A fluorescence quenching technique for the measurement of paramagnetic ion concentrations at the membrane/water interface. Intrinsic and X537A-mediated cobalt fluxes across lipid bilayer membranes

Site-Directed Fluorescence Approaches for Dynamic Structural Biology of Membrane Peptides and Proteins

Membrane proteins mediate a number of cellular functions and are associated with several diseases and also play a crucial role in pathogenicity. Due to their importance in cellular structure and function, they are important drug targets for ~60% of drugs available in the market. Despite the technological advancement and recent successful outcomes in determining the high-resolution structural snapshot of membrane proteins, the mechanistic details underlining the complex functionalities of membrane proteins is least understood. This is largely due to lack of structural dynamics information pertaining to different functional states of membrane proteins in a membrane environment. Fluorescence spectroscopy is a widely used technique in the analysis of functionally-relevant structure and dynamics of membrane protein. This review is focused on various site-directed fluorescence (SDFL) approaches and their applications to explore structural information, conformational changes, hydration dynamics, and lipid-protein interactions of important classes of membrane proteins that include the pore-forming peptides/proteins, ion channels/transporters and G-protein coupled receptors.

Silver Ions Detection via Nucleolipids Self-Assembly

A novel hybrid bioinspired amphiphile featuring a cytosine moiety, which self-assembles into liposomes can be used to detect silver ions in aqueous media. The coordination of Ag⁺ ions by the nucleotide moiety increases membrane rigidity, which enhances the fluorescence of a common reporter, Thioflavin T. Ag⁺ can be sensed even at trace concentrations (3 ppb) with great specificity over other metals ions. These nucleotide based supramolecular structures can be used to detect silver ions in drinking water, demonstrating the robustness of this approach.

Infrared and fluorescence spectroscopic studies of a phospholipid bilayer supported by a soft cationic hydrogel scaffold

Polarized attenuated total reflection (ATR-IR) spectroscopy and fluorescence microscopy techniques were used to characterize a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) membrane supported on porous, cationic hydrogel beads. Fluorescence microscopy images showed that the DPhPC coated the external surface of the hydrogel scaffold. In addition, a fluorescence assay of the emission intensity of the Tb(3+)/dipicolinic acid complex demonstrated that the DPhPC coating acted as a barrier to Tb(3+) efflux from the scaffolded vesicle and successfully sealed the porous hydrogel bead. Fluorescence quenching and ATR-IR spectroscopic measurements revealed that the lipid coating has a bilayer structure. The phytanoyl chains were found to exhibit significant trans-gauche isomerization, characteristic of the fluid liquid phase. However, no lipid lateral mobility was observed by fluorescence recovery after photobleaching (FRAP) measurements. The phosphocholine headgroup was found to be well hydrated and oriented such that the cationic choline group tucked in behind the anionic phosphate group, consistent with an electrostatic attraction between the cationic scaffold and zwitterionic lipid. The absence of lipid lateral mobility may be due to the strength of this attraction.

A highly selective CHEF-type chemosensor for monitoring Zn2+ in aqueous solution and living cells

A new quinolone-based chemosensor was synthesized and successfully applied to quantify and image Zn²⁺ in water samples and living cells.

Orientation and dynamics of melittin in membranes of varying composition utilizing NBD fluorescence

Melittin is a cationic hemolytic peptide isolated from the European honey bee, Apis mellifera. The organization of membrane-bound melittin has earlier been shown to be dependent on the physical state and composition of membranes. In this study, we covalently labeled the N-terminal (Gly-1) and Lys-7 of melittin with an environment-sensitive fluorescent probe, the NBD group, to monitor the influence of negatively charged lipids and cholesterol on the organization and dynamics of membrane-bound melittin. Our results show that the NBD group of melittin labeled at its N-terminal end does not exhibit red edge excitation shift in DOPC and DOPC/DOPG membranes, whereas the NBD group of melittin labeled at Lys-7 exhibits REES of approximately 8 nm. This could be attributed to difference in membrane microenvironment experienced by the NBD groups in these analogs. Interestingly, the membrane environment of the NBD groups is sensitive to the presence of cholesterol, which is supported by time-resolved fluorescence measurements. Importantly, the orientation of melittin is found to be parallel to the membrane surface as determined by membrane penetration depth analysis using the parallax method in all cases. Our results constitute the first report to our knowledge describing the orientation of melittin in cholesterol-containing membranes. These results assume significance in the overall context of the role of membrane lipids in the orientation and function of membrane proteins and peptides.

Liquid-crystalline collapse of pulmonary surfactant monolayers

During exhalation, the surfactant film of lipids and proteins that coats the alveoli in the lung is compressed to high surface pressures, and can remain metastable for prolonged periods at pressures approaching 70 mN/m. Monolayers of calf lung surfactant extract (CLSE), however, collapse in vitro, during an initial compression at approximately 45 mN/m. To gain information on the source of this discrepancy, we investigated how monolayers of CLSE collapse from the interface. Observations with fluorescence, Brewster angle, and light scattering microscopies show that monolayers containing CLSE, CLSE-cholesterol (20%), or binary mixtures of dipalmitoyl phosphatidylcholine(DPPC)-dihydrocholesterol all form bilayer disks that reside above the monolayer. Upon compression and expansion, lipids flow continuously from the monolayer into the disks, and vice versa. In several respects, the mode of collapse resembles the behavior of other amphiphiles that form smectic liquid-crystal phases. These findings suggest that components of surfactent films must collapse collectively rather than being squeezed out individually.

Interpretation of passive permeability measurements on lipid-bilayer vesicles. Effect of fluctuations

A stochastic model for migration dynamics of solute molecules from the bulk aqueous phase to the intravesicular water pool is developed with a goal to interpret recent passive permeability measurements of bilayer membranes. Previously neglected fluctuations of the number of solubilized species in the inner water pool of a vesicle are naturally incorporated into the model. For a homogeneous one-phase bilayer, the model predicts exponential long-time asymptotics of the migration dynamics with a rate constant given by a sum of the frequencies of exit and entry of a solute molecule from/into the intravesicular water pool into/from the bulk aqueous phase. The long-time constant is directly related to the membrane permeability.

Lindane suppresses the lipid-bilayer permeability in the main transition region

The effects of a small molecule, the insecticide lindane, on unilamellar DMPC bilayers in the phase transition region, have been studied by means of differential scanning calorimetry and fluorescence spectroscopy. The calorimetric data show that increasing concentrations of lindane broaden the transition and lower the transition temperature, without changing the transition enthalpy significantly. Lindane therefore enhances the thermal fluctuations of the bilayer. The calorimetric data furthermore suggest that the bilayer structure is intact and not disrupted by even high concentrations (32 mol%) of lindane. Fluorescence spectroscopy was used to measure the passive permeability of unilamellar DMPC bilayers to Co2+ ions. The data show that lindane seals the bilayer for Co2+ penetration and that this effect increases with increasing lindane concentration. The results are discussed in relation to the effects on the permeability of other small molecules, e.g., anesthetics.

Physical state of bulk and protein-associated lipid in nicotinic acetylcholine receptor-rich membrane studied by laurdan generalized polarization and fluorescence energy transfer

The spectral properties of the fluorescent probe laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) were exploited to learn about the physical state of the lipids in the nicotinic acetylcholine receptor (AChR)-rich membrane and compare them with those in reconstituted liposomes prepared from lipids extracted from the native membrane and those formed with synthetic phosphatidylcholines. In all cases redshifts of 50 to 60 nm were observed as a function of temperature in the spectral emission maximum of laurdan embedded in these membranes. The so-called generalized polarization of laurdan exhibited high values (0.6 at 5 degrees C) in AChR-rich membranes, diminishing by approximately 85% as temperature increased, but no phase transitions with a clear Tm were observed. A still unexploited property of laurdan, namely its ability to act as a fluorescence energy transfer acceptor from tryptophan emission, has been used to measure properties of the protein-vicinal lipid. Energy transfer from the protein in the AChR-rich membrane to laurdan molecules could be observed upon excitation at 290 nm. The efficiency of this process was approximately 55% for 1 microM laurdan. A minimum donor-acceptor distance r of 14 +/- 1 A could be calculated considering a distance 0 < H < 10 A for the separation of the planes containing donor and acceptor molecules, respectively. This value of r corresponds closely to the diameter of the first-shell protein-associated lipid. A value of approximately 1 was calculated for Kr, the apparent dissociation constant of laurdan, indicating no preferential affinity for the protein-associated probe, i.e., random distribution in the membrane. From the spectral characteristics of laurdan in the native AChR-rich membrane, differences in the structural and dynamic properties of water penetration in the protein-vicinal and bulk bilayer lipid regions can be deduced. We conclude that 1) the physical state of the bulk lipid in the native AChR-rich membrane is similar to that of the total lipids reconstituted in liposomes, exhibiting a decreasing polarity and an increased solvent dipolar relaxation at the hydrophilic/hydrophobic interface upon increasing the temperature; 2) the wavelength dependence of laurdan generalized polarization spectra indicates the presence of a single, ordered (from the point of view of molecular axis rotation)-liquid (from the point of view of lateral diffusion) lipid phase in the native AChR membrane; 3) laurdan molecules within energy transfer distance of the protein sense protein-associated lipid, which differs structurally and dynamically from the bulk bilayer lipid in terms of polarity and molecular motion and is associated with a lower degree of water penetration.

Probing the binding domain of the NK2 receptor with fluorescent ligands: evidence that heptapeptide agonists and antagonists bind differently

We have investigated the interaction of fluorescent peptide ligands with the G protein-coupled receptor NK2 using novel spectrofluorometric approaches. Several heptapeptide antagonists of structure PhCO-Xaa-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH2 were labelled on position 1 (Xaa) with the environment-sensitive nitrobenzoxadiazole (NBD) probe, differing only in the length of the spacer between the NBD group and the peptide. Upon binding of the labelled antagonist to NK2 receptors stably expressed in Chinese hamster ovary (CHO) cells, an increase in NBD fluorescence was observed when the spacer length was less than 10 A. Collisional quenching experiments using iodide and Co2+ ions were performed to define the accessibility of the NBD group on bound ligands to the solvent. By comparing ligands with spacer arms of varying lengths, we found that the binding pocket is buried at a depth of 5-10 A. In contrast, N-terminally NBD-labelled agonists, decapeptide neurokinin A (NKA) or heptapeptide Nle10-NKA[4-10], bound to the NK2 receptor were accessible to the solvent. Binding of fluorescent ligands to the NK2 receptor was accompanied by an enhancement in the fluorescence anisotropy. The changes in fluorescence properties were used to determine the kinetic parameters of antagonist binding and dissociation. These results indicate that the binding site on the NK2 receptor for the amino-terminal end of the heptapeptide antagonists is buried in the hydrophobic pocket of the receptor protein and clearly distinct from the binding site for the amino-terminal end of agonists, which is accessible to the solvent.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting solute entrapment in phospholipid vesicles prepared by the freeze-thaw extrusion method: a possible general method for improving the efficiency of entrapment

It is often assumed that the internal solute concentrations of phospholipid vesicles are equal to those in the medium in which they were prepared, particularly when freeze-thaw cycles are employed during the procedure. Conditions are reported here which when used to prepare vesicles by the polycarbonate filter extrusion method, produce approximately 12- and approximately 7-fold higher internal concentrations of Ca2+ and sucrose, respectively, than exist in the external medium. Formation of these large gradients is dependent upon the use of freeze-thaw cycles during preparation, on the presence of tetraethylammonium perchlorate in the medium, and is independent of media pH across the region of pH 5-9. Gradient formation is antagonized by high concentrations of an impermeant solute (NaCl). It is proposed that gradients form because solutes are concentrated by exclusion from ice during freezing but that they are normally dissipated by osmotic lysis during thawing. The presence of a permeant solute such as tetraethylammonium perchlorate provides an alternative mechanism to balance osmotic pressure, thereby preserving the gradients of impermeable species.

Chemistry and biology of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-labeled lipids: fluorescent probes of biological and model membranes

Lipids that are covalently labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group are widely used as fluorescent analogues of native lipids in model and biological membranes to study a variety of processes. The fluorescent NBD group may be attached either to the polar or the apolar regions of a wide variety of lipid molecules. Synthetic routes for preparing the lipids, and spectroscopic and ionization properties of these probes are reviewed in this report. The orientation of various NBD-labeled lipids in membranes, as indicated by the location of the NBD group, is also discussed. The NBD group is uncharged at neutral pH in membranes, but loops up to the surface if attached to acyl chains of phospholipids. These lipids find applications in a variety of membrane-related studies which include membrane fusion, lipid motion and dynamics, organization of lipids and proteins in membranes, intracellular lipid transfer, and bilayer to hexagonal phase transition in liposomes. Use of NBD-labeled lipids as analogues of natural lipids is critically evaluated.

Spectroscopic and ionization properties of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-labeled lipids in model membranes

The spectroscopic and ionization properties of various lipids labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group have been studied in model membranes using fluorescence, absorbance and electrophoretic mobility measurements. Electrophoretic measurements show that the NBD group is uncharged at neutral pH. However, at high pH, hydroxyl addition or deprotonation occurs with a pKa, depending upon conditions, of 11.5-11.8 for the NBD group of headgroup-labeled phosphatidylethanolamine (NBD-PE) and 11.1-11.5 for NBD labels placed at the end of one fatty acyl chain of a phosphatidylcholine (6-NBD-PC and 12-NBD-PC). This type of behavior is not observed in the case of a methylated NBD label placed in the flexible 'tail' of cholesterol (NBD-cholesterol). The similarity in pKa for NBD-PE and NBD-PCs suggests that in these cases the NBD group is at a similar depth in the membrane. This was examined further by comparison of the fluorescence emission maximum of the NBD group in model membranes with that in solvents of varying polarity. The apparent polarity experienced by NBD groups in model membranes indicates that for NBD-PE and 12-NBD-PC they are located at the polar region whereas the NBD group of NBD-cholesterol is deeply buried in a nonpolar region of the membrane. This conclusion is supported further by fluorescence quenching experiments measuring NBD exposure to the aqueous quencher Co2+. The results of this study confirm the tentative conclusions of our previous fluorescence quenching studies on the location of NBD groups in model membranes.

Formation of ion-translocating oligomers by nigericin

At pH 4.0, greater than 10(-7) M nigericin was found capable of conducting net charge transfer across bimolecular lecithin membranes, with a stoichiometry of three uncharged ionophore moieties per cation. At neutral or alkaline pH, nigericin catalyzed the transfer of net charge through dimer forms. In agreement with these results, quantitative analysis of nigericin-potassium complexes formed at pH 4.0 showed a 3:1 ratio, and a 2:1 ratio at neutral or alkaline pH. A 1:1 stoichiometry was observed when the ionophore complex was not transferred from methanol-water to chloroform. Moreover, 1H-NMR spectra of nigericin-cation complexes formed at pH 4.0, displayed clear-cut chemical shift variations different to those observed at neutral or alkaline pH. Thus, it is apparent that acid pH causes a transition from dimeric to trimeric forms of nigericin-cation complexes. The membrane conductance increased up to ten times when negatively charged phosphatidyl glycerol was used, while the conductance decreased in positively charged cetylpyridinium containing membranes at pH 4.0. These results suggest that the nigericin-K+ oligomeric complex is positively charged. In this respect, pKa values around 8.0 were obtained for the nigericin carboxylate group in media of different dielectric constant, indicating that this chemical group is undissociated under these conditions. Moreover, the values for the complex formation constants as well as the delta G values calculated for the dimers and trimers indicated that such ionophore cation oligomeric complexes are thermodynamically stable.

The use of cobalt ions as a collisional quencher to probe surface charge and stability of fluorescently labeled bilayer vesicles

Co2+ quenched the fluorescence of the lipid probes NBD-phosphatidylethanolamine (NBD-PE) and lissamine-rhodamine phosphatidylethanolamine (N-Rh-PE) incorporated into lipid vesicles, according to a collisional quenching mechanism in agreement with the Stern-Vollmer law. The quenching coefficient (Q) for NBD-PE, incorporated into uncharged phosphatidylcholine (PC) vesicles was 13.8 M-1. This value was equal to the quenching coefficient of water-soluble NBD-taurine in aqueous solution, indicating that Co2+ was readily accessible to the outer surface of PC vesicles. In phosphatidylserine-phosphatidylethanolamine (PS-PE) (1:1) vesicles, quenching was also proportional to Co2+ concentration but Q was 114 mM-1, some 8000-fold smaller. Using the Gouy-Chapman-Stern model we demonstrated that the surface density of Co2+ bound to lipid was linear with Co2+ concentration in the medium up to 7%. Co2+-associated phospholipid would in turn quench NBD-PE or N-Rh-PE by collisional quenching with lateral diffusion. We investigated the ability of Co2+ to permeate PS-PE (1:1) vesicles. Co2+ quenched fluorophores on the outer surface of large unilamellar vesicles, formed by reverse-phase evaporation. In small unilamellar vesicles Co2+ quenched probes on both outer and inner surfaces, indicating rapid permeation of the ions into the vesicles. Using stopped-flow rapid mixing, we measured the rate of influx of Co2+, and correcting for surface potential using the Gouy-Chapman-Stern model, we calculated a permeability coefficient of 10(-12) cm/s for Co2+ concentrations below 300 microM. Above this concentration, there was a very steep rise in the permeability coefficient, indicating that binding of Co2+ induces defects in the bilayer of these vesicles. This may be related to the ability of the vesicles to undergo membrane fusion. A method for calculating the membrane surface potential from Co2+ quenching data is presented.