Fluorescence depolarization in liquid crystals and membrane bilayers

Influence of Membrane Phase on the Optical Properties of DPH

The fluorescent molecule diphenylhexatriene (DPH) has been often used in combination with fluorescence anisotropy measurements, yet little is known regarding the non-linear optical properties. In the current work, we focus on them and extend the application to fluorescence, while paying attention to the conformational versatility of DPH when it is embedded in different membrane phases. Extensive hybrid quantum mechanics/molecular mechanics calculations were performed to investigate the influence of the phase- and temperature-dependent lipid environment on the probe. Already, the transition dipole moments and one-photon absorption spectra obtained in the liquid ordered mixture of sphingomyelin (SM)-cholesterol (Chol) (2:1) differ largely from the ones calculated in the liquid disordered DOPC and solid gel DPPC membranes. Throughout the work, the molecular conformation in SM:Chol is found to differ from the other environments. The two-photon absorption spectra and the ones obtained by hyper-Rayleigh scattering depend strongly on the environment. Finally, a stringent comparison of the fluorescence anisotropy decay and the fluorescence lifetime confirm the use of DPH to gain information upon the surrounding lipids and lipid phases. DPH might thus open the possibility to detect and analyze different biological environments based on its absorption and emission properties.

Physicochemical properties of L-alpha dipalmitoyl phosphatidylcholine large unilamellar vesicles: Effect of hydrophobic block (PLA/PCL) of amphipathic diblock copolymers

In the present work, we show how amphipathic diblock copolymers affect the physicochemical properties of the lipid bilayer of DPPC liposome. Diblock copolymers proposed for this study are focused in the difference between PLA and PCL hydrophobic block, because PLA and PCL differ in their glass transition temperature, where a higher ratio of PLA, lowers the flexibility of the diblock copolymer. On the contrary, a greater proportion of PCL makes the diblock copolymer more flexible. This flexibility difference between hydrophobic block would affect the physicochemical properties of lipid bilayer of DPPC. The difference of rigidity or flexibility of hydrophobic block and their interaction with DPPC large unilamellar vesicles (LUVs) was evaluated at low and high copolymers concentration. The copolymer concentrations used were chosen based on their respective cmc. We measure (a) Thermotropic behavior from GP of Laurdan and fluorescence anisotropy of DPH; (b) Relation between wavelength excitation and generalized polarization of Laurdan; (c) Time-resolved fluorescence anisotropy of DPH; (d) Water outflow through the lipid bilayer and (e) calcein release from DPPC LUVs. Furthermore, large unilamellar vesicles in the absence and in the presence of different copolymers were characterized by size and zeta-potential. The results show that the diblock copolymer at high PLA/PCL ratio, that is, greater rigidity of hydrophobic block produces an increase of the phase transition temperature (Tm). For DPPC LUVs, Tm increase 3.5 °C at low and about 4.5 °C at high copolymers concentration, sensed by Laurdan and DPH fluorescent probes, although the DPPC/copolymers molar ratio for Cop4 is higher than Cop3, Cop2 and Cop1. In addition, we observed a decrease in the polarity of microenvironments in the bilayer and an increase in the order of the acyl chains in the bilayer to a high proportion of PLA. Furthermore, the presence of diblock copolymer with high proportion of PLA, decreases water outflow from DPPC liposome and water efflux is slower; leading to a decrease in calcein release from DPPC liposomes. Our results clearly show that the greater the stiffness of the hydrophobic block, greater degree of packaging of the lipid bilayer, greater the order of the acyl chains, and greater retention of water and calcein inside the liposome. Therefore, the presence of AB-type diblock copolymers with a more rigid hydrophobic block, stabilizes the lipid bilayer and would allow a more controlled release of water, and encapsulated molecules inside of the DPPC liposome.

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.

Do endotoxin aggregates intercalate into phospholipid membranes in a nonspecific, hydrophobic manner?

The interaction of endotoxin aggregates with phospholipid liposomes of different composition was investigated applying fluorescence polarization spectroscopy with the fluorophore diphenylhexatriene and the resonance energy transfer technique using N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-PE and N-(Rhodamine B sulfonyl)-PE. Fluorescence polarization data at constant temperature could be interpreted in favor of an intercalation of lipopolysaccharide into phospholipid liposomes even in the absence of Ca2+. Intercalation, however, could be clearly excluded from determinations performed as a function of temperature. Experiments employing the resonance energy transfer technique clearly showed that a nonspecific, hydrophobic intercalation of endotoxin aggregates into phospholipid liposomes only takes place in the presence of excess molar concentrations of divalent cations and/or after long-term incubation at elevated temperature (37°C). These findings indicate that under (near) physiological conditions nonspecific intercalation of aggregated lipopolysaccharide into phospholipid membranes represents an unlikely event. The significance of these results for an understanding of the fundamental mechanisms of cell activation by endotoxin is discussed.

Dendrimers complexed with HIV-1 peptides interact with liposomes and lipid monolayers

AIMS

We have investigated the effect of surface charge of model lipid membranes on their interactions with dendriplexes formed by HIV-derived peptides and 2 types of positively charged carbosilane dendrimers (CBD).

METHODS

Interaction of dendriplexes with lipid membranes was measured by fluorescence anisotropy, dynamic light scattering and Langmuir-Blodgett techniques. The morphology of the complexes was examined by transmission electron microscopy.

RESULTS

All dendriplexes independent of the type of peptide interacted with model lipid membranes. Negatively charged vesicles composed of a mixture of DMPC/DPPG interacted more strongly, and it was accompanied by an increase in anisotropy of the fluorescent probe localized in polar domain of lipid bilayers. There was also an increase in surface pressure of the lipid monolayers. Mixing negatively charged liposomes with dendriplexes increased liposome size and made their surface charges more positive.

CONCLUSIONS

HIV-peptide/dendrimer complexes interact with model lipid membranes depending on their surface charge. Carbosilane dendrimers can be useful as non-viral carriers for delivering HIV-peptides into cells.

Dynamics in the interior of AOT lamellae investigated with two-dimensional infrared spectroscopy

The dynamics inside the organic regions of aerosol-OT (AOT)/water mixtures in the lamellar mesophase, bicontinuous cubic (BC) phase, and in an analogous molecule without the charged sulfonate headgroup are investigated by observing spectral diffusion, orientational relaxation and population relaxation using ultrafast two-dimensional infrared (2D IR) vibrational echo spectroscopy and IR pump-probe experiments on the asymmetric CO stretch of a vibrational probe, tungsten hexacarbonyl (W(CO)6). The water layer thickness between the bilayer planes in the lamellar phase was varied. For comparison, the dynamics of W(CO)6 in the normal liquid bis(2-ethylhexyl) succinate (EHS), which is analogous to AOT but has no charged sulfonate headgroup, were also studied. The 2D IR experiments measure spectral diffusion, which results from the structural evolution of the system. Spectral diffusion is quantified by the frequency-frequency correlation function (FFCF). In addition to a homogeneous component, the FFCFs are biexponential decays with fast and slow time components of ∼12.5 and ∼150 ps in the lamellar phase. Both components of the FFCF are independent of the number of water molecules per headgroup for the lamellae, but they slow somewhat in the BC phase. The dynamics in the ordered phases are in sharp contrast to the dynamics in EHS, which displays fast and slow components of the FFCF of 5 and 80 ps, respectively. As the hydration level of AOT increases, vibrational lifetime decreases, suggesting some change in the local environment of W(CO)6 with water content.

Differential dynamic and structural behavior of lipid-cholesterol domains in model membranes

Changes in the cholesterol (Chol) content of biological membranes are known to alter the physicochemical properties of the lipid lamella and consequently the function of membrane-associated enzymes. To characterize these changes, we used steady-state and time resolved fluorescence spectroscopy and two photon-excitation microscopy techniques. The membrane systems were chosen according to the techniques that were used: large unilamellar vesicles (LUVs) for cuvette and giant unilamellar vesicles (GUVs) for microscopy measurements; they were prepared from dipalmitoyl phosphatidylcholine (DPPC) and dioctadecyl phosphatidylcholine (DOPC) in mixtures that are well known to form lipid domains. Two fluorescent probes, which insert into different regions of the bilayer, were selected: 1,6-diphenyl-1,3,5-hexatriene (DPH) was located at the deep hydrophobic core of the acyl chain regions and 2-dimethylamino-6-lauroylnaphthalene (Laurdan) at the hydrophilic-hydrophobic membrane interface. Our spectroscopy results show that (i) the changes induced by cholesterol in the deep hydrophobic phospholipid acyl chain domain are different from the ones observed in the superficial region of the hydrophilic-hydrophobic interface, and these changes depend on the state of the lamella and (ii) the incorporation of cholesterol into the lamella induces an increase in the orientation dynamics in the deep region of the phospholipid acyl chains with a corresponding decrease in the orientation at the region close to the polar lipid headgroups. The microscopy data from DOPC/DPPC/Chol GUVs using Laurdan generalized polarization (Laurdan GP) suggest that a high cholesterol content in the bilayer weakens the stability of the water hydrogen bond network and hence the stability of the liquid-ordered phase (Lo).

Human erythrocytes and neuroblastoma cells are in vitro affected by sodium orthovanadate

Research on biological influence of vanadium has gained major importance because it exerts potent toxic, mutagenic, and genotoxic effects on a wide variety of biological systems. However, hematological toxicity is one of the less studied effects. The lack of information on this issue prompted us to study the structural effects induced on the human erythrocyte membrane by vanadium (V). Sodium orthovanadate was incubated with intact erythrocytes, isolated unsealed human erythrocyte membranes (IUM) and molecular models of the erythrocyte membrane. The latter consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. This report presents evidence in order that orthovanadate interacted with red cell membranes as follows: a) in scanning electron microscopy (SEM) studies it was observed that morphological changes on human erythrocytes were induced; b) fluorescence spectroscopy experiments in isolated unsealed human erythrocyte membranes (IUM) showed that an increase in the molecular dynamics and/or water content at the shallow depth of the lipids glycerol backbone at concentrations as low as 50μM was produced; c) X-ray diffraction studies showed that orthovanadate 0.25-1mM range induced increasing structural perturbation to DMPE; d) somewhat similar effects were observed by differential scanning calorimetry (DSC) with the exception of the fact that DMPC pretransition was shown to be affected; and e) fluorescence spectroscopy experiments performed in DMPC large unilamellar vesicles (LUV) showed that at very low concentrations induced changes in DPH fluorescence anisotropy at 18°C. Additional experiments were performed in mice cholinergic neuroblastoma SN56 cells; a statistically significant decrease of cell viability was observed on orthovanadate in low or moderate concentrations.

Single molecule probes of membrane structure: orientation of BODIPY probes in DPPC as a function of probe structure

Single molecule fluorescence measurements have recently been used to probe the orientation of fluorescent lipid analogs doped into lipid films at trace levels. Using defocused polarized total internal reflection fluorescence microscopy (PTIRF-M), these studies have shown that fluorophore orientation responds to changes in membrane surface pressure and composition, providing a molecular level marker of membrane structure. Here we extend those studies by characterizing the single molecule orientations of six related BODIPY probes doped into monolayers of DPPC. Langmuir-Blodgett monolayers transferred at various surface pressures are used to compare the response from fluorescent lipid analogs in which the location of the BODIPY probe is varied along the length of the acyl chain. For each BODIPY probe location along the chain, comparisons are made between analogs containing phosphocholine and smaller fatty acid headgroups. Together these studies show a general propensity of the BODIPY analogs to insert into membranes with the BODIPY probe aligned along the acyl chains or looped back to interact with the headgroups. For all BODIPY probes studied, a bimodal orientation distribution is observed which is sensitive to surface pressure, with the population of BODIPY probes aligned along the acyl chains increasing with elevated surface pressure. Trends in the single molecule orientations for the six analogs reveal a configuration where optimal placement of the BODIPY probe within the acyl chain maximizes its sensitivity to the surrounding membrane structure. These results are discussed in terms of balancing the effects of headgroup association with acyl chain length in designing the optimal placement of the BODIPY probe.

Effects of an antimalarial quinazoline derivative on human erythrocytes and on cell membrane molecular models

Plasmodium, the parasite which causes malaria in humans multiplies in the liver and then infects circulating erythrocytes. Thus, the role of the erythrocyte cell membrane in antimalarial drug activity and resistance has key importance. The effects of the antiplasmodial N(6)-(4-methoxybenzyl)quinazoline-2,4,6-triamine (M4), and its inclusion complex (M4/HPβCD) with 2-hydroxypropyl-β-cyclodextrin (HPβCD) on human erythrocytes and on cell membrane molecular models are herein reported. This work evidences that M4/HPβCD interacts with red cells as follows: a) in scanning electron microscopy (SEM) studies on human erythrocytes induced shape changes at a 10μM concentration; b) in isolated unsealed human erythrocyte membranes (IUM) a concentration as low as 1μM induced sharp DPH fluorescence anisotropy decrease whereas increasing concentrations produced a monotonically decrease of DPH fluorescence lifetime at 37°C; c) X-ray diffraction studies showed that 200μM induced a complete structural perturbation of dimyristoylphosphatidylcholine (DMPC) bilayers whereas no significant effects were detected in dimyristoylphosphatidylethanolamine (DMPE) bilayers, classes of lipids present in the outer and inner monolayers of the human erythrocyte membrane, respectively; d) fluorescence spectroscopy data showed that increasing concentrations of the complex interacted with the deep hydrophobic core of DMPC large unilamellar vesicles (LUV) at 18°C. All these experiments are consistent with the insertion of M4/HPβCD in the outer monolayer of the human erythrocyte membrane; thus, it can be considered a promising and novel antimalarial agent.

Physicochemical mechanism for the enhanced ability of lipid membrane penetration of polyarginine

Arginine-rich, cell-penetrating peptides (e.g., Tat-peptide, penetratin, and polyarginine) are used to carry therapeutic molecules such as oligonucleotides, DNA, peptides, and proteins across cell membranes. Two types of processes are being considered to cross the cell membranes: one is an endocytic pathway, and another is an energy-independent, nonendocytic pathway. However, the latter is still not known in detail. Here, we studied the effects of the chain length of polyarginine on its interaction with an anionic phospholipid large unilamellar vesicle (LUV) or a giant vesicle using poly-l-arginine composed of 69 (PLA69), 293 (PLA293), or 554 (PLA554) arginine residues, together with octaarginine (R8). ζ-potential measurements confirmed that polyarginine binds to LUV via electrostatic interactions. Circular dichroism analysis demonstrated that the transition from the random coil to the α-helix structure upon binding to LUV occurred for PLA293 and PLA554, whereas no structural change was observed for PLA69 and R8. Fluorescence studies using membrane probes revealed that the binding of polyarginine to LUV affects the hydration and packing of the membrane interface region, in which the degree of membrane insertion is greater for the longer polyarginine. Isothermal titration calorimetry measurements demonstrated that although the binding affinity (i.e., the Gibbs free energy of binding) per arginine residue is similar among all polyarginines the contribution of enthalpy to the energetics of binding of polyarginine increases with increasing polymer chain length. In addition, confocal laser scanning microscopy showed that all polyarginines penetrate across giant vesicle membranes, and the order of the amount of membrane penetration is R8 ≈ PLA69 < PLA293 ≈ PLA554. These results suggest that the formation of α-helical structure upon lipid binding drives the insertion of polyarginine into the membrane interior, which appears to enhance the membrane penetration of polyarginine.

Acyl-chain mismatch driven superlattice arrangements in DPPC/DLPC/cholesterol bilayers

Fluorescence and infrared spectroscopy and cholesterol oxidase activity were employed to investigate the effect of phosphatidylcholine (PC) acyl chain length mismatch on the lateral organizations of lipids in liquid-ordered dipalmitoyl-PC/dilauroyl-PC/cholesterol (DPPC/DLPC/CHOL) bilayers. Plots of steady-state fluorescence emission anisotropy of diphenylhexatriene (DPH) labeled PC (DPH-PC) embedded in the DPPC/DLPC/CHOL bilayers revealed significant peaks at several DPPC mole fractions (Y(DPPC)) when the cholesterol mole fraction (X(CHOL)) was fixed to particular values. Analogously, the DPH-PC anisotropy peaked at several critical X(CHOL)'s when Y(DPPC) was fixed. Acyl chain C-H and C horizontal lineO vibrational peak frequencies of native PC as well as the activity of cholesterol oxidase also revealed dips and peaks at similar Y(DPPC)'s. Importantly, most of the observed peaks/dips coincide with the critical mole fractions predicted by the Superlattice (SL) model. A three-dimensional map of DPH-PC anisotropy versus composition in the range 0.32 <or= X(CHOL) <or= 0.50; 0.54 <or= Y(DPPC) <or= 0.72 revealed a prominent peak at (X(CHOL), Y(DPPC)) approximately (0.42, 0.64). This suggests a simultaneous presence of two different types of superlattices, one where cholesterol is the quest molecule in a PC host lattice and another where DPPC is the guest in the DLPC host lattice. Time-resolved measurements of DPH-PC fluorescence indicated the existence of an ordered, rotationally hindered environment of acyl chains at that "critical" composition consistent with the existence of SL arrangements. We propose that beside CHOL/PC superlattices, DPPC, and DLPC as well tend to adopt regular SL-like lateral distributions relative to each other, presumably because the less hydrophobic DLPC molecule is slightly displaced toward the aqueous phase, thus allowing more room and mobility for the head groups of both DPPC and DLPC as well as for the acyl chain tails of DPPC. The parallel presence of two kinds of superlattices, that is, CHOL/PC-SL and DPPC/DLPC-SL as demonstrated here, has intriguing implications regarding lipid homeostasis of eukaryote membranes.

Multiphoton polarization imaging of steady-state molecular order in ternary lipid vesicles for the purpose of lipid phase assignment

We have investigated lipid acyl chain order parameters of giant unilamellar vesicles (GUVs) using multiphoton fluorescence microscopy. We compare two widely used models of lipid acyl chain order parameters: the "wobble-on-a-cone" model and the Gaussian distribution model. For the first time, we systematically address a ternary system for which the phase diagram encompassing both composition and temperature space has been mapped in order to determine tie-line directions and thus phase assignment. In addition, because miscibility and chain melting transitions can be observed directly and simultaneously with multiphoton microscopy, our technique is applicable to determining the extent of the coupling between chain order and miscibility; thus, it provides a more robust platform for comparison with theory.

Localisation of BODIPY-labelled phosphatidylcholines in lipid bilayers

A series of sn-2 acyl-labelled phosphatidyl-cholines (PC), bearing 4,4-difluoro-1-3-5-7-tetra-methyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me(4)-BODIPY) at the end of the C(n)-acyl chains were solubilised in unilamellar vesicles and studied with respect to the order and location of the Me(4)-BODIPY (denoted: B) group. The obtained results are based on time-resolved electronic energy transfer from donors (2-(9-anthroyloxy)-stearic acid) localised in the lipid-water interface to acceptors BnPC (n = 3, 5, 7, 9, 11, 13, 15), as well as the energy migration among the Me(4)-BODIPY groups of BnPC:s. The donor-acceptor and the donor-donor experiments strongly suggest that the Me(4)-BODIPY group in BnPC tends to loop back close to the lipid-water interface. The Me(4)-BODIPY groups, residing in the two bilayer leaflets, are located at approximately the same depth, and transversally separated by ca. 27 A for all n-values. Close to the interface, the optimal transversal distribution widens somewhat with increasing length of the sn-2 acyl chain. The obtained order parameter profile of the BnPC:s is also compatible with such a location.

Alteration of retinal rod outer segment membrane fluidity in a rat model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is caused by an inherited defect in the last step in cholesterol (Chol) biosynthesis, leading to abnormal accumulation of 7-dehydrocholesterol and decreased Chol levels. Progressive retinal degeneration occurs in an animal model of SLOS, induced by treating rats with AY9944, a selective inhibitor of the enzyme affected in SLOS. Here we evaluated alterations in the biochemical and physical properties of retinal rod outer segment (ROS) membranes in this animal model. At 1 month of AY9944 treatment, there were modest alterations in fatty acid composition, but no significant differences in cis-parinaric acid (cPA) spectroscopic parameters in ROS membranes from treated versus control rats. However, at 3 months, ROS docosahexaenoic acid (DHA) content was dramatically reduced, and cPA fluorescence anisotropy values were decreased, relative to controls. Also, 1,6-diphenyl-1,3,5-hexatriene exhibited decreased rotational motion and increased orientational order in ROS membranes from 3 month-old AY9944-treated rats, relative to controls. No significant changes in protein:lipid ratios were observed; however, rhodopsin regenerability was compromised by 3 months of treatment. These findings are consistent with reduced ROS membrane fluidity in the SLOS rat model, relative to controls, primarily due to the dramatic reduction in membrane DHA levels, rather than altered sterol composition.

Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions

Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.

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.

Lipid membrane-induced optimization for ligand-receptor docking: recent tools and insights for the "membrane catalysis" model

Cells in living organisms are regulated by chemical and physical stimuli from their environment. Often, ligands interact with membrane receptors to trigger responses and Sargent and Schwyzer conceived a model to describe this process, "membrane catalysis". There is a notion that the physical organization of membranes can control the response of cells by speeding up reactions. We revisit the "membrane catalysis" model in the light of recent technical, methodological and theoretical advances and how they can be exploited to highlight the details of membrane mediated ligand-receptor interactions. We examine the possible effects that ligand concentration causes in the membrane catalysis and focus our attention in techniques used to determine the partition constant. The hypothetical diffusional advantage associated with membrane catalysis is discussed and the applicability of existing models is assessed. The role of in-depth location and orientation of ligands is explored emphasizing the contribution of new analysis methods and spectroscopic techniques. Results suggest that membranes can optimize the interaction between ligands and receptors through several different effects but the relative contribution of each must be carefully investigated. We certainly hope that the conjugation of the methodological and technical advances here reported will revive the interest in the membrane catalysis model.

Measurement of single macromolecule orientation by total internal reflection fluorescence polarization microscopy

A new approach is presented for measuring the three-dimensional orientation of individual macromolecules using single molecule fluorescence polarization (SMFP) microscopy. The technique uses the unique polarizations of evanescent waves generated by total internal reflection to excite the dipole moment of individual fluorophores. To evaluate the new SMFP technique, single molecule orientation measurements from sparsely labeled F-actin are compared to ensemble-averaged orientation data from similarly prepared densely labeled F-actin. Standard deviations of the SMFP measurements taken at 40 ms time intervals indicate that the uncertainty for individual measurements of axial and azimuthal angles is approximately 10 degrees at 40 ms time resolution. Comparison with ensemble data shows there are no substantial systematic errors associated with the single molecule measurements. In addition to evaluating the technique, the data also provide a new measurement of the torsional rigidity of F-actin. These measurements support the smaller of two values of the torsional rigidity of F-actin previously reported.

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.

Time-resolved fluorescence and fourier transform infrared spectroscopic investigations of lateral packing defects and superlattice domains in compositionally uniform cholesterol/phosphatidylcholine bilayers

Time-resolved fluorescence and Fourier transform infrared spectroscopies were used to investigate the lateral organization of lipids in compositionally uniform and fully equilibrated 1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPC/CHOL) liposomes prepared by a recently devised low-temperature trapping method. Independent fluorescence decay lifetime and rotational dynamics parameters of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) in these liposomes were recovered from the time-resolved fluorescence measurements as a function of cholesterol molar fraction (X(CHOL)) at 23 degrees C. The results indicate significantly greater lifetime heterogeneity, shorter average lifetime, rotational correlation time, and lower order parameter of the DPH moiety at X(CHOL) approximately 0.40 and 0.50 as compared to the adjacent cholesterol concentrations. Less prominent changes were also detected at, for example, X(CHOL) approximately 0.20 and 0.33. These X(CHOL)'s coincide with the "critical" X(CHOL)'s predicted by the previously proposed superlattice (SL) model, thus indicating that POPC and cholesterol molecules tend to form SL domains where the components tend to be regularly distributed. The data also support another prediction of the SL model, namely that lateral packing defects coexist with the ordered SL domains. It appears that unfavorable interaction of the DPH-moiety of DPH-PC with cholesterol results in a preferential partition of DPH-PC to the defect regions. Fourier transform infrared analysis of the native lipid O=P=O, C=O, and C-H vibrational bands of POPC/CHOL liposomes in the absence of DPH-PC revealed an increase in the conformational order of the acyl chains and a decrease in the conformational order (or increased hydration) of the interfacial and headgroup regions at or close to the predicted critical X(CHOL)'s. This provides additional but probe-independent evidence for SL domain formations in the POPC/CHOL bilayers. We propose that the defect regions surrounding the putative SL domains could play an important role in modulating the activity of various membrane-associated enzymes, e.g., those regulating the lipid compositions of cell membranes.

Relationship between conformation of polysaccharides -in the dilute regime and their interaction with a phospholipid bilayer

Interactions between polysaccharides and phospholipid bilayers have already been demonstrated in the literature but little is known about the influence of macromolecule conformations related to the solvent characteristics (pH, ions, ionic strength). In this study we have investigated the conformation of iono- and thermo-sensitive polysaccharides, iota- and kappa-carrageenans, and their interaction with a dimyristoylphosphatidylcholine (DMPC) model bilayer. The study was performed in two different media (NaCl 150 mmol/L, pH 6.5, and NaCl 300 mmol/L, pH 6.5). In the first part, the iota- and kappa-carrageenan samples have been characterized by size exclusion chromatography (SEC) coupled with a multi-angle laser light-scattering detector (MALLS). The SEC-MALLS results clearly show polysaccharide chain association at high ionic strength. In the second part, the polysaccharide-membrane interaction has been studied, using fluorescent probes embedded in the membrane. The thermotropic properties of the membrane were investigated by fluorescence depolarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The membrane surface accessibility was evaluated by fluorescence quenching of 2-(9-anthroyloxy) stearic acid (2-AS). Whatever the ionic strength tested, the polysaccharide presence notably enhances the membrane fluidity below the T(m). This sign of an interaction in the polar level of the membrane is more marked at low NaCl concentration. In contrast, the liposomes bilayer accessibility is drastically lowered when increasing the ionic strength. This is induced by macromolecular chain adsorption on the liposome surface, enhanced by the polysaccharide chain association. An ionic strength enhancement induces a conformational modification of the polysaccharide chains which modifies their ability to interact with the bilayer.

A maximum entropy analysis of protein orientations using fluorescence polarization data from multiple probes

Techniques have recently become available to label protein subunits with fluorescent probes at predetermined orientation relative to the protein coordinates. The known local orientation enables quantitative interpretation of fluorescence polarization experiments in terms of orientation and motions of the protein within a larger macromolecular assembly. Combining data obtained from probes placed at several distinct orientations relative to the protein structure reveals functionally relevant information about the axial and azimuthal orientation of the labeled protein segment relative to its surroundings. Here we present an analytical method to determine the protein orientational distribution from such data. The method produces the broadest distribution compatible with the data by maximizing its informational entropy. The key advantages of this approach are that no a priori assumptions are required about the shape of the distribution and that a unique, exact fit to the data is obtained. The relative orientations of the probes used for the experiments have great influence on information content of the maximum entropy distribution. Therefore, the choice of probe orientations is crucial. In particular, the probes must access independent aspects of the protein orientation, and two-fold rotational symmetries must be avoided. For a set of probes, a "figure of merit" is proposed, based on the independence among the probe orientations. With simulated fluorescence polarization data, we tested the capacity of maximum entropy analysis to recover specific protein orientational distributions and found that it is capable of recovering orientational distributions with one and two peaks. The similarity between the maximum entropy distribution and the test distribution improves gradually as the number of independent probe orientations increases. As a practical example, ME distributions were determined with experimental data from muscle fibers labeled with bifunctional rhodamine at known orientations with respect to the myosin regulatory light chain (RLC). These distributions show a complex relationship between the axial orientation of the RLC relative to the fiber axis and the azimuthal orientation of the RLC about its own axis. Maximum entropy analysis reveals limitations in available experimental data and supports the design of further probe angles to resolve details of the orientational distribution.

Studies of archaebacterial bipolar tetraether liposomes by perylene fluorescence

Membrane packing and dynamics of bipolar tetraether liposomes composed of the polar lipid fraction E (PLFE) from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius have been studied by perylene fluorescence. At a probe-to-PLFE lipid ratio of 1:400, we have detected an unusual fluorescence intensity increase with increasing temperature, while the fluorescence lifetime changed little. As the ratio was decreased, the intensity anomaly was diminished. At 1:3200 and 1:6400, the anomaly disappeared. A remarkable perylene intensity anomaly was also observed in bilayers composed of saturated monopolar diester phosphatidylcholines at their main phase transition temperatures. These results suggest that the intensity anomaly may be due to probe aggregation caused by tight membrane packing. At the same probe-to-lipid ratio (1:400), however, 1, 2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and 1, 2-diphytanoyl-sn-glycero-3-phosphoglycerol (DPhPG) liposomes did not exhibit any intensity anomaly with increasing temperature. This suggests that DPhPC and DPhPG liposomes are more loosely packed than PLFE liposomes; thus the branched methyl groups are not the contributing factor of the tight membrane packing found in PLFE liposomes. Using a multiexcitation method, we have also determined the average (R), in-plane (R(ip)), and out-of-plane (R(op)) rotational rates of perylene in PLFE liposomes at various temperatures (20-65 degrees C). R and R(ip), determined at two different probe-to-lipid ratios (1:400 and 1:3200), both undergo an abrupt increase when the temperature is elevated to approximately 48 degrees C. These data suggest that PLFE liposomes are rigid and tightly packed at low temperatures, but they begin to possess appreciable "membrane fluidity" at temperatures close to the minimum growth temperature ( approximately 50 degrees C) of thermoacidophilic archaebacteria.

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.

Plasma membrane fluidity and polarity of polymorphonuclear leukocytes from children with type I diabetes mellitus

Polymorphonuclear leukocytes (PMN) from diabetic subjects have been found to be abnormal in various functional activities. These activities are mediated by the plasma membrane. This study was designed to evaluate plasma membrane fluidity and polarity in children with type I diabetes mellitus using fluorescence spectroscopy. PMN membrane fluidity and polarity were assessed in a group of 32 diabetic children. Membrane fluidity was investigated by measuring steady-state fluorescence anisotropy and fluorescence decay of 1-[4-trimethylammonium-phenyl]-6-phenyl- 1,3,5-hexatriene (TMA-DPH), whereas membrane polarity was studied by measuring the steady-state fluorescence emission and excitation spectra of 2-dimethylamino[6-lauroyl]-naphthalene (Laurdan). TMA-DPH and Laurdan are known to be incorporated at the hydrophobic-hydrophilic interface of the bilayer. Our data show a significant increase in steady-state fluorescence anisotropy in diabetic PMN that reflects a decrease in membrane fluidity, and a decrease in TMA-DPH lifetime distribution indicating a decrease in membrane heterogeneity. Laurdan shows a blue shift of the fluorescence emission and a red shift of the excitation spectra in diabetic PMN with respect to the control group, indicating a decrease in membrane polarity. The results demonstrate a decrease in the phospholipid order at the membrane surface and a decrease in membrane polarity in diabetic PMN. These alterations in the physico-chemical properties of the plasma membrane could be the basis of the modifications in functional activities of PMN. The changes in the plasma membrane of PMN could be the result of metabolic and chemical modification associated with type I diabetes.

Model-independent analysis of the orientation of fluorescent probes with restricted mobility in muscle fibers

The orientation of proteins in ordered biological samples can be investigated using steady-state polarized fluorescence from probes conjugated to the protein. A general limitation of this approach is that the probes typically exhibit rapid orientational motion ("wobble") with respect to the protein backbone. Here we present a method for characterizing the extent of this wobble and for removing its effects from the available information about the static orientational distribution of the probes. The analysis depends on four assumptions: 1) the probe wobble is fast compared with the nanosecond time scale of its excited-state decay; 2) the orientational distributions of the absorption and emission transition dipole moments are cylindrically symmetrical about a common axis c fixed in the protein; 3) protein motions are negligible during the excited-state decay; 4) the distribution of c is cylindrically symmetrical about the director of the experimental sample. In a muscle fiber, the director is the fiber axis, F. All of the information on the orientational order of the probe that is available from measurements of linearly polarized fluorescence is contained in five independent polarized fluorescence intensities measured with excitation and emission polarizers parallel or perpendicular to F and with the propagation axis of the detected fluorescence parallel or perpendicular to that of the excitation. The analysis then yields the average second-rank and fourth-rank order parameters (<P2> and <P4>) of the angular distribution of c relative to F, and <P2a> and <P2e>, the average second-rank order parameters of the angular distribution for wobble of the absorption and emission transition dipole moments relative to c. The method can also be applied to other cylindrically ordered systems such as oriented lipid bilayer membranes and to processes slower than fluorescence that may be observed using longer-lived optically excited states.

Removal of dissolved O2 by a fluid membrane of Fomblin Y (perfluoro polyether): fluorescence of diphenylhexatriene in Fomblin Y

O2 can be gently removed from solutions by contact with a "bulk fluid membrane" of the viscous and nearly inert perfluoro polyether Fomblin Y. A volume of Fomblin dissolves approximately 20 times more O2 than an equal volume of water. Hence, when a volume of aqueous solution which was in equilibrium with air is enclosed with an equal volume of Fomblin which had been flushed with argon, oxygen would diffuse into the Fomblin, leaving in the water only 5% of the oxygen that was there. When the Fomblin is stirred, diffusion is rather rapid. The residue can be removed either by placing an oxygen scavenger on the other side of the Fomblin or by flowing a trickle of deoxygenated Fomblin through the sample. Diphenylhexatriene, a fluorescent probe of cell membranes, can be dissolved in Fomblin Y and has a fluorescence lifetimes extending from 12 ns in O2 saturation to 30 ns (!) in the absence of oxygen. Stern-Volmer plots calibrated against a Clark electrode validate this system for oximetry. A general purpose anaerobic cuvette for fluorescence spectroscopy, containing the sample solution, a Fomblin compartment and the oxygen scavenger Na-dithionite is demonstrated.