Probes, macromolecular, diffusion

The nature of polymer motion in semidilute and concentrated solutions remains a major question of macromolecular science. Extant models describe polymer dynamics very differently 3-11). Many experimental methods have been used to study polymer dynamics (12). One meAod is probe diffusion, in which inferences about polymer dynamics are made by observing the motions of dilute mesoscopic probe particles diffusing in the polymer solution of interest. Probe diffusion can be observed by several experimental techniques, for example, quasi-elastic light scattering spectroscopy (QELSS), fluorescence recovery after photobleaching (FRAP), and forced Rayleigh scattering (FRS). 

A number of macromolecular diffusion and conformational properties can be studied using fluorescence anisotropy, fluorescence correlation spectroscopy (ECS), and fluorescence recovery after photobleaching (FRAP). These techniques most commonly are applied to proteins labelled with highly fluorescent probes, but can exploit intrinsic fluorescence in some instances. In fluorescence anisotropy studies, polarized light is used to selectively excite molecules whose transition dipole moments are aligned with the electric field vector. Steady-state... 

Local motions which occur in macromolecular systems can be probed from the diffusion process of small molecules in concentrated polymeric solutions. The translational diffusion is detected from NMR over a time scale which may vary from about 1 to 100 ms. Such a time interval corresponds to a very large number of elementary collisions and a long random path consequently, details about mechanisms of molecular jump are not disclosed from this NMR approach. However, the dynamical behaviour of small solvent molecules, immersed in a polymer melt and observed over a long time interval, permits the determination of characteristic parameters of the diffusion process. Applying the Langevin s equation, the self-diffusion coefficient Ds is defined as... 

Diffusion of Water Vapor in Newborn Rat Stratum Corneum. Measurement and interpretation of diffusion in heterogenous biological systems such as the stratum corneum are difficult compared with similar measurements for well-defined synthetic polymer systems, but studies of water diflFusion in stratum corneum are essential to a better understanding of those factors which contribute to the barrier function of the corneum. Water diffusion measurements under both equilibrium and non-equilibrium conditions are useful to probe the influence of temperature and other factors on stratum corneum macromolecular structure. 

Thus, if ti is in a time range accessible to autocorrelation (roughly 1—10—7 sec), fluorescence fluctuations may be used to measure macromolecular translational diffusion coefficients. The presence of a fluorescent label enables this method to measure the translational diffusion coefficient of a molecule in a complex mixture. Such a measurement would be very difficult in an ordinary light-scattering experiment because all components of the mixture contribute.17 The advantage of fluorescent probes is that they allow particular species to be labeled and thereby separately studied. For or of the order of 10 4 cm and for a particle with a diffusion coefficient of the order 10 5 cm2/sec, tt = 10 3 sec, well within our ability to measure. This leads to the interesting possibility of measuring diffusion coefficients of labeled molecules in membranes, and in cells in vivo. 

Translational diffusion within polymer brushes can be also accessed by FCS studies. It depends on the local viscosity within the brush and on probe size, thus causing different results for small and macromolecular probes [156]. For charged polymers, pH and ionic strength plays an additional role. If the dyes and the brushes are oppositely charged a dynamic association of dye molecules with the polymer... 

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