Membranes excimer formation

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

Vauhkonen M., Sassaroli M., Somerharju P. and Eisinger J. (1990) Dipyrenyl-phosphatidylcholines as Membrane Fluidity Probes. Relationship between Intramolecular and Intermolecular Excimer Formation Rates, Biophys. J. 57, 291-300. 

Lateral Mobility(Fluidity) of Sulfonate A and B Micelles. The ratio of excimer to monomer fluorescence intensity of pyrene had previously been used to measure the fluidity of biological membranes (8). The ease of excimer formation was correlated with the fluidity of the membrane. The same principle may be applied to the measurement of fluidity in inverted micelles. To this end, we used three pyrene carboxylic acid probes of varying chain length PVA, PNA and... 

DiaryIpropanes. Excimer formation with diarylpropanes in solution talces place intramolecularly, making this process independent of concentration (44). Therefore, these molecules, especially 1Py(3)1Py (45), have been used extensively to probe the fluidity of micelles and artificial and biological membranes (17-19,46-50). Here, they are expected to be convenient indicators of the degree of mobility freedom of adsorbed molecules. 

The fluidity in the neighborhood of probe molecules can be tested by use of probes capable of intramolecular excimer formation. The probe molecules contain the two excimer-forming moieties linked by an alkyl chain. The extent of excimer formation depends on the viscosity of the environment and can be monitored by measuring the excimer/monomer fluorescence intensity ratio. The dependence of this ratio on reciprocal viscosity for the probe molecule dipyrenylpropane is shown in Fig. 18, in which the obtained microfluidities for surfactant systems are indicated. The fluidities decrease in the order SHS microemulsion, SDS, CTAC, Triton X-100 cf. Ref. 167 (for abbreviations see Tables 6 and 7). The same sequence order was found by Kano et al. (68). In systems containing heavy counterions the method leads to data that must be evaluated carefully, since heavy atom interactions may be different with excited monomers and excimers. The intramolecular excimer technique is also useful in biological studies. For instance, Almeida et al. investigated the sarcoplasmic reticulum membrane in which the activity of the Ca -pumping enzyme is modulated by the membrane fluidity (197). 

Since increasing temperature leads to increased fluidity and thus to a faster probe diffusion, pyrene Hpids have been frequently used to study phase transition in membranes [161,162]. Phospholipid phase separation increases the local concentration of dye labeled Hpids and can, therefore, be investigated via the characterization of exdmer formation. The binding of proteins or ions, however, may induce phase separation as well as decreasing lateral lipid diffusion. Since these two effects are opposing in terms of excimer formation, the binding of such proteins or ions cannot be studied by the (Ex/Mo)-ratio. The time-resolved analysis of the monomer fluorescence of the labeled lipid, however, allows for the separation of... 

The concept of quasi-three-dimensionil diffusion in membranes is illustrated by pyrene exclmer fonnation. Although excimer formation is not strictly quenching, the monomer emission is decreased by diffusive encounters of excited memomers with ground-atate monomers. Hence, the quenching of the monomer emission is described by Stem-Volmer kinetics. 

A disadvantage of the use of pyrene is that the extent of excimer formation depends on the bulk concentration of pyrene in the membranes. For this reason, several groups have synthesized covalently linked pyrenes or pyrene-amine conjugates. " Examples of such molecules are... 

The term exdmer is used when the excited dye forms a transient fluorescent dimeric complex with another fluorophore of the same kind. The exdmer fluorescence is usually red shifted with respect to that of the monomer (see Fig. 6.28) The most widely used types of eocdmer-forming probes are pyrene (see Fig. 6.28) and perylene and their derivatives. The ratio of the maxima of the excimer to the monomer spectra can be used to judge the efficiency of exdmer formation. This (Ex/Mo)-ratio depends on the concentration of the dye and is controlled by the diffusion properties. It allows, when using pyrene or perylene labeled fatty acids or phospholipids (see Fig. 6.28), the estimation of the probe s lateral diffusion coefficients in lipid bilayer membranes. Thus, membrane fluidity can be measured by monitoring the fluorescence spectra of such an exdmer probe. 

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