Binding and diffusion kinetics of the interaction of a hydrophobic potential-sensitive dye with lipid vesicles

Interaction between DMPC liposomes and HM-PNIPAM polymer

The interactions of hydrophobically-modified poly-(N-isopropylacrylamides) (HM-PNIPAM) and dimyristoylphosphatidylcholine (DMPC) vesicles were investigated by the effect of the polymer on the binding of a fluorescent dye, oxonol VI, to DMPC vesicles, and on its diffusion across the membrane. On mixing with the vesicles, the dye exhibits an increase in fluorescence, which occurs in a two-stage process. The process was monitored by stopped-flow fluorescence spectrophotometry. According to the dependence of the reciprocal relaxation time on vesicle concentration, the rapid stage seems to be due to the second-order binding of the dye to the lipid membrane, a process that is almost diffusion-controlled, whereas the slow process is attributed to movement of the dye within the membrane phase. The polymer did not significantly affect the rate constant of the binding step, but it slowed down slightly the dissociation process of the dye from the membrane. However, the polymer affected the second stage, causing an increase in the reciprocal of its relaxation time, which suggests that the polymer makes the vesicle membrane more fluid.

Effects of bilayer surface charge density on molecular adsorption and transport across liposome bilayers

Second harmonic generation (SHG) was used to study both the adsorption of malachite green (MG), a positively charged organic dye, onto liposomes of different lipid compositions, and the transport kinetics of MG across the liposome bilayer in real time. We found that the dye adsorption increased linearly with the fraction of negatively charged lipids in the bilayer. Similarly, the transport rate constant for crossing the bilayer increased linearly with the fraction of charged lipid in the bilayer.

Slow fluorescent indicators of membrane potential: a survey of different approaches to probe response analysis

Basic tenets related to the use of three main classes of potentiometric redistribution fluorescent dyes (carbocyanines, oxonols, and rhodamines) are discussed in detail. They include the structure/function relationship, formation of nonfluorescent (H-type) and fluorescent (J-type) dimers and higher aggregates, probe partitioning between membranes and medium and binding to membranes and intracellular components (with attendant changes in absorption and emission spectra, fluorescence quantum yield and lifetime). The crucial importance of suitable probe-to-cell concentration ratio and selection of optimum monitored fluorescence wavelength is illustrated in schematic diagrams and possible artifacts or puzzling results stemming from faulty experimental protocol are pointed out. Special attention is paid to procedures used for probe-response calibration (potential clamping by potassium in the presence of valinomycin, use of gramicidin D in combination with N-methylglucamine, activation of Ca-dependent K-channels by A23187, the null-point technique). Among other problems treated are dye toxicity, interaction with mitochondria and other organelles, and possible effects of intracellular pH and the quantity of cytosolic proteins and/or RNA on probe response. Individual techniques using redistribution dyes (fluorescence measurements in cuvettes, flow cytometry and microfluorimetry of individual cells including fluorescence confocal microscopy) are discussed in terms of reliability, limitations and drawbacks, and selection of suitable probes. Up-to-date examples of application of slow dyes illustrate the broad range of problems in which these probes can be used.

A theoretical description of non-steady-state diffusion of hydrophobic ions across lipid vesicle membranes including effects of ion-ion interactions in the aqueous phase

A theoretical model of hydrophobic ion diffusion across vesicular membranes is presented, which is based upon activated rate theory. The model is applicable to the sudden addition of hydrophobic ions to a vesicle suspension, for example in a stopped-flow experiment. The time course of diffusion is calculated by numerical integration of differential rate equations for the ion concentrations and electrical potential differences across the membrane. The model utilizes the three-capacitor model of the membrane and an extended Debye-Hückel theory, taking into account non-neutrality on each side of the membrane. At low ionic strengths good agreement is found between the infinite time diffusion potential and the equilibrium Nernst potential. At large excess of inert electrolyte discrepancies are found, but under such conditions the membrane potential is negligible due to screening.

Influx and efflux kinetics of cationic dye binding to respiring mitochondria

We have investigated the kinetics of interaction of cationic fluorescent lipophiles (dyes) rhodamine 123, rhodamine 6G, tetramethyl rhodamine ethyl ester, safranine O, 1,1'-diethyloxacarbocyanine, 1,1'-diethyloxadicarbocyanine, and 1,1'-diethylthiadicarbocyanine iodide with isolated respiring rat-liver mitochondria (RLM). Dye flux across the RLM inner membrane was measured by following the kinetics of fluorescence signal change after mixing of dye and RLM. The time course of fluorescence was analysed in terms of a kinetic model of the binding and transport processes involved. The rate constants of dye influx and efflux were extracted from the observed effect on the apparent time constant of fluorescence change to equilibrium intensity upon mixing dye with increasing concentrations of RLM. From the influx rate constants obtained, the apparent permeability constants for dye influx (at zero potential) across the membrane were calculated and ranged from 3 to 140 x 10(-4) cm/s. The influx rate constant was found to be linearly related to relative dye lipophilicity, as predicted by the model. As another test of the model, from the ratio of the influx and efflux rate constants, the apparent trans-membrane potential, psi, was calculated and found generally to agree with reported values, but to depend on the lipophilicity of the dye used. Not predicted by the simple model was a dissymmtry observed in the influx and efflux time constants for fluorescence change to equilibrium intensity. Inferences are made relating to the utility of these dyes as probes of psi.

An adsorption isotherm for the interaction of membrane-permeable hydrophobic ions with lipid vesicles

An adsorption isotherm is presented which describes the association of membrane-permeable hydrophobic ions with lipid vesicles. The theory is based upon the Langmuir adsorption isotherm, but is has been significantly extended to take into account diffusion of ions across the membrane and dissociation into the intravesicular space as well as electrical effects due to the build up of boundary potentials within the membrane. The boundary potentials are calculated according to the three-capacitor model of the lipid membrane. In contrast to the Gouy-Chapman theory, which is only applicable when the charged group is located directly at the membrane-solution interface, the theory presented here allows the charge to be located at any position within the membrane.