Liposomes microviscosities

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]. 

In oitr own studies, we modified membrane microviscosity by incubating nomoal platelets overnight as 22 with cholesterol-rich, cholesterol-normal and cholesterol or liposomes. Under these conditions, and considering only high affinity sites since moderate and low affinity sites were combined in this analysis, the number of receptors was proportional to membrane microviscosity while the affinity at those receptors was inversely proportional to membrane microviscosity (Table 2). 

Increased content of conjugated dienes in linoleate acyls in the mixed liposomes composed 95% of DPPC and 5% of DLPC caused the increase in their microviscosity (Figure 11, curve 1). The microviscosity of liposome membranes containing 100% DLPC was considerably decreased upon the enzymatic oxidation by C-15 reticulocyte lipoxygenase (Figure 11, curve 2). 

Figure 11. Effect of the hydroperoxy and hydroxy derivatives of fiee PUFA and phospholipids on the microviscosity of liposomes composed of saturated and unsaturated phosphatidylcholine ...

The results of two series of independent experiments (3-5 measurements for each experimental point) are given the difference between microviscosity values of the modified and initial membranes (the initial phosphatidylcholine microviscosity was taken as 1 for every type of liposomes) was significant at... 

If the medium is highly ordered or anisotropic, the Perrin equation does not hold and microviscosity can be used only qualitatively. In this case, a fluorophore is restricted to certain motions that will occur to different extents in different dimensions. An example of such a phenomenon would be a fluorescent molecule that would align itself along the fatty acid chains of phospholipids in the phospholipid bilayer of a liposome. For fluorophores in an anisotropic medium, the time-resolved fluorescence anisotropy does not fall to zero (see Fig. 4, curve a), that is, the depolarizing rotations of the fluorophore will not attain an isotropic distribution at infinite time. The anisotropy will decay to an infinite-time anisotropy (r ) rather than to zero and will have the form... 

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