Methods for probing lateral diffusion of membrane components: triplet—triplet annihilation and triplet—triplet energy transfer

Diffusional Encounter Rate Constants for Xanthone and 2-Naphthoic Acid by Flash Photolysis Experiments and Brownian Dynamics Simulations: Substantial Effects of Polarizability of the Triplet State

Diffusional encounter rate constants, for xanthone and 2-naphthoic acid molecules in their triplet states with xanthone or 2-naphthoic acid molecules in their triplet or singlet states, were determined using nanosecond laser flash photolysis spectroscopy. Simultaneously, Brownian dynamics simulations were used to compute these rate constants for assumed models of encountering molecules. Altogether, a global fit to transient absorption progress curves, reporting populations of triplet state xanthone and triplet state 2-naphthoic acid molecules, allowed us to determine six diffusional encounter rate constants from our experiments. The most important result of this study is the detection of substantial effects of the electric polarizability of molecules in their triplet state, visible for xanthone triplet and 2-naphthoic acid ground states, a homo triplet-triplet annihilation of 2-naphthoic acid, and a hetero triplet-triplet annihilation for xanthone and 2-naphthoic acid.

Lipid Bilayers: The Effect of Force Field on Ordering and Dynamics

The sensitivity of the structure and dynamics of a fully hydrated pure bilayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in molecular dynamics simulations to changes in force-field and simulation parameters has been assessed. Three related force fields (the Gromos 54A7 force field, a Gromos 53A6-derived parameter set and a variant of the Berger parameters) in combination with either particle-mesh Ewald (PME) or a reaction field (RF) were compared. Structural properties such as the area per lipid, carbon-deuterium order parameters, electron density profile and bilayer thicknesses, are reproduced by all the parameter sets within the uncertainty of the available experimental data. However, there are clear differences in the ordering of the glycerol backbone and choline headgroup, and the orientation of the headgroup dipole. In some cases, the degree of ordering was reminiscent of a liquid-ordered phase. It is also shown that, although the lateral diffusion of the lipids in the plane of the bilayer is often used to validate lipid force fields, because of the uncertainty in the experimental measurements and the fact that the lateral diffusion is dependent on the choice of the simulation conditions, it should not be employed as a measure of quality. Finally, the simulations show that the effect of small changes in force-field parameters on the structure and dynamics of a bilayer is more significant than the treatment of the long-range electrostatic interactions using RF or PME. Overall, the Gromos 54A7 best reproduced the range of experimental data examined.

A modified FCCS procedure applied to Ly49A-MHC class I cis-interaction studies in cell membranes

The activity of natural killer (NK) cells is regulated by a fine-tuned balance between activating and inhibitory receptors. Dual-color fluorescence cross-correlation spectroscopy (FCCS) was used to directly demonstrate a so-called cis-interaction between a member of the inhibitory NK cell receptor family Ly49 (Ly49A), and its ligand, the major histocompatibility complex (MHC) class I, within the plasma membrane of the same cell. By a refined FCCS model, calibrated by positive and negative control experiments on cells from the same lymphoid cell line, concentrations and diffusion coefficients of free and interacting proteins could be determined on a collection of cells. Using the intrinsic intercellular variation of their expression levels for titration, it was found that the fraction of Ly49A receptors bound in cis increase with increasing amounts of MHC class I ligand. This increase shows a tendency to be more abrupt than for a diffusion limited - three dimensional bimolecular reaction, which most likely reflects the two-dimensional confinement of the reaction. For the Ly49A- MHC class I interaction it indicates that within a critical concentration range the local concentration level of MHC class I can provide a distinct regulation mechanism of the NK cell activity.

Quenching of triplet state fluorophores for studying diffusion-mediated reactions in lipid membranes

An approach to study bimolecular interactions in model lipid bilayers and biological membranes is introduced, exploiting the influence of membrane-associated electron spin resonance labels on the triplet state kinetics of membrane-bound fluorophores. Singlet-triplet state transitions within the dye Lissamine Rhodamine B (LRB) were studied, when free in aqueous solutions, with LRB bound to a lipid in a liposome, and in the presence of different local concentrations of the electron spin resonance label TEMPO. By monitoring the triplet state kinetics via variations in the fluorescence signal, in this study using fluorescence correlation spectroscopy, a strong fluorescence signal can be combined with the ability to monitor low-frequency molecular interactions, at timescales much longer than the fluorescence lifetimes. Both in solution and in membranes, the measured relative changes in the singlet-triplet transitions rates were found to well reflect the expected collisional frequencies between the LRB and TEMPO molecules. These collisional rates could also be monitored at local TEMPO concentrations where practically no quenching of the excited state of the fluorophores can be detected. The proposed strategy is broadly applicable, in terms of possible read-out means, types of molecular interactions that can be followed, and in what environments these interactions can be measured.

Diffusion and partitioning of fluorescent lipid probes in phospholipid monolayers

The pressure-dependent diffusion and partitioning of single lipid fluorophores in DMPC and DPPC monolayers were investigated with the use of a custom-made monolayer trough mounted on a combined fluorescence correlation spectroscopy (FCS) and wide-field microscopy setup. It is shown that lipid diffusion, which is essential for the function of biological membranes, is heavily influenced by the lateral pressure and phase of the lipid structure. Both of these may change dynamically during, e.g., protein adsorption and desorption processes. Using FCS, we measured lipid diffusion coefficients over a wide range of lateral pressures in DMPC monolayers and fitted them to a free-area model as well as the direct experimental observable mean molecular area. FCS measurements on DPPC monolayers were also performed below the onset of the phase transition (Pi < 5 mN/m). At higher pressures, FCS was not applicable for measuring diffusion coefficients in DPPC monolayers. Single-molecule fluorescence microscopy and differential scanning calorimetry clearly showed that this was due to heterogeneous partitioning of the lipid fluorophores in condensed phases. The results were compared with dye partitioning in giant lipid vesicles. These findings are significant in relation to the application of lipid fluorophores to study diffusion in both model systems and biological systems.

Sticky overhangs enhance siRNA-mediated gene silencing

siRNA delivery to cells offers a convenient and powerful means of gene silencing that bypasses several barriers met by gene delivery. However, nonviral vectors, and especially polymers, form looser complexes with siRNA than with plasmid DNA. As a consequence, exchange of siRNA for larger polymeric anions such as proteoglycans found outside cells and at their surface may occur and lower delivery. We show here that making siRNAs "gene-like," via short complementary A(5-8)/T(5-8) 3' overhangs, increases complex stability, and hence RNase protection and gene silencing in vitro up to 10-fold. After decomplexation in the cytoplasm, sticky siRNA (ssiRNA) concatemers fall apart. ssiRNAs are therefore not inducing antiviral responses, as shown by the absence of IFN-beta production. Finally, transfection experiments in the mouse lung show that ssiRNA should be particularly suited to silencing with linear polyethylenimine in vivo.

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

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

High-pressure proton NMR study of lateral self-diffusion of phosphatidylcholines in sonicated unilamellar vesicles

Effects of pressure on the lateral diffusion of phospholipid molecules in sonicated pure 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles (15 wt%) in D2O were examined using the high-pressure proton NMR rotating frame spin-lattice relaxation time (T1rho) method. Proton T1rho were measured at pressures from 1 bar to 5000 bar and at temperatures of 50 degrees C to 70 degrees C for DPPC and 5 degrees C to 35 degrees C for POPC. The T(-1)1rho values were plotted as a function of the square root of the spin-locking field angular frequency (omega1(1/2) and the lateral diffusion coefficient (D) calculated from the slope. Pressure effects on lateral diffusion were observed in the liquid-crystalline (LC) phase. The lateral diffusion coefficient exhibited sharp decreases in response to the various pressure-induced phase transitions encountered. However, pressure had little, if any, effect on lateral diffusion in the pressure-induced gel I (GI) phase and pressure-induced interdigitated gel (Gi) phase. The activation volumes for diffusion were calculated from the slopes from plots of In D versus pressure for both DPPC (37 ml/mol at 50 degrees C, 34 ml/mol at 60 degrees C and 25 ml/mol at 70 degrees C) and POPC (16 ml/mol at 5 degrees C, 9 ml/mol at 20 degrees C and 6 ml/mol at 35 degrees C) sonicated vesicles in the LC phase. The activation energy for diffusion (Ea) was calculated using the slopes from plots of In D versus the inverse of the temperature (1/T) for both DPPC and POPC in the LC phase (3.5 kcal/mol and 3.9 kcal/mol, respectively) and for both DPPC and POPC in the GI phase (6.0 kcal/mol and 4.4 kcal/mol, respectively). From the lateral diffusion coefficient and line width data pressure-temperature phase diagrams for sonicated pure DPPC and POPC vesicles were constructed. The values of the temperature to pressure equivalence of DPPC (dTm/dP) were estimated to be 22.1 degrees C/kbar for the LC to GI phase transition and 28.6 degrees C/kbar for the GI to Gi phase transition. The value of the temperature to pressure equivalence of POPC for the LC to GI phase transition was estimated to be 19.0 degrees C/kbar.

Influence of the gel-liquid phase transition on hematoporphyrin triplet deactivation in liposomes

The deactivation of the triplet state of hematoporphyrin and its dimethyl ester in unilamellar liposomes of dipalmitoyl phosphatidylcholine was studied by nanosecond laser flash photolysis. It was found that the rate of deactivation increases abruptly on raising the temperature in the region of the gel-liquid phase transition of the lipid bilayer (41 degrees C). The rate of change has its maximum at 38.4 +/- 0.5 degree C for both porphyrins. This variation is due to the high lateral mobility of the porphyrins in the liquid-crystal bilayer, which enhances the rates of concentration triplet quenching and triplet-triplet annihilation.

Is ubiquinone diffusion rate-limiting for electron transfer?

The different possible dispositions of the electron transfer components in electron transfer chains are discussed: random distribution of complexes and ubiquinone with diffusion-controlled collisions of ubiquinone with the complexes, random distribution as above, but with ubiquinone diffusion not rate-limiting, diffusion and collision of protein complexes carrying bound ubiquinone, and solid-state assembly. Discrimination among these possibilities requires knowledge of the mobility of the electron transfer chain components. The collisional frequency of ubiquinone-10 with the fluorescent probe 12-(9-anthroyl)stearate, investigated by fluorescence quenching, is 2.3 X 10(9) M-1 sec-1 corresponding to a diffusion coefficient in the range of 10(-6) cm2/sec (Fato, R., Battino, M., Degli Esposti, M., Parenti Castelli, G., and Lenaz, G., Biochemistry, 25, 3378-3390, 1986); the long-range diffusion of a short-chain polar Q derivative measured by fluorescence photobleaching recovery (FRAP) (Gupte, S., Wu, E. S., Höchli, L., Höchli, M., Jacobson, K., Sowers, A. E., and Hackenbrock, C. R., Proc. Natl. Acad. Sci. USA 81, 2606-2610, 1984) is 3 X 10(-9) cm2/sec. The discrepancy between these results is carefully scrutinized, and is mainly ascribed to the differences in diffusion ranges measured by the two techniques; it is proposed that short-range diffusion, measured by fluorescence quenching, is more meaningful for electron transfer than long-range diffusion measured by FRAP, or microcollisions, which are not sensed by either method. Calculation of the distances traveled by random walk of ubiquinone in the membrane allows a large excess of collisions per turnover of the respiratory chain. Moreover, the second-order rate constants of NADH-ubiquinone reductase and ubiquinol-cytochrome c reductase are at least three orders of magnitude lower than the second-order collisional constant calculated from the diffusion of ubiquinone. The activation energies of either the above activities or integrated electron transfer (NADH-cytochrome c reductase) are well above that for diffusion (found to be ca. 1 kcal/mol). Cholesterol incorporation in liposomes, increasing bilayer viscosity, lowers the diffusion coefficients of ubiquinone but not ubiquinol-cytochrome c reductase or succinate-cytochrome c reductase activities. The decrease of activity by ubiquinone dilution in the membrane is explained by its concentration falling below the Km of the partner enzymes. It is calculated that ubiquinone diffusion is not rate-limiting, favoring a random model of the respiratory chain organization.(ABSTRACT TRUNCATED AT 400 WORDS)

Pressure variation of the lateral diffusion in lipid bilayer membranes

A pressure-induced decrease of the lateral diffusion in pure and cholesterol containing phosphatidylcholine bilayer membranes has been determined by the excimer formation technique using pyrene as probe molecule. The experimental results at pressures up to 150 bars are described satisfactorily by the free volume theory of a molecular transport in liquids. A pressure increase of extrapolated 575 bars decreases the lateral diffusion of lipids by a factor of two in pure dipalmitoylphosphatidylcholine membranes. Higher pressures are necessary to induce the same effect in cholesterol containing membranes. This result is interpreted by the condensing effect of cholesterol in fluid bilayer membranes.

Excimer-forming lipids in membrane research

Pyrenedecanoic acid and pyrene lecithin are optical probes well suited to investigate lipid bilayer membranes. The method is based on the determination of the formation of excited dimers or excimers. The rate of excimer formation yields information on the dynamic molecular properties of artificial as well as of natural membranes. This article will review applications of the excimer-forming probes. Pyrene lipid probes are used to determine the coefficient of the lateral diffusion in fluid lipid membranes. Results in artificial membranes are comparable to the values obtained in erythrocyte membranes. Moreover, the excimer formation rate is a very sensitive measure of changes in membrane fluidity. Membrane fluidity is an important regulator of membrane functional proteins. For example, there is a correlation between membrane fluidity and enzyme activities of the adenylate cyclase system. The excimer formation technique is not restricted to the measurement of lateral mobility in membranes. It can also be used to determine the transversal mobility, that is, the lipid exchange between the lipid layers of one bilayer or between bilayers of different vesicles. Again, artificial as well as natural membranes can be investigated by this technique. Another important area of investigation in membrane research is the interaction between lipids and proteins. Lipids, in the presence of a protein, show a different dynamic behavior from free lipids. Because of changes in fluidity and a modified solubility of the pyrene probes within different membrane regions, our methods could also be applied to the examination of phase separation phenomena and to lipid-protein interactions.

Permeability of lipid bilayers to methylmercuric chloride: quantification by fluorescence quenching of a carbazole-labeled phospholipid

We investigated the permeabilities of lipid bilayers to the neurotoxin methylmercuric chloride (MMC). This mercurial is an efficient collisional quencher of the fluorescence of N-alkyl carbazole derivatives. Quenching of the fluorescence of beta-(3-(9-carbazole)-propionyl-L-alpha-phosphatidylcholine (CPA-PC) in vesicles of dimyristoyl phosphatidylcholine and of dioleoyl phosphatidylcholine reveal rapid diffusion of MMC in the alkyl side chain regions of these bilayers. By a combination of (1) the lipid concentration dependence of the apparent quenching constants, (2) the solubility of MMC in concentrated lipid dispersions and (3) the 270 MHz proton magnetic resonance of methylmercury in the presence of lipid bilayers we conclude that the lipid-water partition coefficient of this mercurial is less than or equal to two. Using the fluorescence quenching and the partitioning data we estimate the diffusion coefficient of MMC in these bilayers to range from 0.13 to 0.31 X 10(-5) cm2/sec, or 20--47% of its diffusion coefficient in ethanol. These data indicate that lipid bilayers do not pose a significant permeability barrier to the diffusional transport of MMC.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

On two-dimensional passive random walk in lipid bilayers and fluid pathways in biomembranes

The lateral mobility of pyrene, pyrene decanoic acid, and 1-palmitoyl-2-pyrene decanoyl-phosphatidyl choline (pyrene lecithin) in lipid bilayers is determined by the excimer formation technique. This method is applied to vesicles of lecithins differing in chain length and in the degree of saturation of the hydrocarbon chains. These values are compared with results in cephalins of different chain length and in dipalmitoyl phosphatidic acid at variable pH. The influence of cholesterol is investigated. The results are analyzed in terms of the Montroll model of two-dimensional random walk. The jump frequency of the probe molecule within the lipid lattice is obtained. The advantage of this measure of transport in lipid layers is that it does not involve lipid lattice parameters. The main results of the present work are: (i) The lateral mobility of a given solute molecule in lamellae of saturated lecithins is independent of hydrocarbon chain length and rather a universal function of temperature. (ii) In unsaturated dioleyl lecithin the amphiphatic molecules have lateral mobilities of the same size as in saturated lipids. The jump frequency of pyrene, however, is by a factor of two larger in the unsaturated lecithin. (iii) The jump frequencies in phosphatidyl ethanolamines are about equal to those in lecithins. (iv) In phosphatidic acid layers the hopping frequencies depend on the charges of the head groups of both the lipids and the probes. (v) Cholesterol strongly reduces the jump frequency in fluid layers. (vi) The lateral mobility in biological membranes is comparable to that in artificial lipid bilayers. The experimental results are discussed in terms of the free volume model of diffusion in fluids. Good agreement with the predictions made from this model is found. A striking result is the observation of a tilt in dioleyl-lecithin bilayer membranes from the hopping frequencies of pyrene and pyrene lecithin. A tilt angle of phi = 17 degrees is estimated.

Diffusion and partitioning of a pesticide, lindane, into phosphatidylcholine bilayers. A new fluorescence quenching method to study chlorinated hydrocarbon-membrane interactions

Chlorinated hydrocarbons, such as the pesticide lindane (gamma-hexachlorocyclohexane), quench the fluorescence of carbazole. The observed quenching is a result of the molecular contacts which occur upon diffusional collisions. Because the amount of quenching depends upon the collisional frequency between carbazole and pesticide, this phenomenon provides a measure of both the diffusional rate of lindane and its local concentration. The carbazole fluorophore is localized within phosphatidylcholine bilayers by cosonicating the lipid with a newly synthesized phospholipid, beta-(11-(9-carbazole)-undecanoyl)-L-alpha-phosphatidylcholine. Using this probe in dimyristoyl-L-alpha-phosphatidylcholine vesicles, and the above mentioned quenching phenomena, we determined the lindane diffusion rate within the bilayer to be 5.7.10-7 cm2/s at 37 degrees C. Measurement of the apparent quenching constant at various dimyristoyl phosphatidylcholine concentrations yielded a lipid-water partition coefficient for lindane of 9500, which is in agreement with the value of 8980 obtained by our equilibrium dialysis experiments. Vesicles of dimyristoyl-L-alpha-phosphatidylcholine become saturated with lindane at a pesticide to lipid molar ratio of approx. 0.28. These results demonstrate the possibility of using the quenching of carbazole fluorescence to investigate the transport and partitioning of pesticides within biological membranes. This ability should prove useful in studies of the interactions of chlorinated hydrocarbons with cell membranes.

Amphiphile diffusion in model membrane systems studied by pulsed NMR

The translational diffusion of the amphiphilic molecules in a number of lyotropic liquid crystalline phases has been measured with the pulsed NMR pulsed magnetic field gradient method. The amphiphiles studied were soaps, monoglycerids and lecithins. Measurements were performed both for oriented lamellar and for cubic phases. The order of magnitude of the diffusion coefficients was found to be the same as in neat liquids of analogous compounds. It was also found that the difussion coefficient depend markedly on the amphiphile end group in a way that parallels the area per polar head group as determined in X-ray studies. When corrections for geometrical factors has been made the diffusion rate is approximately equal in cubic and lamellar phases containing the same amphiphile.

Proton NMR bandshape studies of lamellar liquid crystals and gel phases containing lecithins and cholesterol

Proton NMR spectra for gel and liquid crystalline samples, composed of dimyristoyl and/or dipalmitoyl lecithin, cholesterol and water, can be consistently interpreted in terms of mesophase symmetry and molecular diffusion according to a model proposed by Wennerstrom (Wennerstrom, H. (1973) Chem. Phys. Lett. 18, 41-44). It is shown by computer simulation that the characteristic "super-lorentzian" bandshape of the lamellar mesophase can be described by the superposition of three gaussian curves. The NMR signal of the gel phase can be simulated by the superposition of two gaussian curves with widths at half height of 2.5 kHz and 19 kHz. An upper limit of the lateral diffusion coefficient of the lecithin molecules in the gel phase is calculated to be about 5-10(-15) m-2/s. It is therefore concluded that the static intermolecular dipolar couplings average to zero in the lamellar mesophase. An estimation of the order parameter of the liquid crystalline phase is made from experimental data and a calculated "rigid lattice" linewidth. A two phase system is shown to exist in the temperature range 28-34 degrees C for a mesophase of a mixture of dimyristoyl and dipalmitoyl lecithin. The presence of cholesterol results in enhanced lateral diffusion of the lecithin molecules at temperatures below the Chapman transition point.

Phase transitions and fluidity characteristics of lipids and cell membranes

The concept that the liquid crystalline or mesomorphic condition was of importance to biological systems is a relatively old idea. Thus Bernal (1933) when discussing the different types of arrangements of molecules in liquid crystals commented ‘Such structures belong to the liquid crystal as a unit and not to its molecules which may be replaced by others without destroying them and they persist in spite of the complete fluidity of the substance. These are just the properties to be required for a degree of organization between that of the continuous substance, liquid or crystalline solid and even the simplest living cell.’ Stewart (1961) some thirty years later also stated that ‘It is this property – the combination of flow and lability with a preferred and relatively stable molecular orientation – that makes the mesomorphic (i.e. liquid crystal) phase uniquely appropriate to the structure of protoplasm and living tissue.’