Diffusion in two dimensions: Comparison between diffusional fluorescence quenching in phospholipid vesicles and in isotropic solution

Fluorescence decay of 1-methylpyrene in small unilamellar l-alpha-dimyristoylphosphatidylcholine vesicles. A temperature and concentration dependence study

The fluorescence decay kinetics of 1-methylpyrene in small unilamellar l-alpha-dimyristoylphosphatidylcholine vesicles above the phase transition temperature has been studied as a function of concentration and temperature. When the 1-methylpyrene/phospholipid ratio equals 1:2000 no excimer is observed and the fluorescence decay is monoexponential. When this ratio is equal to or higher than 1 200, excimer is observed and the monomer and excimer decays can be adequately described by two exponential terms. The deviation of the monomer decays from monoexponentiality cannot be described by a model where the diffusion-controlled excimer formation is time dependent. The observed decays are compatible with the excimer formation scheme which is valid in an isotropic medium. The activation energy of excimer formation is found to be 29-9 +/-1.4 kJ mol . The (apparent) excimer formation constant and the excimer lifetime at different temperatures have been determined. The diffusion coefficient associated with the excimer formation process varies between 2 x 10(-10) m(2)/s at 70 degrees C to 4 x 10(-11) m(2)/s at 25 degrees C.

Kinetics of bimolecular reactions in model bilayers and biological membranes. A critical review

The quantitative study of the probability of molecular encounters giving rise to a reaction in membranes is a challenging discipline. Model systems, model in the sense that they use model bilayers and model reactants, have been widely used for this purpose, but the methodologies employed for the analysis of the results obtained in experiments, and for experimental design, are so disparate that a concerned experimentalist has difficulty in deciding about the value of each approach. This review intends to examine the several approaches that can be found in the literature showing, when feasible, the weakness, strengths and limits of application of each of them. There is not, so far, a full experimental validation of the most promising theories for the analysis of reactions in two dimensions, what leaves open a large field for new research. The major challenge resides in the time range in which the processes take place, but the possibilities of the existing techniques for these studies are far from exhausted. We review also the attempts of several authors to quantitatively analyze the kinetics of reactions in biological membranes. Especially in this field, the recently developed microspectroscopies enclose a still unexplored potential.

Diffusion of α-Tocopherol in Membrane Models: Probing the Kinetics of Vitamin E Antioxidant Action by Fluorescence in Real Time

The new fluorescent membrane probe Fluorazophore-L, a lipophilic derivative of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene, is employed to study the quenching of alpha-tocopherol (alpha-Toc) by time-resolved fluorescence in the microheterogeneous environments of Triton XR-100 and SDS micelles, as well as POPC liposomes. Fluorazophore-L has a small nonaromatic fluorescent polar headgroup and an exceedingly long-lived fluorescence (e.g., 140 ns in aerated SDS micelles), which is efficiently quenched by alpha-Toc (3.9 x 10(9) M(-1) s(-1) in benzene). Based on solvatochromic effects and the accessibility by water-soluble quenchers, the reactive headgroup of Fluorazophore-L, along with the chromanol group of alpha-Toc, resides at the water-lipid interface, which allows for a diffusion-controlled quenching in the lipidic environments. The quenching experiments represent an immobile or stationary case; that is, interparticle probe or quencher exchange during the excited-state lifetime is insignificant. Different quenching models are used to characterize the dynamics and antioxidant action of alpha-Toc in terms of diffusion coefficients or, where applicable, rate constants. The ideal micellar quenching model is suitable to describe the fluorescence quenching in SDS micelles and affords a pseudo-unimolecular quenching rate constant of 2.4 (+/- 0.4) x 10(7) s(-1) for a single quencher per micelle along with a mean aggregation number of 63 +/- 3. In Triton micelles as well as in unilamellar POPC liposomes, a two-dimensional (lateral) diffusion model is most appropriate. The mutual lateral diffusion coefficient D(L) for alpha-Toc and Fluorazophore-L in POPC liposomes is found to be 1.8 (+/- 0.1) x 10(-7) cm(2) s(-1), about a factor of 2 larger than for mutual diffusion of POPC, but more than 1 order of magnitude lower than a previously reported value. The comparison of the different environments suggests a quenching efficiency in the order benzene >> SDS micelles > Triton micelles > POPC liposomes, in line with expectations from microviscosity. The kinetic measurements provide important benchmark values for the modeling of oxidative stress in membranes and other lipidic assemblies. The special case of small lipidic assemblies (SDS micelles), for which the net antioxidant efficacy of alpha-Toc may be lower than expected on the grounds of its diffusional behavior, is discussed.

Annexin V interaction with phosphatidylserine-containing vesicles at low and neutral pH

Annexin V belongs to a class of Ca2+-binding proteins for which different functions in the cell are discussed, e.g., involvement in exocytosis, inhibition of protein kinase C, and calcium channel activity in cartilage matrix vesicles. All these functions are related to the ability of the annexins to bind to acidic phospholipids. In this study, the interaction of annexin V with large unilamellar vesicles (LUV) prepared from phosphatidylserine (PS) at low pH was compared to that at neutral pH. Annexin V strongly binds to PS LUV at low pH, whereas at neutral pH 20-100 microM Ca2+ are required to induce binding. This is caused by the different electric charge of the protein. The binding ability of the PS LUV for annexin V is higher at low pH. Binding of annexin V induces dehydration of the vesicle surface and a decrease of the lateral diffusion within the bilayer. While this dehydration is due to vesicle contact at pH 4, at pH 7.4 it is due to surface covering by annexin V. Annexin V promotes the phospholipid intermixing between LUVs at pH 5 and below but inhibits it at pH 7.4. A substitution of up to 80% of the PS by the uncharged phosphatidylcholine does not impair the extent of phospholipid intermixing at pH 4. The high binding capacity of PS LUV, the disappearance of the inhibiting action, and a calculated increase of the annexin V hydrophobicity make it likely that annexin V is able to penetrate into the membrane at low pH. At neutral pH, annexin V molecules act as steric barriers, preventing close apposition of two vesicles. At pH 5, annexin V lowers the threshold concentration of the Ca2+-induced phospholipid intermixing. Such a promotion is well-known for annexin VII (synexin). The effect may be related to the isoelectric points of the two annexins which have been reported as 4.8 (annexin V) and 7.0 (annexin VII), respectively.

Fluorescence study of a derivatized diacylglycerol incorporated in model membranes

A fluorescence study of a diacylglycerol derivatized with the n-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) chromophore (NBD) was carried out. Fluorescence self-quenching was observed for this probe in lecithin model membranes due to collisional interaction rather than to an aggregational behaviour of the probe. The efficient energy migration (Ro = 28 A) of the NBD fluorophore was studied via the fluorescence depolarization upon increase of probe concentration in membranes, and the results are compared with a model where a random distribution of the probes is assumed. A surface location of the chromophore was concluded for the NBD derivative of diacylglycerol, both from the fluorescence parameters and from the study of its fluorescence quenching by spin label probes. Very high lateral diffusion coefficients were obtained for these probes, both from the self-quenching (D = 2-6 x 10(-6) cm2 s-1) and from the spin probe quenching (D = 3.5 x 10(-6) cm2 s-1) studies. A concomitant fluorescence study of the related probe NBD-phosphatidylcholine revealed that its photophysical behaviour is similar to the derivatized diacylglycerol.

Determination of fluorescent probes localization in membranes by nonradiative energy transfer

One of the new methods of studying the structure and dimensions of biological membranes is based on the Förster's nonradiative energy transfer between special molecules, the so-called 'membrane fluorescent probes'. Further development of the approach is presented in this article. It consists of the combined use of the time-resolved and steady-state fluorescence data with subsequent computer simulation of the energy transfer in membranes. Anthracene as an energy donor, and 4-p-(dimethylamino)styryl-N-dodecylpyridinium (DSP-12) or 4-dimethylaminochalcone (DMC) as energy acceptors were bound with artificial phospholipid membrane vesicles ('liposomes'). The synchrotron radiation was used as an impulse source for the excitation light. The steady-state fluorescence data permit the area of possible probe localization in membranes to be distinguished, while the kinetic data allow them to be narrowed significantly. There is a good agreement between the obtained localization and our present-day knowledge of lipid bilayer structure. The accuracy of the method is ca. several Angströms.

Effect of some polypeptide hormones on the lateral mobility in plasma membranes of rat testes, L-cells and liver. Influence of development and ageing

1. The effect of different doses of six polypeptide hormones on pyrene diffusion in rat testes and liver plasma membranes was tested. Pyrene mobility was reduced in membranes possessing respective receptors. 2. An incubation time of 15 min of testes plasma membranes with 10(-5) M lutropin (LH) reduced pyrene and diphenyl hexatriene mobilities by 10-20%. 3. The addition of 10(-5) M LH to a suspension of intact L-cells from rats at different ages decreased the diffusion of membrane fluorescently labelled lipids and proteins by ca 60%. Diffusion was measured by fluorescence recovery after photobleaching. 4. Observed LH effects were independent of development and ageing.

Micropolarities of lipid bilayers and micelles. 5. Localization of pyrene in small unilamellar phosphatidylcholine vesicles

The excimer/monomer ratio of emission intensities (IE/IM) and the enhancement of the 0-0 vibronic transition in the fluorescence spectra of pyrene (PY) and 16-(1-pyrenyl)hexadecanoic acid (C16PY) were used to investigate the localization of PY in the bilayers of small unilamellar vesicles constituted of phosphatidylcholine (SUV-PC). First, from comparison of the fluorescence characteristics of PY in water with those of PY incorporated into the SUV-PC membranes, we concluded that the probe is incorporated preferentially in the lipid phase of the vesicles and not in the bulk aqueous phase. In addition, we found that, contrary to what happens with the pyrenyl moiety of C16PY, the location of PY varies with its relative concentration in the membrane space. The critical concentration was observed to be around 1.0 mol% of incorporated PY. At concentrations below this value, PY is located in the hydrocarbon core of the lipid bilayers. Above 1.0 mol%, the PY molecules reside preferentially in the neighbourhood of the glyceryl moiety region of the PC vesicles.

Dynamic fluorescence quenching studies on lipid mobilities in phosphatidylcholine-cholesterol membranes

Bimolecular collision rate of 8-anilinonaphthalene-1-sulfonic acid (ANS) and the nitroxide doxyl group attached to various carbons on stearic acid spin labels (n-SASL) in phosphatidylcholine-cholesterol membranes in the fluid phase was studied by observing dynamic quenching of ANS fluorescence by n-SASL's. The excited-state lifetime of ANS and its reduction by the n-SASL doxyl group were directly measured by the time-correlated single photon counting technique to observe only dynamic quenching separately from static quenching and were analyzed by using Stern-Volmer relations. The collision rate of ANS with the n-SASL doxyl group ranges between 1 X 10(7) and 6 X 10(7), and the extent of dynamic quenching by n-SASL is in the order of 5-much much greater than 6- greater than 7- less than 9- less than 10- less than 12- less than 16-SASL (less than 5-SASL) in dimyristoylphosphatidylcholine (DMPC) membranes. Collision rate of 16-SASL is only 10% less than that of 5-SASL. Since the naphthalene ring of ANS is located in the near-surface region of the membrane, these results indicate that the methyl terminal of SASL appears in the near surface area frequently, probably due to extensive gauche-trans isomerism of the methylene chain. The presence of 30 mol% cholesterol decreases the collision rate of ANS with 12- and 16-SASL doxyl groups but not with the 5-SASL doxyl group in DMPC membranes. On the other hand, in egg-yolk phosphatidylcholine membranes, inclusion of 30 mol% cholesterol does not affect the collision of ANS with either 5-SASL or 16-SASL doxyl groups, in agreement with our previous observation that alkyl chain unsaturation moderates cholesterol effects on lipid motion in the membrane (Kusumi et al., Biochim. Biophys. Acta 854, 307-317). It is suggested that dynamic quenching of ANS fluorescence by lipid-type spin labels is a useful new monitor of membrane fluidity that reports on various lipid mobilities in the membrane; a class of motion can be preferentially observed over others by selecting a proper spin label, i.e., rotational diffusion of lipid about its long axis and translational diffusion by using 5-SASL, wobbling motion of the lipid long axis by using 7-SASL or androstane spin label, and gauche-trans isomerism by using 16-SASL.

Localization of the galactoside binding site in the lactose carrier of Escherichia coli

The location of flurophores specifically bound to the lactose/H+ carrier of Escherichia coli was ascertained by the use of various collisional quenchers. The reporter groups were (1) the pyrenyl residue of N-(1-pyrenyl)maleimide attached to the essential cysteine residue 148, which is presumably at or near the galactoside binding site, and (2) the dansyl moieties of a series of fluorescent substrate molecules. The accessibility of these fluorophores from the lipid phase was assessed by nitroxyl-labelled fatty acids and phospholipids. By using a series of nitroxyl-labelled fatty acids carrying the quencher at different positions in the acyl chain, the position of a quenchable fluorophore with respect to the membrane normal can be determined. The accessibility of fluophores from the aqueous phase was assessed by using a water-soluble quencher, the N-methylpicolinium ion. The results of quenching studies suggest that the galactoside binding site is located within the carrier and that this binding site communicates with the aqueous phase through a pore.

Microenvironment changes of human blood platelet membranes associated with fibrinogen binding

Alterations in the membrane organization caused by fibrinogen binding to human blood platelets and their isolated membranes were analyzed by fluorescence and electron spin resonance measurements. The degree of fluorescent anisotropy of DPH, ANS and fluorescamine increased significantly when fibrinogen reacted with its membrane receptors. Both fluorescence and ESR analyses showed that fibrinogen binding to platelet membranes is accompanied by an increase of the membrane lipid rigidity. This effect seems to be indirect in nature and is mediated by altered membrane protein interactions. As it has been shown that an increased membrane lipid rigidity leads to a greater exposure of membrane proteins, including fibrinogen receptors, this might facilitate a formation of molecular linkages between neighboring platelets. On the other hand, changes of fluorescence anisotropy of membrane tryptophans and N-(3-pyrene) maleimide suggest the augmented mobility of the membrane proteins. Evidence is presented which indicated that the binding of fibrinogen to the membrane receptors is not accompanied by any changes in the fluorescence intensity of ANS attached to the membranes. It may suggest that the covering of platelets with fibrinogen does not influence the surface membrane charge. In contrast to fibrinogen, calcium ions caused an increase of the fluorescence intensity resulting from the more efficient binding of ANS to the platelet membranes.

Fluorescence decay kinetics of pyrene in membrane vesicles

The fluorescence decay kinetics of pyrene incorporated into artificial and natural membrane vesicles has been studied by pulse fluorimetry. The emission of monomeric pyrene and its excimer embedded in sonicated liposomes prepared from dipalmitoylphosphatidylcholine and a mixture of this phospholipid and dipalmitoylphosphatidylserine follows a multiple exponential decay law at temperatures both below and above their thermal transitions (10--48 degrees C). When pyrene is incorporated into fragmented skeletal sarcoplasmic reticulum vesicles, the emission decay exhibits similar multiple exponential character. The decay of the monomer in the phospholipid vesicles can be adequately described by three exponential terms. The experimental decays observed with both types of vesicles deviate significantly from a previously proposed model in which departure of the decay of pyrene monomer from monoexponentiality is qualitatively related to a time dependence in the diffuslipid vesicles can be adequately described by three exponential terms. The experimental decays observed with both types of vesicles deviate significantly from a previously proposed model in which departure of the decay of pyrene monomer from monoexponentiality is qualitatively related to a time dependence in the diffuslipid vesicles can be adequately described by three exponential terms. The experimental decays observed with both types of vesicles deviate significantly from a previously proposed model in which departure of the decay of pyrene monomer from monoexponentiality is qualitatively related to a time dependence in the diffusion-controlled formation of excimers from ground state and excited monomers. It is suggested that the observed decays are compatible with a reaction scheme involving excited state interaction.

Temperature dependence of the fluorescence of pyrene labeled crab nerve membranes

A method, using albumin-pyrene complexes, has been developed for labeling, in a controlled manner, crab leg nerves whose excitability was preserved. The excimer-to-monomer fluorescence intensity ratio of pyrene, embedded in nerve membrane lipids and in their crude lipid extracts, is a fluidity parameter which displayed the following features with temperatures. a--a temperature-dependent increase of fluidity b--three breaks (6 degrees, 19 degrees and 37 degrees C) in the physiological medium c--In Ca++-depleted sea water, the 37 degrees characteristic temperature vanished. These breaks may reflect some lateral phase separations of the lipid components of nerve membranes. The calcium dependent temperature break may involve a segregation of acidic phospholipids while the other two breaks (6 degrees and 19 degrees C) may be due to neutral lipids phase separation. The relationship of these findings to nerve function is discussed.

Variations in microviscosity values induced by different rotational behaviour of fluorescent probes in some aliphatic environments

Recently there have been large developments in the indirect estimation of phospholipid bilayer and membrane microviscosities through the use of fluorescent probes with the help of paraffin oils as standard solvents. Before applying this semi-empirical method to membrane systems, it seemed necessary to test: first, a large variety of probes (this has been done by many authors) and secondly, a large variety of aliphatic oils (there is little literature on these tests). The present paper shows the variations of the rotational relaxation rates of three probes in relation to the viscosities of some aliphatic oils. When changing the oil but keeping constant the macroscopic viscosity, large differences appear in the relaxation rates of a given fluorphore (a ratio of 30/1 is observed in the extreme cases). The microviscosities of membranes deduced from the probe motion will consequently exhibit large uncertainty, as is shown with dipalmitoyl phosphatidylcholine. The cause of these different behaviours must be looked for in the properties of the oil. Particularly, the anisotropy of the solute-solvent interaction in the site where the probe is located depends in part on the internal order of the solvent which is used as a reference.