Partitioning of (+-)-5,6-dihydro-6-phenyl-2-n-alkyl-imidazo- [2,1-b]thiazoles into large unilamellar liposomes: a steady-state fluorescence quenching study

Interaction of polyphenols with model membranes: Putative implications to mouthfeel perception

Food polyphenols in fruits juices, tea, coffee, wine and beer confer sensory properties such as colour, astringency and bitterness. The development of functional healthy drinks without the unpleasant sensory feeling is boosting research for a clearer understanding on the interactions of polyphenols within the oral mucosa. In this study we investigated the interaction of astringent polyphenols, namely ECG, EGCG, procyanidin B4 and PGG, with lipids in model membranes by spectroscopic techniques. The membrane model was built varying the cholesterol content to mimic mouth regions and experiments were conducted at pH 5 to mimic the pH drop at the moment of beverage (e.g. green tea, red wine) intake. Fluorescence quenching results conducted on LUVs with cholesterol molar fractions ranging between 0.34 < χ_chol < 0.74 and similar size distributions (122.9 ± 3.7 nm) showed that interaction of polyphenols is structure- and concentration-dependent. Also, the decrease of partition constants (K_p) with increasing cholesterol content (χchol) suggest that the affinity of polyphenols is weaker in cholesterol-rich liposomes. STD results revealed that the interaction of EGCG and PGG with membrane lipids involved mainly galloyl residues. Overall, spectroscopic data show that polyphenols interact to higher extent with more polar regions found in buccal, flour of the mouth and gingiva regions than with more hydrophobic regions located in the palate and tongue supporting that lipid microenvironments play a role in oral sensory perception.

Interactions of a non-fluorescent fluoroquinolone with biological membrane models: A multi-technique approach

Fluoroquinolones are antibiotics which act by penetrating into bacterial cells and inhibiting enzymes related to DNA replication, and metal complexes of these drugs have recently been investigated as one approach to counteracting bacterial resistance. In this work, we apply a multi-technique approach to studying the partition coefficient (Kp) for the non-fluorescent third-generation fluoroquinolone sparfloxacin or its copper-complex with lipid membrane models of Gram-negative bacteria. The techniques investigated are UV-vis absorption and (19)F NMR spectroscopies together with quenching of a fluorescent probe present in the lipids (using steady-state and time-resolved methods). (19)F NMR spectroscopy has previously been used to determine the Kp values of fluorinated drugs but in the case of sparfloxacin did not yield useful data. However, similar Kp values for sparfloxacin or its copper-complex were obtained for the absorption and fluorescence quenching methods confirming the usefulness of a multi-technique approach. The Kp values measured for sparfloxacin were significantly higher than those found for other fluoroquinolones. In addition, similar Kp values were found for sparfloxacin and copper-complex suggesting that in contrast to other fluoroquinolones hydrophobic diffusion occurs readily for both of these molecules.

Effects of diclofenac on EPC liposome membrane properties

In this work the interaction of a non-steroidal anti-inflammatory drug (NSAID), diclofenac, with egg yolk phosphatidylcoline (EPC) liposomes, used as cell-membrane models, was quantified by determination of the partition coefficient. The liposome/aqueous phase partition coefficient was determined by derivative spectrophotometry, fluorescence quenching, and measurement of zeta-potential. Theoretical models based on simple partition of the diclofenac between two different media, were used to fit the experimental data, enabling the determination of K(p). The three techniques used yielded similar results. The effects of the interaction on the membrane's characteristics were further evaluated, either by studying membrane potential changes or by effects on membrane fluidity. The liposome membrane potential and the size and size-homogeneity of liposomes were measured by light scattering. The effects of diclofenac on the internal viscosity or fluidity of the membrane were determined by use of spectroscopic probes-a series of n-(9-anthroyloxy) fatty acids in which the carboxyl terminal group is located at the interfacial region of the membrane and the fluorescent anthracene group is attached at different positions along the fatty acid chain. The location of the diclofenac on the membrane was also evaluated, by fluorescence quenching using the same series of fluorescent probes. Because the fluorescent anthracene group is attached at different positions along the fatty acid chain, it is possible to label at a graded series of depths in the bilayer. The interactions between the drug and the probe are a means of predicting the location of the drug on the membrane.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Modelling of interaction of basic lipophilic ligands with cytochrome P-450 reconstituted in liposomes. Determination of membrane partition coefficients of S-(-)-nicotine and N,N-diethylaniline from spectral binding studies and fluorescence quenching

The spectral interaction of N,N-diethylaniline and S-(-)-nicotine with cytochrome P450IIB4 reconstituted into large unilamellar vesicles could properly be described by a model for interaction of basic lipid-soluble ligands with membrane-bound acceptor sites in which linear partitioning of non-ionized ligand in the membrane is postulated. Apparent spectral dissociation constants Ksapp for type I binding of N,N-diethylaniline and for type II binding of S-(-)-nicotine increased linearly with increasing lipid volume fraction alpha L of the proteoliposomes. From plots of Ksapp vs. alpha L, the membrane partition coefficient of each ligand was calculated. The apparent affinity of cytochrome P450IIB4 for the ligands increased as the pH was raised from 6.0 to 8.5. However, effective dissociation constants were virtually independent of the pH, indicating that only the uncharged form of the basic ligands interact with cytochrome P450IIB4. For each compound, the apparent quenching rate constants kqapp derived from the Stern-Volmer plots for dynamic quenching of the fluorescence intensity of 8-(2-anthryl)octanoic acid in liposomes, decreased with increasing liposomal concentration. Plots of (kqapp)-1 vs. alpha L of the liposomes yielded the overall bimolecular quenching rate constant kq of each quencher. The kq value for S-(-)-nicotine was about three orders of magnitude less than that for N,N-diethylaniline. The values of the partition coefficient of N,N-diethylaniline, obtained from the binding studies and the fluorescence quenching measurements, were identical (on average, Kp amounted to 383). Analysis of the quenching data of N,N-diethylaniline with Scatchard plots likewise revealed that the association of the compound with liposomal membranes is a pure partition process.