Effect of orientational order on the decay of the fluorescence anisotropy in membrane suspensions. Experimental verification on unilamellar vesicles and lipid/alpha-lactalbumin complexes

Generalized polarization and time-resolved fluorescence provide evidence for different populations of Laurdan in lipid vesicles

The solvatochromic dye Laurdan is widely used in sensing the lipid packing of both model and biological membranes. The fluorescence emission maximum shifts from about 440 nm (blue channel) in condensed membranes (So) to about 490 nm (green channel) in the liquid-crystalline phase (Lα). Although the fluorescence intensity based generalized polarization (GP) is widely used to characterize lipid membranes, the fluorescence lifetime of Laurdan, in the blue and the green channel, is less used for that purpose. Here we explore the correlation between GP and fluorescence lifetimes by spectroscopic measurements on the So and Lα phases of large unilamellar vesicles of DMPC and DPPC. A positive correlation between GP and the lifetimes is observed in each of the optical channels for the two lipid phases. Microfluorimetric determinations on giant unilamellar vesicles of DPPC and DOPC at room temperature are performed under linearly polarized two-photon excitation to disentangle possible subpopulations of Laurdan at a scale below the optical resolution. Fluorescence intensities, GP and fluorescence lifetimes depend on the angle between the orientation of the linear polarization of the excitation light and the local normal to the membrane of the optical cross-section. This angular variation depends on the lipid phase and the emission channel. GP and fluorescence intensities in the blue and green channel in So and in the blue channel in Lα exhibit a minimum near 90o. Surprisingly, the intensity in the green channel in Lα reaches a maximum near 90o. The fluorescence lifetimes in the two optical channels also reach a pronounced minimum near 90o in So and Lα, apart from the lifetime in the blue channel in Lα where the lifetime is short with minimal angular variation. To our knowledge, these experimental observations are the first to demonstrate the existence of a bent conformation of Laurdan in lipid membranes, as previously suggested by molecular dynamics calculations.

Dual photoisomerization mechanism of azobenzene embedded in a lipid membrane

The photoisomerization of chromophores embedded in biological environments is of high importance for biomedical applications, but it is still challenging to define the photoisomerization mechanism both experimentally and computationally. We present here a computational study of the azobenzene molecule embedded in a DPPC lipid membrane, and assess the photoisomerization mechanism by means of the quantum mechanics/molecular mechanics surface hopping (QM/MM-SH) method. We observe that while the trans-to-cis isomerization is a slow process governed by a torsional mechanism due to the strong interaction with the environment, the cis-to-trans mechanism is completed in sub-ps time scale and is governed by a pedal-like mechanism in which both weaker interactions with the environment and a different geometry of the potential energy surface play a key role.

Decoding the Kinetic Pathways toward a Lipid/DNA Complex of Alkyl Alcohol Cationic Lipids Formed in a Microfluidic Channel

Complexes of cationic liposomes with DNA have emerged as promising nonviral vectors for delivering genetic information into cells for gene therapy. Kinetics of the liposome/DNA complex (lipoplex) formation on a millisecond time scale are studied by monitoring time evolution of fluorescence of 8-anilino-1-naphthalene sulfonic acid (ANS) and ethidium bromide (EtBr) in a continuous flow microfluidic channel coupled to a fluorescence microscope. The formation of lipoplexes between calf thymus DNA and liposomes based on two novel cationic lipids (Lip1810 and Lip1814) are found to follow a two-step process with kinetic constants for the Lip1814/DNA complex (k₁ = 1120-1383 s^-1, k₂ = 0.227-1.45 s^-1) being significantly different from those (k₁ = 68.53-98.5 s^-1, k₂ = 32.3-60.19 s^-1) corresponding to formation of the Lip1810/DNA complex. The kinetic pathway leading to the formation of Lip1814/DNA complex is diffusion-controlled whereas the formation of Lip1810/DNA complex occurs by a conformational rearrangement-controlled pathway. The observed difference in the kinetics of lipoplex formation likely originates from different structures of the lipid/DNA complexes.

Deciphering the response of asymmetry in the hydrophobic chains of novel cationic lipids towards biological function

Cationic liposomes, a type of non-viral vectors, often play the important biological function of delivering nucleic acids during cell transfection. Variations in the molecular architecture of di-alkyl dihydroxy ethyl ammonium chloride-based cationic lipids involving hydrophobic tails have been found to influence their biological function in terms of cell transfection efficiency. For example, liposomes based on a cationic lipid (Lip1814) with asymmetry in the hydrophobic chains were found to display higher transfection efficacy in cultured mammalian cell lines than those comprising of symmetric Lip1818 or asymmetric Lip1810. The effect of variations in the molecular architecture of the cationic lipids on the biological activity of liposomes has been explored here via the photophysical studies of 8-anilino-1-naphthalenesulphonate (ANS) and Nile Red (NR) in three cationic liposomes, namely Lip1810, Lip1814 and Lip1818. Time-resolved fluorescence of ANS revealed reduced hydration at the lipid-water interface and enhanced relaxation dynamics of surface water (lipid headgroup bound water molecules) in Lip1810- and Lip1814-based liposomes in the presence of cholesterol. As the probe ANS failed to be incorporated into the lipid-water interface of Lip1818 due to the significantly high rigidity of these liposomes, no information concerning the extent of hydration of the lipid-water interface or the interfacial water dynamics could be obtained. Time-resolved polarization-gated anisotropy measurements of NR in the presence of cholesterol revealed the rigidity of the cationic liposomes to be increasing in the order of Lip1810 < Lip1814 < Lip1818. In the presence of cholesterol, moderately higher rigidity, reduced membrane hydration and enhanced relaxation dynamics of the interfacial water molecules gave rise to the superior cell transfection efficacy of Lip1814-based cationic liposomes than those of the highly flexible Lip1810 or the highly rigid Lip1818.

Behavior of the DPH fluorescence probe in membranes perturbed by drugs

1,6-Diphenyl-1,3,5-hexatriene (DPH) is one of the most commonly used fluorescent probes to study dynamical and structural properties of lipid bilayers and cellular membranes via measuring steady-state or time-resolved fluorescence anisotropy. In this study, we present a limitation in the use of DPH to predict the order of lipid acyl chains when the lipid bilayer is doped with itraconazole (ITZ), an antifungal drug. Our steady-state fluorescence anisotropy measurements showed a significant decrease in fluorescence anisotropy of DPH embedded in the ITZ-containing membrane, suggesting a substantial increase in membrane fluidity, which indirectly indicates a decrease in the order of the hydrocarbon chains. This result or its interpretation is in disagreement with the fluorescence recovery after photobleaching measurements and molecular dynamics (MD) simulation data. The results of these experiments and calculations indicate an increase in the hydrocarbon chain order. The MD simulations of the bilayer containing both ITZ and DPH provide explanations for these observations. Apparently, in the presence of the drug, the DPH molecules are pushed deeper into the hydrophobic membrane core below the lipid double bonds, and the probe predominately adopts the orientation of the ITZ molecules that is parallel to the membrane surface, instead of orienting parallel to the lipid acyl chains. For this reason, DPH anisotropy provides information related to the less ordered central region of the membrane rather than reporting the properties of the upper segments of the lipid acyl chains.

Orientational distribution of DPH in lipid membranes: a comparison of molecular dynamics calculations and experimental time-resolved anisotropy experiments

The behavior of the fluorescent probe diphenylhexatriene (DPH) in different lipid phases is investigated. The rotational autocorrelation functions are calculated in order to model the time-resolved fluorescence anisotropy decay. The role of the order parameters is discussed.

Atomistic Picture of Fluorescent Probes with Hydrocarbon Tails in Lipid Bilayer Membranes: An Investigation of Selective Affinities and Fluorescent Anisotropies in Different Environmental Phases

By reverting to spectroscopy, changes in the biological environment of a fluorescent probe can be monitored and the presence of various phases of the surrounding lipid bilayer membranes can be detected. However, it is currently not always clear in which phase the probe resides. The well-known orange 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbo-cyanine perchlorate (DiI-C18(5)) fluorophore, for instance, and the new, blue BODIPY (4,4-difluoro-4-bora-3 a,4 a-diaza- s-indacene) derivative were experimentally seen to target and highlight identical parts of giant unilamellar vesicles of various compositions, comprising mixtures of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), sphingomyelin (SM), and cholesterol (Chol). However, it was not clear which of the coexisting membrane phases were visualized (Bacalum et al., Langmuir. 2016, 32, 3495). The present study addresses this issue by utilizing large-scale molecular dynamics simulations and the z-constraint method, which allows evaluating Gibbs free-energy profiles. The current calculations give an indication why, at room temperature, both BODIPY and DiI-C18(5) probes prefer the gel (S_o) phase in DOPC/DPPC (2:3 molar ratio) and the liquid-ordered (L_o) phase in DOPC/SM/Chol (1:2:1 molar ratio) mixtures. This study highlights the important differences in orientation and location and therefore in efficiency between the probes when they are used in fluorescence microscopy to screen various lipid bilayer membrane phases. Dependent on the lipid composition, the angle between the transition-state dipole moments of both probes and the normal to the membrane is found to deviate clearly from 90°. It is seen that the DiI-C18(5) probe is located in the headgroup region of the SM/Chol mixture, in close contact with water molecules. A fluorescence anisotropy study also indicates that DiI-C18(5) gives rise to a distinctive behavior in the SM/Chol membrane compared to the other considered membranes. The latter behavior has not been seen for the studied BODIPY probe, which is located deeper in the membrane.

Diphenylhexatriene membrane probes DPH and TMA-DPH: A comparative molecular dynamics simulation study

Fluorescence spectroscopy and microscopy have been utilized as tools in membrane biophysics for decades now. Because phospholipids are non-fluorescent, the use of extrinsic membrane probes in this context is commonplace. Among the latter, 1,6-diphenylhexatriene (DPH) and its trimethylammonium derivative (TMA-DPH) have been extensively used. It is widely believed that, owing to its additional charged group, TMA-DPH is anchored at the lipid/water interface and reports on a bilayer region that is distinct from that of the hydrophobic DPH. In this study, we employ atomistic MD simulations to characterize the behavior of DPH and TMA-DPH in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and POPC/cholesterol (4:1) bilayers. We show that although the dynamics of TMA-DPH in these membranes is noticeably more hindered than that of DPH, the location of the average fluorophore of TMA-DPH is only ~3-4Å more shallow than that of DPH. The hindrance observed in the translational and rotational motions of TMA-DPH compared to DPH is mainly not due to significant differences in depth, but to the favorable electrostatic interactions of the former with electronegative lipid atoms instead. By revealing detailed insights on the behavior of these two probes, our results are useful both in the interpretation of past work and in the planning of future experiments using them as membrane reporters.

A Blue-Light-Emitting BODIPY Probe for Lipid Membranes

Here we describe a new BODIPY-based membrane probe (1) that provides an alternative to dialkylcarbocyanine dyes, such as DiI-C18, that can be excited in the blue spectral region. Compound 1 has unbranched octadecyl chains at the 3,5-positions and a meso-amino function. In organic solvents, the absorption and emission maxima of 1 are determined mainly by solvent acidity and dipolarity. The fluorescence quantum yield is high and reaches 0.93 in 2-propanol. The fluorescence decays are well fitted with a single-exponential in pure solvents and in small and giant unilamellar vesicles (GUV) with a lifetime of ca. 4 ns. Probe 1 partitions in the same lipid phase as DiI-C18(5) for lipid mixtures containing sphingomyelin and for binary mixtures of dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC). The lipid phase has no effect on the fluorescence lifetime but influences the fluorescence anisotropy. The translational diffusion coefficients of 1 in GUVs and OLN-93 cells are of the same order as those reported for DiI-C18. The directions of the absorption and emission transition dipole moments of 1 are calculated to be parallel. This is reflected in the high steady-state fluorescence anisotropy of 1 in high ordered lipid phases. Molecular dynamic simulations of 1 in a model of the DOPC bilayer indicate that the average angle of the transition moments with respect to membrane normal is ca. 70°, which is comparable with the value reported for DiI-C18.

Molecular dynamics simulation of a dye molecule in the interior of a bilayer: 1,6-diphenyl-1,3,5-hexatriene in dipalmitoylphosphatidylcholine

A molecular dynamics simulation was carried out for a dipalmitoylphosphatidylcholine (DPPC) membrane in its liquid crystalline state containing different concentrations of the dye molecule 1,6-diphenyl-1,3,5-hexatriene (DPH). From a numerical analysis of the trajectories, we obtained information concerning structural changes of the membrane due to the presence of the probe and some hydrodynamic information concerning the probe itself. The hydrodynamic properties regarding dye molecules that have been reported in this article are: rotational and translational diffusion coefficient and relaxation times. From this analysis, we estimated a range of values of 0.6-0.9 cP for the micro-viscosity in the mid-membrane. These simulations also afforded us some information regarding structural changes in the membrane as a consequence of the presence of the fluorescent dyes at different concentrations. Thus, the disorder inside the membrane, the surface area per lipid and thickness of the membrane were also investigated.

Combination of Polarized TIRF and ATR Spectroscopies for Determination of the Second and Fourth Order Parameters of Molecular Orientation in Thin Films and Construction of an Orientation Distribution Based on the Maximum Entropy Method

This article describes two mathematical formalisms for the determination of the second and fourth order parameters of molecular films using optical spectroscopy. Method A uses polarized total internal reflection fluorescence (TIRF) to calculate the second and fourth order parameters, {P2(cos theta)} and {P4(cos theta)}, using an independently determined value for the angle between the absorption and emission dipoles, gamma. Method B uses {P2(cos theta)} obtained from attenuated total reflectance (ATR) data, along with polarized TIRF measurements to calculate {P4(cos theta)} and {cos2 gamma}. The choice of a specific method should rely on experimental considerations. We also present a method to separate the contributions of substrate surface roughness and dipole orientation with respect to the molecular axis from the spectroscopically determined second and fourth order parameters. Finally, a maximum entropy approach for construction of an orientation distribution from order parameters is compared with the commonly used delta and Gaussian distributions.

Pseudo real-time method for monitoring of the limiting anisotropy in membranes

Data acquisition and analysis of the time-resolved fluorescence anisotropy is typically a time consuming process preventing usage of this experimental method for monitoring of time-dependent phenomena. We describe a method for pseudo real-time monitoring of the limiting fluorescence anisotropy r(infinity) allowing to track changes of the membrane order occurring on the time scale of minutes. Principle and performance of the method is demonstrated in the time domain with the time-correlated single photon counting detection. DMPC liposomes stained with 1,6-diphenyl-1,3,5-hexatriene (DPH) have been used to test influence of the diffusion membrane potential on the membrane order during the temperature-induced phase transition in DMPC membranes. It has been found that the transmembrane field of the order of -70 mV increases the phase transition temperature by about 1.5 degrees C-2 degrees C. It is proposed that the full advantage of the method can be utilized with a gated detection, which besides a faster data acquisition brings additional advantage of excitation light suppression. The method can be also used for imaging.

Orientation distribution functions based upon both (P1), (P3) order parameters and upon the four (P1) up to (P4) values: application to an electrically poled nonlinear optical azopolymer film

The most probable orientational distribution functions of rod-like polar molecules contained in a noncentrosymmetric uniaxial system are established using the first-rank and third-rank Legendre polynomials, (P1(cos theta)) and (P3(cos theta)) order parameters, and the maximum entropy method. Emphasis is put on the different domains of existence in the ((P1), (P3)) plane for the various shapes of the distributions: it is thus shown that, for any positive (P1(cos theta)) value and for decreasing (P3(cos theta)) values, the distribution function may exhibit either a distorted oblate form with an intense maximum at 0 degrees, or a three-leaved rose curve with maxima at 60 degrees, 180 degrees, and 300 degrees, and finally another markedly oblate shape with a strong maximum at 180 degrees. As an illustrative example, we have considered the azobenzene molecular orientations in an electrically poled p(DR1M) homopolymer thin film after a thermal process and several relaxation periods. We have made use not only of the (P1) and (P3) parameters determined from polarized second-harmonic generation (SHG) measurements, but also of the (P2) values extracted from UV-visible spectra and of the (P4) values adjusted according to the information entropy theory. In such a thin film with very large nonlinear properties (d33 coefficients were varying from 437.0 to 117.0 pm/V at 1064 nm) it is evidenced that a strong polar order is maintained even after a long relaxation period of 42 days. So, the distribution functions demonstrate that the poling treatment was quite efficient and they emphasize the importance in the determination of both couples of odd and even order parameters in such uniaxially oriented optical elements.

Fluorescence Anisotropy Study of Aqueous Dispersions of Block Ionomer Complexes

Block ionomer complexes (BIC) of "dual hydrophilic" block copolymers containing ionic and nonionic blocks and oppositely charged surfactants spontaneously form colloidal particles of ca. 80 nm in diameter stable in aqueous dispersions at every composition of the mixture. Packing and dynamics of aliphatic groups of the surfactant in BIC were examined by using the quenching-resolved fluorescence anisotropy (QRFA) method with 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. The values of the order parameter and rotational relaxation time in the BIC were higher than those in the surfactant micelles. Incorporation of aliphatic alcohols in the BIC decreased the order parameter and increased the rotational relaxation time. The effects on the order parameter were explained by changes in the surfactant aliphatic group conformation to "fill the gaps" induced by insertion of shorter alcohol molecules. The effects on the relaxation time were attributed to a decrease in repulsion of the surfactant headgroups and expulsion of water from the BIC hydrophobic interior as evidenced by the decrease in micropolarity. The results of this study have implications for potential use of the BIC in pharmaceutics and other fields.

Short-lived fluorescence component of DPH reports on lipid--water interface of biological membranes

Fluorescence measurements of 1,6-diphenyl-1,3,5-hexatriene (DPH) in large unilamellar phospholipid vesicles were performed to characterize the influence of the membrane physical properties on the short-lived lifetime component of the fluorescence decay. We have found that the short-lived component of DPH significantly shortens when the membrane undergoes a temperature-induced phase transition as it is known for the long-lived component of DPH. We induced membrane phase transitions also by alcohols, which are reported to be distributed different way in the membrane--ethanol close to the membrane-water interface and benzyl alcohol in the membrane core. A different effect of the respective alcohol on the short and long decay component was observed. Both the time-resolved fluorescence spectra of DPH taken during lipid vesicle staining and the lifetime dependences caused by changes of temperature and/or induced by the alcohols show that the short-lived fluorescence originates from the population of dye molecules distributed at the membrane-water interface.

Correlation of membrane order and dynamics derived from time-resolved fluorescence measurements with solute permeability

The relevance of order and dynamics of phospholipid bilayer membranes as detected with fluorescent probe molecules embedded in the membranes for describing their permeability properties was studied. Order parameters (S) and rotational diffusion coefficients (Dperpendicular) of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) in unilamellar vesicles were determined by time-resolved fluorescence spectroscopy. Vesicles consisting of combinations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine, 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine, egg sphingomyelin and cholesterol were studied at 288, 298, and 308 degrees K. Permeability coefficients (P) of the model permeant D-[14C]mannitol were determined. A model was proposed for correlating P with both S and Dperpendicular, where S is linked to the average free surface area per lipid molecule and Dperpendicular reflects lipid thermal motion and, thus, redistribution rate of free surface area of the bilayer. P values ranging from 0.9 to 12.4 x 10(-11) cm/s were well described by the model. This supports the notion that permeation depends on membrane structural and dynamic properties. While changes in both S and Dperpendicular, at relative significance varying with the situation, appeared responsible for the effect of lipid composition on permeability, the effect of temperature on P was related primarily to Dperpendicular. P correlated better with S and Dperpendicular obtained with TMA-DPH rather than DPH. The location of the fluorescent probe molecules within the membranes is discussed as the cause for this difference.

Quantitative analysis of the fluorescence properties of intrinsically fluorescent proteins in living cells

The main potential of intrinsically fluorescent proteins (IFPs), as noninvasive and site-specific markers, lies in biological applications such as intracellular visualization and molecular genetics. However, photophysical studies of IFPs have been carried out mainly in aqueous solution. Here, we provide a comprehensive analysis of the intracellular environmental effects on the steady-state spectroscopy and excited-state dynamics of green (EGFP) and red (DsRed) fluorescent proteins, using both one- and two-photon excitation. EGFP and DsRed are expressed either in the cytoplasm of rat basophilic leukemia (RBL-2H3) mucosal mast cells or anchored (via LynB protein) to the inner leaflet of the plasma membrane. The fluorescence lifetimes (within approximately 10%) and spectra in live cells are basically the same as in aqueous solution, which indicate the absence of both IFP aggregation and cellular environmental effects on the protein folding under our experimental conditions. However, comparative time-resolved anisotropy measurements of EGFP reveal a cytoplasmic viscosity 2.5 +/- 0.3 times larger than that of aqueous solution at room temperature, and also provide some insights into the LynB-EGFP structure and the heterogeneity of the cytoplasmic viscosity. Further, the oligomer configuration and internal depolarization of DsRed, previously observed in solution, persists upon expression in these cells. DsRed also undergoes an instantaneous three-photon induced color change under 740-nm excitation, with efficiently nonradiative green species. These results confirm the implicit assumption that in vitro fluorescence properties of IFPs are essentially valid for in vivo applications, presumably due to the beta-barrel protection of the embodied chromophore. We also discuss the relevance of LynB-EGFP anisotropy for specialized domains studies in plasma membranes.

Oxidation of unsaturated phospholipids in membrane bilayer mixtures is accompanied by membrane fluidity changes

Steady-state and time-resolved fluorescence spectroscopy has been used to obtain information on oxidation processes and associated dynamical and structural changes in model membrane bilayers made from single unilamellar vesicles (SUV's) of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) mixed with increasing amounts of 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (SAPC). The highly unsaturated arachidonoyl chain containing four double bonds is prone to oxidation. Lipid oxidation was initiated chemically by a proper oxidant and could be followed on line via the fluorescence changes of an incorporated fluorescent lipophilic fatty acid: 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (BP-C11). The oxidation rate increases with an increasing amount of SAPC. Size measurements of different SUV's incorporated with a trace amount of a phosphatidylcholine analogue of BP-C11 using fluorescence correlation spectroscopy have demonstrated that an increase of lipid unsaturation results in smaller sized SUV's and therefore to a larger curvature of the outer bilayer leaflet. This suggests that the lipid-lipid spacing has increased and that the unsaturated fatty acyl chains are better accessible for the oxidant. Oxidation results in some characteristic physical changes in membrane dynamics and structure, as indicated by the use of specific fluorescence probes. Fluorescence measurements of both dipyrenyl- and diphenylhexatriene-labelled PC introduced in non-oxidised and oxidised DOPC-SAPC membranes clearly show that the microfluidity (local fluidity at the very site of the probes) significantly decreases when the oxidised SAPC content increases in the lipid mixture. A similar effect is observed from the lateral diffusion experiments using monopyrenyl PC in the same membrane systems: the lateral diffusion is distinctly slower in oxidised membranes.

Location and orientation of DODCI in lipid bilayer membranes: effects of lipid chain length and unsaturation

The location and orientation of a linear dye molecule, DODCI, in lipid bilayer membrane were determined by the effect of viscosity and refractive index of the aqueous medium on the fluorescence properties of the dye bound to the membrane. The membrane-bound dye is solubilized in two sites, one near the surface (short fluorescence lifetime) and another in the interior of the membrane (long lifetime). The ratio of the dye in the two locations and the orientation of the dye (parallel or perpendicular to the membrane) are sensitive to the lipid chain length and unsaturation in the alkyl chain. The fraction of the dye in the interior region is higher for short alkyl chains (C12>C14>C16>>C18C20) and in unsaturated lipids (C14:1>C14:0, C16:1>C16:0). These experimental results are consistent with the general principle that the penetration of an amphiphilic organic molecule in the interior region of the membrane is more when the structure of th bilayer is more fluid-like.

Simulation of the distribution and diffusion of a rigid amphipathic particle embedded in a model membrane

We simulate, by Brownian dynamics, the distribution, orientation and diffusion of a rigid molecule, represented as a dumbbell, with amphipathic nature, embedded in a model membrane. The significant features of a biological membrane are reproduced by means of a Maier-Saupe orienting potential, an enclosing potential and a lipophobic potential. We also evaluate the equilibrium quantities, such as order parameter, and dynamic features, such as rotational or translational diffusivity, of the embedded molecule in terms of the system parameters and compare the obtained results with those obtained from model independent theory.

Ligand-dependent conformational equilibria of serum albumin revealed by tryptophan fluorescence quenching

Ligand-dependent structural changes in serum albumin are suggested to underlie its role in physiological solute transport and receptor-mediated cellular selection. Evidence of ligand-induced (oleic acid) structural changes in serum albumin are shown in both time-resolved and steady-state fluorescence quenching and anisotropy measurements of tryptophan 214 (Trp214). These studies were augmented with column chromatography separations. It was found that both the steady-state and time-resolved Stern-Volmer collisional quenching studies of Trp214 with acrylamide pointed to the existence of an oleate-dependent structural transformation. The bimolecular quenching rate constant of defatted human serum albumin, 1.96 x 10(9) M-1 s-1, decreased to 0.94 x 10(9) M-1 s-1 after incubation with oleic acid (9:1). Furthermore, Stern-Volmer quenching studies following fractionation of the structural forms by hydrophobic interaction chromatography were in accordance with this interpretation. Time-resolved fluorescence anisotropy measurements of the Trp214 residue yielded information of motion within the protein together with the whole protein molecule. Characteristic changes in these motions were observed after the binding of oleate to albumin. The addition of oleate was accompanied by an increase in the rotational diffusion time of the albumin molecule from approximately 22 to 33.6 ns. Within the body of the protein, however, the rotational diffusion time for Trp214 exhibited a slight decrease from 191 to 182 ps and was accompanied by a decrease in the extent of the angular motion of Trp214, indicating a transition after oleate binding to a more spatially restricted but less viscous environment.

Analysis of the fluorescence anisotropy of labelled membranes submitted to a shear stress

Human erythrocyte membranes are elastic and undergo a deformation under shear stress. The phenomenon has been analysed by recording the fluorescence anisotropy of labelled isolated membranes. A model has been developed which assumes an orientation correlation function of a molecular probe incorporated in an elongated membrane. This model has been successfully used to analyse quantitatively data obtained with (1-trimethylamino)-(1,6-diphenyl)-1,3,5-hexatriene (TMA-DPH) and 6-(9-anthroyloxy)-stearic acid (6-AS). In agreement with the model, the effect of the membrane deformation is opposite for these two probes, which corroborates the concept that the alteration of the fluorescence anisotropy reflects mainly the deformation of the membrane and not the rotational freedom of the molecular probe.

Molecular order and dynamics in bilayers consisting of highly polyunsaturated phospholipids

The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic bilayer structural properties of symmetrically substituted phosphatidylcholines (PCs) with acyl chains containing no, one, four, or six double bonds and mixed-chain phosphatidylcholines with a saturated sn-1 chain and one, four, or six double bonds in the sn-2 chain. Both the Brownian rotational diffusion (BRD) model and the wobble-in-cone model were fit to all differential polarization data, and the descriptions of the data provided by the BRD model were found to be statistically superior. Global analysis of differential polarization data revealed two statistically equivalent solutions. The solution corresponding to a bimodal orientational distribution function, f(theta), was selected based on the effects of temperature on f(theta) and previous measurements on fixed, oriented bilayers. The overall equilibrium acyl chain order in these bilayers was analyzed by comparing the orientational probability distribution for DPH, f(theta) sin theta, with a random orientational distribution. Orientational order decreased and probe dynamics increased in mixed-chain species as the unsaturation of the sn-2 chain was increased. The degree of orientational order dropped dramatically in the dipolyunsaturated species compared with the mixed-chain phosphatidylcholines, which contained a polyunsaturated sn-2 chain. In terms of both orientational order and probe dynamics, the differences between the highly polyunsaturated species and the monounsaturated species were much greater than the differences between the monounsaturated species and a disaturated PC.

Intrinsic fluorescence properties and structural analysis of p13(suc1) from Schizosaccharomyces pombe

p13(suc1) acts in the fission yeast cell division cycle as a component of p34(cdc2). In the present work, structural information contained in the intrinsic fluorescence of p13(suc1) has been extracted by steady-state and time-resolved fluorescence techniques. In its native form, the steady-state emission spectrum of p13(suc1) is centered at 336 nm. Upon denaturation by guanidine HCl (4.0 M), the emission spectrum is shifted to 355-360 nm and the fluorescence intensity decreases 70%. The same changes are not obtained with p13(suc1) at 56 degrees C or after incubation at 100 degrees C, and the protein appears to be substantially temperature-stable. The fluorescence decay of p13(suc1) is best described by three discrete lifetimes of 0.6 ns (tau1), 2.9 ns (tau2), and 6.1 ns (tau3), with amplitudes that are dependent on the native or unfolded state of the protein. Under native conditions, the two predominant decay-associated spectra, DAS-tau2 (lambdamax = 332 nm) and DAS-tau3 (lambdamax = 340 nm), derive from two different excitation DAS. Moreover distinct quenching mechanisms and collisional accessibilities (kq(tau2)>>kq(tau3)) are resolved for each lifetime. An interpretation in terms of specific tryptophan residue (or protein conformer)-lifetime assignments is presented. The decay of the fluorescence anisotropy of native p13(suc1) is best described by a double exponential decay. The longer correlation time recovered (9 ns </= phi2 </= 15ns) can be associated with the rotational motion of the protein as a whole and a Stokes radius of 21.2 A has been calculated for p13(suc1). Anisotropy measurements obtained as a function of temperature indicate that, in solution, the protein exists exclusively as a prolate monomer. In 1 mM zinc, changes of the anisotropy decay parameters are compatible with subunits oligomerization.

New fluorescent octadecapentaenoic acids as probes of lipid membranes and protein-lipid interactions

The chemical and spectroscopic properties of the new fluorescent acids all(E)-8, 10, 12, 14, 16-octadecapentaenoic acid (t-COPA) and its (8Z)-isomer (c-COPA) have been characterized in solvents of different polarity, synthetic lipid bilayers, and lipid/protein systems. These compounds are reasonably photostable in solution, present an intense UV absorption band (epsilon(350 nm) approximately 10(5) M(-1) cm(-1)) strongly overlapped by tryptophan fluorescence and their emission, centered at 470 nm, is strongly polarized (r(O) = 0.385 +/- 0.005) and decays with a major component (85%) of lifetime 23 ns and a faster minor one of lifetime 2 ns (D,L-alpha-dimyristoylphosphatidylcholine (DMPC), 15 degrees C). Both COPA isomers incorporate readily into vesicles and membranes (K(p) approximately 10(6)) and align parallel to the lipids. t-COPA distributes homogeneously between gel and fluid lipid domains and the changes in polarization accurately reflect the lipid T(m) values. From the decay of the fluorescence anisotropy in spherical bilayers of DMPC and POPC it is shown that t-COPA also correctly reflects the lipid order parameters, determined by 2H NMR techniques. Resonance energy transfer from tryptophan to the bound pentaenoic acid in serum albumin in solution, and from the tryptophan residues of gramicidin in lipid bilayers also containing the pentaenoic acid, show that this probe is a useful acceptor of protein tryptophan excitation, with R(O) values of 30-34 A.

Distribution and diffusivity of a hydrophobic probe molecule in the interior of a membrane: theory and simulation

We propose a simple model for the distribution of position and orientation and the diffusion of a hydrophobic probe molecule embedded in a membrane. The molecule experiences both a Maier-Saupe orienting potential as well as an enclosing potential of repulsion from the membrane walls. A statistical thermodynamics treatment of the model provides predictions of the location and orientation of the molecule within the membrane. In particular, we evaluate the order parameter of the molecule in terms of the model constants. The diffusivity of the probe is studied by Brownian dynamics simulation. For rotational diffusion, we check an available analytical approximate treatment that allows for the prediction of the dynamics in terms of equilibrium quantities. We also pay attention to quantities related to the initial and mean reorientational rate of the probe. For translational diffusion, we use the simulation results to analyze some general aspects of lateral and transversal diffusion.

Membrane-bound states of alpha-lactalbumin: implications for the protein stability and conformation

alpha-Lactalbumin (alpha-LA) associates with dimyristoylphosphatidylcholine (DMPC) or egg lecithin (EPC) liposomes. Thermal denaturation of isolated DMPC or EPC alpha-LA complexes was dependent on the metal bound state of the protein. The intrinsic fluorescence of thermally denatured DMPC-alpha-LA was sensitive to two thermal transitions: the Tc of the lipid vesicles, and the denaturation of the protein. Quenching experiments suggested that tryptophan accessibility increased upon protein-DMPC association, in contrast with earlier suggestions that the limited emission red shift upon association with the liposome was due to partial insertion of tryptophan into the apolar phase of the bilayer (Hanssens I et al., 1985, Biochim Biophys Acta 817:154-166). On the other hand, above the protein transition (70 degrees C), the spectral blue shifts and reduced accessibility to quencher suggested that tryptophan interacts significantly with the apolar phase of either DMPC and EPC. At pH 2, where the protein inserts into the bilayer rapidly, the isolated DMPC-alpha-LA complex showed a distinct fluorescence thermal transition between 40 and 60 degrees C, consistent with a partially inserted form that possesses some degree of tertiary structure and unfolds cooperatively. This result is significant in light of earlier findings of increased helicity for the acid form, i.e., molten globule state of the protein (Hanssens I et al., 1985, Biochim Biophys Acta 817:154-166). These results suggest a model where a limited expansion of conformation occurs upon association with the membrane at neutral pH and physiological temperatures, with a concomitant increase in the exposure of tryptophan to external quenchers; i.e., the current data do not support a model where an apolar, tryptophan-containing surface is covered by the lipid phase of the bilayer.

A role for phospholipid polyunsaturation in modulating membrane protein function

Visual transduction is one of the best characterized G protein--coupled signalling systems. In addition, about 50% of the disk membrane phospholipid acyl chains are 22:6n-3, making this system ideal for determining the role of polyunsaturation in modulating membrane-signalling systems. The extent of formation of metarhodopsin II (MII), the G protein--activating photointermediate of rhodopsin, was studied in phospholipid vesicles composed of a variety of phosphatidylcholines, differing in their acyl chain composition at the sn-2 position. The amount of MII formed increased progressively with the level of acyl chain unsaturation at the sn-2 position. The effect of added cholesterol was to reduce the amount of MII formed. The acyl chain packing free volume of the rhodopsin containing lipid vesicles was characterized by a fractional volume parameter fv derived from measurements of the time-resolved fluorescence anisotropy decay of the hydrophobic membrane probe 1,6-diphenyl-1,3,5-hexatriene. The relationship among sn-2 acyl chain unsaturation, cholesterol content, and MII formation is explained on the basis of variation in fv with bilayer lipid composition and a novel model for the packing of phospholipids containing polyenoic acyl chains, such as 22:6n-3.

Reorientational properties of fluorescent analogues of the protein kinase C cofactors diacylglycerol and phorbol ester

The reorientational properties of the fluorescently labelled protein kinase C (PKC) cofactors diacylglycerol (DG) and phorbol ester (PMA) in vesicles and mixed micelles have been investigated using time-resolved polarised fluorescence. The sn-2 acyl chain of DG was replaced by diphenylhexatriene- (DPH) propionic acid, while a dansyl labelled analogue of phorbol ester was used. The extent of ordering of DPH-DG in vesicles turned out to be slightly different from that of the control choline lipid DPH-PC. Addition of PKC to vesicles containing 30 mole% brain PS considerably slowed down the DPH-DG anisotropy decay. This was not observed when DPH-DG was replaced by DPH-PC. Analysis of the fluorescence anisotropy decays of these DPH-lipids in micelles polyoxyethylene-9-laurylether mixed with 10 mole% of the essential phosphatidylserine allowed estimation of their lateral diffusion, orientation distribution and reorientational dynamics within the micelles. Addition of PKC resulted in a significantly slower decay of the fluorescence anisotropy of both DPH-DG and DPH-PC even in the absence of calcium, indicating a calcium independent complexation of PKC with the PS containing micelles. Addition of calcium resulted in a further reduction of the decay of anisotropy of DPH-DG but not of DPH-PC indicating that the Ca2+ dependent immobilisation is cofactor-specific. Similar specific interactions with PKC resulted in a slower decay of dansylated PMA when calcium and PS were present.

Cholesterol effect on the physical state of lipid multibilayers from the platelet plasma membrane by time-resolved fluorescence

There are indications that the plasma membrane lipid composition and, in particular, the cholesterol/phospholipid (C/PL) ratio, affects platelet function. As a first approximation to the molecular characterization of the effect of cholesterol on the order, fluidity and lateral heterogeneity of the platelet plasma membrane, the steady-state and time-resolved fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH) and trans-parinaric acid (tPnA) has been studied in multibilayer vesicles of phospholipids extracted from human platelet plasma membrane with different cholesterol/phospholipid molar ratios modified in vitro from 0.07 to 0.9. The DPH studies show that the increased presence of cholesterol has a stronger effect on the order than on the fluidity of the bilayer, as has been previously observed in other lipid membranes. On the other hand, from the analysis of the fluorescence kinetics of tPnA we conclude that a higher cholesterol content gives rise to an increase of the heterogeneity of the bilayer, due to a larger fraction of solid-like lipid domains. These domains contain a cholesterol concentration much higher than the macroscopic average value.