Rotational diffusion rate

PTFE is known to undergo two crystalline first-order transitions at 19° and 30° and three viscoelastic relaxations have been observed. The temperature dependence of the relaxations (a,8,Y) as determined by dielectric and anelastic measurements is shown in the relaxation map of Figure 10 along with the temperatures of the crystalline phase transitions (vertical dotted lines) Also indicated in Figure 10 are the results of rotational diffusion rates obtained in this study. 

We should note that, given the difference in quantum yield between the free and bound probe, the fractional intensities utilized in Fig. 7 actually represent small percentages of bound probe on a molar basis. In fact, considering the accuracy of the differential phase measurement (better than O.r) one can detect, in this system, on the order of 0.1% bound probe. This phenomenon also occurs in time-domain measurements. Specifically, if one monitors the anisotropy decay of a system which displays multiple lifetimes associated with multiple rotational diffusion rates then one may observe a decline at short times of the anisotropy followed by a rise at latter times and subsequent decrease. This dip and rise effect has been observed by Millar and co-workers in studies on protein-DNA interactions, specifically in the case of the interaction of a fluorescent DNA duplex with the Klenow fragment of DNA polymerase. 

Rotational diffusion rates of 2,2,6,6-tetramethyl-4-piperidinol-l-oxyl, bound to 2% cross-linked polystyrene with DF 0.02, arc slower than those of the soluble nit-roxyl (331. The rates (as the inverse rotational correlation time v ) increase with increased swelling of the polymer, from 3 x 10 s with no solvent or with the nonsolvents ethanol and 2-propanol, to 3 x 10 s i with benzene, to >10 s for a benzene solution of the corresponding soluble polystyrene. Increased cross-linking (4% and 12% DVB) gives decreased swelling a decreased rotational diffusion rates. 

With respect to fiber-optic-based fluorescence sensing, most of the past approaches have employed either excitation or emission wavelength selectivity (10,11). The other selectivity parameters fluorescence lifetime, steady-state polarization, and rotational diffusion rates have received little if any attention. The acquisition of fluorescence lifetime information via optical fibers has been demonstrated previously (12) however, the time resolution and ability to resolve multiexponential decays of fluorescence were neither demonstrated nor possible with this earlier instrument. Recently, our own group (13,1 ) has developed and described the first fiber-optic-based fluorescence lifetime instrumentation capable of unequivocally determining single, double, and/or triple exponential decays of fluorescence in remotely-located samples. 

A A R , Rl djf, djfprp Anisotropic viscosity rotational diffusion rate constants parallel and perpendicular to director... 

Fluidity in membranes is an easily visualized phenomenon and is a term which is widely used. However, it can be misleading. For example, it is commonly assumed (and stated) that making membrane lipids more saturated or adding cholesterol makes a membrane less fluid. The assumption is that such alterations will reduce the speed of movement of lipids. However, introduction of cholesterol into phosphatidylcholine model membranes has no effect on lateral movement and may actually increase rotational diffusion rates. What cholesterol and increased saturation do is to increase the order in the hydrocarbon matrix and this is what can be measured easily by NMR or ESR order parameters. 

Considering that the measured fluorescence anisotropy is based on the difference in rotational diffusion rates of the bound and free forms of the chiral molecule in the presence of a chiral selector, it stands that in a solution of mixed enantiomeric composition the anisotropy will vary between two extreme values for the each of the pure enantiomers. For a given chiral compound in the presence of a chiral selector, the measured anisotropy is then a function of the enantiomeric composition. 

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