Depolarization, concentration rotational

Davydov splitting, 206 de Broglie wavelength, 17 Depolarization, concentration, 120 rotational, 120... 

Molecular Rotational Diffusion. Rotational diffusion is the dominant intrinsic cause of depolarization under conditions of low solution viscosity and low fluorophore concentration. Polarization measurements are accurate indicators of molecular size. Two types of measurements are used steady-state depolarization and time-dependent (dynamic) depolarization. 

The values of /labs were determined from the experimentally measured absorption anisotropy with Eq. 6. In viscous polyTHF, the rotational movement of dye molecules on a lOOps time scale is assumed to be negligible, and thus, does not reduce the limiting value of anisotropy. For the concentrated fluorene solutions (5 x 10 M, 1 mm cuvette), the anisotropy Tabs was not affected by FOrster depolarization mechanisms [13] due to the short time delay, tD, between probe and pump pulses when td/ti 1 [35]. 

Fredrickson3 has formulated expressions for the concentration depolarization of fluorescence in the presence of molecular rotation. A theoretical examination of diffusion influenced fluorescence quenching by nearest possible quenching neighbours in liquids has been made35. A modified version of Smoluchowski - Collins - Kimball formulation of the Stern - Volmer equations has been matched with experimental data for quenching of anthraquinone derivatives by N,N-dimethyl- -toluidine. Another paper discusses this work on the basis of the kinetics of partly diffusion controlled reactions3 . 

The DMF module (PaU Corp., New York) which consists of several disks mounted on the same shaft [2, 135]. The reported studies of the apphcation of the rotating disk dynamic membrane indicate that high shear-induced filtration is much less sensitive to the solids concentration. Advantage of the rotational system is that it permits operation at both very low transmembrane pressure-drop, and low upstream mass velocity, without loss in depolarization efficiency. It is thus possible with this system to achieve cleaner separation of solute components, than is achievable with conventional systems. Equipment for this process is, unfortunately, significandy more costly, and maintenance costs much higher also, than those for conventional membrane systems. 

The effects of rotational difhision and energy transfer ate easily sqiarated fey juifidous choice of the experimental conditions. For example, Brownian rotations cause negligible depolarization when the rotational rate is much slower than the rate of fluorescence emisaon. In contrast, RET occurs only in concentrated solution where the average distance between the fluorophore molecules is comparable to a characteristic distance Rg, which is typically near 40 A. One may readily calculate that millimolar concentrations are required to obtain this average distance (Chapter 13). Hence, since the usual concentradons requited for fluotesoence measurements ate about l0r U, RET is easily avdded by the use of dilute solutions. 

Raman Depolarization Ratio. Thomley and Shepherd suggest that measuronent of Raman depolarization ratios (p) offer a sensitive probe for polymer conformations and yields valuable information even in concentrated solutions where, in contrast to other techniques, it does not suffer from difficulties of either measurement or interpretation. They use computations of the probability of a rotational isomer being trans in order to relate/> to free energy of mixing for poly(dimethyl siloxane)-benzene. 

At present it is not yet clear which properties of protein molecules are best revealed by a study of the depolarization of the fluorescence of conjugates. The relaxation times of the rotation observed by this method appear to give a reliable estimate of the molecular size when the shape of the molecule is taken into consideration. Several advantages of this method are (1) It may be applied to very dilute solutions. The author has studied 0.02% serum albumin solutions, and even smaller concentrations could be used with little improvement of the experimental tech-... 

The depolarized scattering is related to orientation fluctuations, thereby being determined by rotational diffusion in dilute solutions, but dependent on translational modes as well in moderately concentrated solutions. In general,... 

Semidilute solutions of rod-like polymers are also of basic interest in polymer science. In semidilute solutions, even at low concentrations, the rod molecules are overlapping and entangled but are still isotropic, having no liquid crystalline order. Combinations of static and dynamic, and polarized and depolarized light scattering could reveal characteristic features of the solution over the concentration range from the isotropic to lyotropic liquid crystalline regions. By DLS of depolarized components based on eqn [126], Zero and Pecora observed translational and rotational diffusion of polypeptides in semidilute solutions. 

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