Dielectric relaxation rotational diffusion coefficient

Dielectric dispersion measurements also provide a means of determining rotational diffusion coefficients or mean rotational relaxation times of solute molecules. In principle, data for these hydrodynamic quantities can be used for a... 

Owing to their definite structures, most biomolecules have an appreciable permanent dipole moment which must lead to dielectric polarization via the rotational mechanism of preferential orientation. Thus pertinent experimental investigation permits a direct determination of the molecular dipole moments and rotational relaxation times (or rotational diffusion coefficients, respectively). These are characteristic factors for many macro-molecules and give valuable information regarding structural properties such as length, shape, and mass. 

Experimental dipole moments may be used to check the validity of electrostatic calculations. In the past, dipole moments of proteins were often characterized by measurements of dielectric relaxation. More information may be obtained by measurements of the electric dichroism because these measurements provide not only the magnitude of the dipole moment but also the optical anisotropy with respect to the dipole vector. Thus, measurements of the electric dichroism provide a more rigorous test for calculations of electrostatic parameters of proteins. Using the calculations described earlier for pK s of titratable groups, one can predict dipole moments of proteins and their axes given by the principal axes of the rotational diffusion tensor and compare them with electrooptical data. ° One important aspect of comparison of computed and experimental dipole moment is that computations of dipole moments, optical anisotropy, and rotational diffusion coefficients can be used in combination with experimental electrooptical procedures to determine the long-range structure of biomacromolecular assemblies, such as the complexes of DNA and proteins described by Pbrschke et al. so... 

The analysis of the dynamics and dielectric relaxation is made by means of the collective dipole time-correlation function (t) = (M(/).M(0)> /( M(0) 2), from which one can obtain the far-infrared spectrum by a Fourier-Laplace transformation and the main dielectric relaxation time by fitting < >(/) by exponential or multi-exponentials in the long-time rotational-diffusion regime. Results for (t) and the corresponding frequency-dependent absorption coefficient, A" = ilf < >(/) cos (cot)dt are shown in Figure 16-6 for several simulated states. The main spectra capture essentially the microwave region whereas the insert shows the far-infrared spectral region. 

Dielectric relaxation T2, correlation time for rotation about the dipolar axis Tc, rotational correlation time for solvation complexes Z>s, self-diffusion coefficient Ti, correlation time for rotation of the dipole... 

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