the overall relaxation process. Larger molecules have slower tumbling motions that lead to higher Xc values. However, local dynamics and independent domains can modulate the relaxation parameters, which account for differences in their flexibility and mobility. 

The anisotropies of both Trp-residues of apacid glycoprotein measured at 20°C are very close (Table 8.7), (a result identical to that obtained with the Weber method, see Table 8.6) indicating that the two Trp residues are highly mobile. This result is confirmed by the values of the rotational correlation times (3 and 5 ns) lower than the global rotational correlation time of a i-acid glycoprotein. When the Trp residue is embedded in the protein core and does not show any residual motions such as in the Lens culinaris agglutinin, its rotational con elation time will be close to that of the protein and its anisotropy will be higher than that of the surface Trp residue. 

Fig. 2 Simulated EPR spectra for nitroxides in X band (9.7 GHz) assuming different (isotropic) rotational mobilities, (a) Very fast rotational mobility corresponding to the isotropic limit, rotational mobilities corresponding to rotational correlation times of (b)...

In order to analyze the rotational mobility of the spin label quantitatively, spectral simulations are required. Simulation programs for CW EPR spectra are available [26, 36-38]. For the case of fast isotropic motion, approximate values of the rotational correlation time can be calculated from the fine height ratios [39]. 

We have observed that the rotational correlation times (it) depend strongly on the composition of the micellar solution and that thi > for all the investigated microemulsion solutions. This shows clearly that the polar region of the interface is more rigid that the nonpolar part of the microemulsion. In addition, it is observed that the mobility of the interface (t. ) is related linearly to the [CTAB]/[hexanol] ratio at the interface at constant [CTAB]/[water] ratio [3]. When the former ratio decreases, the rotational correlation time also decreases, indicating a dilution of the interface. This is turn leads to decreased interaction between the CTAB molecules and results in a more labile interface. 

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 ... 

Fig.8. ESR-spectrum of 4-hydroxy-2,2,6,6-tetramethylpiperidine -N-oxyl in solution, a) free rotation, correlation time —10 s b) restricted mobility, correlation time —10 s. 

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