Microviscosity probes

R. E. Dale, L. A. Chen, and L. Brand, Rotational relaxation of the microviscosity probe diphenylhexatriene in paraffin oil and egg lecithin vesicles, J. Biol. Chem. 252, 7500-7510... 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Lukac S (1984) Thermally induced variations in polarity and microviscosity of phospholipid and surfactant vesicles monitored with a probe forming an intramolecular charge-transfer complex. J Am Chem Soc 106 4386 -392... 

Microviscosity, fluidity, molecular mobility. Estimation by means of fluorescent probes... 

We should first emphasize that viscosity is a macroscopic parameter which loses its physical meaning on a molecular scale. Therefore, the term microviscosity should be used with caution, and the term fluidity can be alternatively used to characterize, in a very general way, the effects of viscous drag and cohesion of the probed microenvironment (polymers, micelles, gels, lipid bilayers of vesicles or biological membranes, etc.). 

The term microviscosity is often used, but again no absolute values can be given, and the best we can do is to speak of an equivalent viscosity, i.e. the viscosity of a homogeneous medium in which the response of the probe is the same. But a difficulty arises as to the choice of the reference solvent because the rotational relaxation rate of a probe in various solvents of the same macroscopic viscosity depends on the nature of the solvent (chemical structure and possible internal order). 

In conclusion, the method of intramolecular excimer formation is rapid and convenient, but the above discussion has shown that great care is needed for a reliable interpretation of the experimental results. In some cases it has been demonstrated that the results in terms of equivalent microviscosity are consistent with those obtained by the fluorescence polarization method (described in Section 8.5), but this is not a general rule. Nevertheless, the relative changes in fluidity and local dynamics upon an external perturbation are less dependent on the probe, and useful applications to the study of temperature or pressure effects have been reported. 

Viriot M. L., Bouchy M., Donner M. and Andre J.-C. (1983) Kinetics of Partly Diffusion-Controlled Reactions. XII. Intramolecular Exdmers as Fluorescent Probes of the Microviscosity of Living Cells. Photohiochem. Photobiophys. 5, 293— 306. 

Microviscosity imaging Identify viscosity-lifetime probes... 

The diffusion-related molecular processes occurring within a Cig stationary-phase have also been investigated using pyrene as a fluorescent probe [169]. Particular spectral bands were attributed to pyrene excimers formed in a diffusion-limited reaction. Rate constants for this formation were then used to estimate the microviscosity of the stationary-phase. A similar application of total internal reflection fluorescence... 

Less frequently used at present is electron spin resonance spectroscopy, which is based on the use of spin probes as model componnds or covalent spin labeling of drugs. Microviscosity and micropolarity of the molecnlar environment of the probe can be derived from electron spin resonance spectra. Moreover, the spectra allow us to differentiate isotropic and anisotropic movements, which result from the incorporation of the probe into liposomal structures. Quantitative distribution of the spin probes between the internal lipid layer, the snrfactant, and the external water phase is to be determined noninvasively. On the basis of the chemical degradation of drugs released from the lipid compartment, agents with reductive features (e.g., ascorbic acid) allow us to measure the exchange rate of the drugs between lipophilic compartments and the water phase [27,28]. 

Miyagishi, S., Kurimoto, H., Ishihara, Y., and Asakawa, T. (1994). Determination of the critical micelle concentrations and microviscosity with a fluorescence probe, auramine. Bull. Chem. Soc. Jpn., 67, 2398-2402. 

Microviscosity experiments using a molecular probe indicated a greater viscosity in micelles than in the bulk. The viscosity decreases rapidly up to go 10 and then decreases slowly as the micelle size increases. Additional studies using fluorescence probes revealed two different solvation rates inside the micelle. Zhong attributed the different solvation rates to water bound to the polar head group of AOT and bulk water inside the micelle (Zhong et al., 2002). This increased viscosity and extended solvation sphere limits the size of particles that can be synthesized using reverse micelles. 

Actually, the apparent rate constant of the photoisomerisation in a viscous media, like biological membranes, was found to be dependent upon the medium relaxation rate. Hence, it is possible to study the dynamics of proteins and biological membranes in the vicinity of the incorporated stilbene probe by monitoring the steady-state fluorescence decay of the stilbene probe with the conventional constant-illumination spectrofluorimeter. The experimental values of a (j>n Icx can be measured independently or can be omitted by comparison with photoisomerisation kinetics of the same probe and similar conditions in a medium with known macro- and microviscosity. A combined analysis of the trans-cis photoisomerisation kinetics of a stilbene probe and its polarization allows the establishment of the mechanism and the estimation of the... 

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