Anisotropy decays order parameters

FIG. 11 Order parameter variation along acyl chains in red cell ghosts ( ), small unilamellar vesicles of egg phosphatidylcholine (V), and paraffin oil (+), as determined by the fluorescence anisotropy decay of the w-anthroyloxy fatty acid probes. (Reprinted by permission from Ref. 12.)... 

The diffusion constant D with the underlying microviscosity , and the two order parameters , <(P4> reflecting the degree of orientational constraint have been successfully determined from the fluorescence anisotropy decay in... 

Extensions of the analysis of time-resolved fluorescence anisotropy decay data in terms of two order parameters have also been developed (see, e.g., Refs. 51-54). Thus, the corresponding higher order parameter term is <7%) given by(53)... 

An important advantage of the depolarization technique is that it allows one to measure the molecular ordering, as well as the motional parameters. For this purpose, it is necessary to detect the time dependence of the anisotropy. In the presence of ordering constraints, the r value does not decay to zero, but to some limiting value foo r = (ro — roo)e / c - - poo. The rate of decay defines a rotational correlation time, and Poo is a direct measure of the order parameter through the following relation s = Poo/ o (29). The fluorescence depolarization method works well as long as fluorescence lifetimes, which are typically 10 s, are not too different from the rotation relaxation times to be measured. When the rotational correlation time... 

Order parameters are used to interpret data on order and fluidity of a number of probes in lipid membranes obtained by measurements of fluorescence anisotropy decay 32 Ambiguities in the interpretation of time resolved fluorescence anisotropy measurements in lipid vesicle systems with DPH or TMA-DPH probes are attributed to the unsatisfactory models being used to interpret the data . The solubilisation of diphenylpolyenes in lipid bilayers has been critically examined33. It is concluded that such probes are satisfactory if used at low concentrations. 

Figure 25. (a) Dependence of the absorbance of linearly polarized probe light at 360 nm ( ) and 450 nm ( ), on the angle F between the probe and the UV irradiation lights polarization. This behavior is fitted with a law in cos F with an amplitude that decays in accordance with the cis => trans thermal back reaction. The theoretical fits are indicated by dotted curves. Evolution of (b) the anisotropy, AA" -, and (c) the order parameter S defined in relation 10, for the wavelengths 360 nm ( ) and 450 nm ( ), with the time of UV (360 nm) irradiation. 

Because the molecule s motion is restricted, the anisotropy does not decay to zero but reaches some final constant value, r . Equation 19 is typically used to model the anisotropy decay of a rod-shaped fluorophore such as l- 4-trimethylammomumphenyl)-6-phenyl-l,3,5-hexatriene (TMA-DPH) embedded in a phospholipid membrane, where it reports on the order and dynamics of the latter. The so-called order parameter S of the membrane is defined as... 

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