Long-range lipid-protein interactions. Evidence from time-resolved fluorescence depolarization and energy-transfer experiments with bacteriorhodopsin-dimyristoylphosphatidylcholine vesicles

Zeolitic imidazolate frameworks as intrinsic light harvesting and charge separation materials for photocatalysis

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal organic frameworks that have attracted considerable attention in the past years and have found many applications including heterogeneous catalysis due to their highly ordered porous structure, large surface area, and structural flexibility. However, ZIFs are largely utilized as simple hosts or passive media for dispersing other catalytically active species, resembling the roles of zeolites in catalysis. In contrast, our recent findings show that ZIFs not only have broad absorption across the UV-visible and near IR spectral region but also have an exceptionally long-lived excited charge separated state, suggesting that ZIFs may be used as intrinsic light harvesting and photocatalytic materials rather than as inert hosts. This Perspective will focus on the recent progress on the fundamental studies of the intrinsic light absorption, charge separation, and photocatalytic properties of ZIFs and will discuss the outlook for future development.

Distance dependent energy transfer dynamics from a molecular donor to a zeolitic imidazolate framework acceptor

Zeolitic Imidazolate frameworks (ZIFs) have been demonstrated as promising light harvesting and photocatalytic materials for solar energy conversion. To facilitate their application in photocatalysis, it is essential to develop a fundamental understanding of their light absorption properties and energy transfer dynamics. In this work, we report distance-dependent energy transfer dynamics from a molecular photosensitizer (RuN3) to ZIF-67, where the distance between RuN3 and ZIF-67 is finely tuned by depositing an ultrathin Al2O3 layer on the ZIF-67 surface using an atomic layer deposition (ALD) method. We show that energy transfer time decreases with increasing distance between RuN3 and ZIF-67 and the Förster radius is estimated to be 14.4 nm.

Detection of nanosized ordered domains in DOPC/DPPC and DOPC/Ch binary lipid mixture systems of large unilamellar vesicles using a TEMPO quenching method

Nanosized ordered domains formed in 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DOPC/DPPC) and DOPC/cholesterol (Ch) liposomes were characterized using a newly developed (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) quenching method. The membrane fluidity of the DOPC/DPPC liposomes, evaluated by the use of 1,6-diphenyl-1,3,5-hexatriene (DPH), increased significantly above their phase-transition temperature. The fluorescence spectra of 6-lauroyl-2-dimethylamino naphthalene (Laurdan) indicated the formation of an immiscible ordered phase in the DOPC/DPPC (50/50) liposomal membrane at 30 °C. The analysis of the membrane polarity indicated that the surface of the liquid-disordered phase was hydrated whereas that of the ordered phase was dehydrated. DOPC/DPPC and DOPC/Ch (70/30) liposomes exhibited heterogeneous membranes, indicating that nanosized ordered domains formed on the surface of the DOPC/DPPC liposomes. The size of these nanosized ordered domains was estimated using the TEMPO quenching method. Because TEMPO can quench DPH distributed in the disordered phases, the remaining fluorescence from DPH is proportional to the size of the ordered domain. The domain sizes calculated for DOPC/DPPC (50/50), DOPC/DPPC (25/75), DOPC/Ch (70/30), and DOPC/DPPC/Ch (40/40/20) were 13.9, 36.2, 13.2, and 35.5 Å, respectively.

Structure and physical properties of biomembranes and model membranes

Biomembranes belong to the most important structures of the cell and the cell organels. They play not only structural role of the barrier separating the external and internal part of the membrane but contain also various functional molecules, like receptors, ionic channels, carriers and enzymes. The cell membrane also preserves non-equillibrium state in a cell which is crucial for maintaining its excitability and other signaling functions. The growing interest to the biomembranes is also due to their unique physical properties. From physical point of view the biomembranes, that are composed of lipid bilayer into which are incorporated integral proteins and on their surface are anchored peripheral proteins and polysaccharides, represent liquid scrystal of smectic type. The biomembranes are characterized by anisotropy of structural and physical properties. The complex structure of biomembranes makes the study of their physical properties rather difficult. Therefore several model systems that mimic the structure of biomembranes were developed. Among them the lipid monolayers at an air-water interphase, bilayer lipid membranes (BLM), supported bilayer lipid membranes (sBLM) and liposomes are most known. This work is focused on the introduction into the "physical word" of the biomembranes and their models. After introduction to the membrane structure and the history of its establishment, the physical properties of the biomembranes and their models areare stepwise presented. The most focus is on the properties of lipid monolayers, BLM, sBLM and liposomes that were most detailed studied. This contribution has tutorial character that may be usefull for undergraduate and graduate students in the area of biophysics, biochemistry, molecular biology and bioengineering, however it contains also original work of the author and his co-worker and PhD students, that may be usefull also for specialists working in the field of biomembranes and model membranes.

Modelling of proteins in membranes

This review describes some recent theories and simulations of mesoscopic and microscopic models of lipid membranes with embedded or attached proteins. We summarize results supporting our understanding of phenomena for which the activities of proteins in membranes are expected to be significantly affected by the lipid environment. Theoretical predictions are pointed out, and compared to experimental findings, if available. Among others, the following phenomena are discussed: interactions of interfacially adsorbed peptides, pore-forming amphipathic peptides, adsorption of charged proteins onto oppositely charged lipid membranes, lipid-induced tilting of proteins embedded in lipid bilayers, protein-induced bilayer deformations, protein insertion and assembly, and lipid-controlled functioning of membrane proteins.

Simulation studies of protein-induced bilayer deformations, and lipid-induced protein tilting, on a mesoscopic model for lipid bilayers with embedded proteins

Biological membranes are complex and highly cooperative structures. To relate biomembrane structure to their biological function it is often necessary to consider simpler systems. Lipid bilayers composed of one or two lipid species, and with embedded proteins, provide a model system for biological membranes. Here we present a mesoscopic model for lipid bilayers with embedded proteins, which we have studied with the help of the dissipative particle dynamics simulation technique. Because hydrophobic matching is believed to be one of the main physical mechanisms regulating lipid-protein interactions in membranes, we considered proteins of different hydrophobic length (as well as different sizes). We studied the cooperative behavior of the lipid-protein system at mesoscopic time- and lengthscales. In particular, we correlated in a systematic way the protein-induced bilayer perturbation, and the lipid-induced protein tilt, with the hydrophobic mismatch (positive and negative) between the protein hydrophobic length and the pure lipid bilayer hydrophobic thickness. The protein-induced bilayer perturbation was quantified in terms of a coherence length, xi(P), of the lipid bilayer hydrophobic thickness profile around the protein. The dependence on temperature of xi(P), and the protein tilt-angle, were studied above the main-transition temperature of the pure system, i.e., in the fluid phase. We found that xi(P) depends on mismatch, i.e., the higher the mismatch is, the longer xi(P) becomes, at least for positive values of mismatch; a dependence on the protein size appears as well. In the case of large model proteins experiencing extreme mismatch conditions, in the region next to the so-called lipid annulus, there appears an undershooting (or overshooting) region where the bilayer hydrophobic thickness is locally lower (or higher) than in the unperturbed bilayer, depending on whether the protein hydrophobic length is longer (or shorter) than the pure lipid bilayer hydrophobic thickness. Proteins may tilt when embedded in a too-thin bilayer. Our simulation data suggest that, when the embedded protein has a small size, the main mechanism to compensate for a large hydrophobic mismatch is the tilt, whereas large proteins react to negative mismatch by causing an increase of the hydrophobic thickness of the nearby bilayer. Furthermore, for the case of small, peptidelike proteins, we found the same type of functional dependence of the protein tilt-angle on mismatch, as was recently detected by fluorescence spectroscopy measurements.

Modulation of glycophorin A transmembrane helix interactions by lipid bilayers: molecular dynamics calculations

Starting from the glycophorin A dimer structure determined by NMR, we performed simulations of both dimer and monomer forms in explicit lipid bilayers with constant normal pressure, lateral area, and temperature using the CHARMM potential. Analysis of the trajectories in four different lipids reveals how lipid chain length and saturation modulate the structural and energetic properties of transmembrane helices. Helix tilt, helix-helix crossing angle, and helix accessible volume depend on lipid type in a manner consistent with hydrophobic matching concepts: the most relevant lipid property appears to be the bilayer thickness. Although the net helix-helix interaction enthalpy is strongly attractive, analysis of residue-residue interactions reveals significant unfavorable electrostatic repulsion between interfacial glycine residues previously shown to be critical for dimerization. Peptide volume is nearly conserved upon dimerization in all lipid types, indicating that the monomeric helices pack equally well with lipid as dimer helices do with one another. Enthalpy calculations indicate that the helix-environment interaction energy is lower in the dimer than in the monomer form, when solvated by unsaturated lipids. In all lipid environments there is a marked preference for lipids to interact with peptide predominantly through one rather than both acyl chains. Although our trajectories are not long enough to allow a full thermodynamic treatment, these results demonstrate that molecular dynamics simulations are a powerful method for investigating the protein-protein, protein-lipid, and lipid-lipid interactions that determine the structure, stability and dynamics of transmembrane alpha-helices in membranes.

The role of chromophore in the lipid-protein interactions in bacteriorhodopsin-phosphatidylcholine vesicles

By fluorescence and phase properties of a 1-acyl-2-[8-(2-anthroyl)-octanoyl]-sn-glycero-3-phosphocholine probe, the influence of the chromophore on the phase transition of bacteriorhodopsin-lipid vesicles was investigated. It was observed that removal of the chromophore led to the down-shifting of the phase transition temperatures. The temperatures corresponding to the beginning and ending of the gel-liquid phase transition were also influenced. This demonstrated that the liquid phase is reached more easily when the chromophore is bleached. The results indicate that removal of the chromophore alters the protein-lipid interactions. It is suggested that this alteration might be related to the change in the lipid molecular packing.

The transmembrane protein bacterioopsin affects the polarity of the hydrophobic core of the host lipid bilayer

Influence of the transmembrane protein bacterioopsin (the retinal-free form of bacteriorhodopsin) on the polarity of egg-phosphatidylcholine bilayers was studied by means of a steady-state and time-resolved fluorescence approach exploiting the solvatochromic properties of the 2-anthroyl fluorophore. Introduced in phosphatidylcholine molecules in the form of 8-(2-anthroyl)octanoic acid, this fluorophore probed the hydrocarbon core of the lipid bilayer. As previously shown (E. Pérochon et al., Biochemistry 31 (1992) 7672-7682), water molecules were detected in this region of the terminal part of the lipid acyl chains. Their number was considerably reduced upon addition of bacterioopsin to the lipids. This was assessed by a blue shift in the fluorescence emission spectra of the probe and a marked decrease in the fractional population of fluorophores interacting with water, to the benefit of those experiencing a hydrophobic environment. In agreement with current theories, this decrease in the hydration of the bilayer may be linked to an increase in the acyl chain order and a decrease in the lateral diffusion coefficient of lipids near the protein. The data obtained at high protein concentration accounts for a protein/lipid interface which is much less hydrated than the hydrophobic core of a protein-free lipid bilayer.

Is the protein/lipid hydrophobic matching principle relevant to membrane organization and functions?

Biological membranes are complex and well-organized multimolecular assemblies composed of a wide variety of protein and lipid molecular species. If such a diversity in protein and lipid polar headgroup structures may easily be related to a large panel of functions, the wide dispersion in acyl chain length and structure which the lipids display is more difficult to understand. It is not required for maintaining bilayer assembly and fluidity. Direct information on the lateral distribution of these various molecular species, on their potential specificity for interaction between themselves and with proteins and on the functional implications of these interactions is also still lacking. Because hydrophobic interactions play a major role in stabilizing membrane structures, we suggest considering the problem from the point of view of the matching of the hydrophobic surface of proteins by the acyl chains of the lipids. After a brief introduction to the hydrophobic matching principle, we will present experimental results which demonstrate the predictive power of the current theories and then, we will introduce the new and important concept of protein/lipid sorting in membranes. Finally, we will show how the hydrophobic matching condition may play a key role in the membrane organization and function.

Lipid domains and lipid/protein interactions in biological membranes

In the fluid mosaic model of membranes, lipids are organized in the form of a bilayer supporting peripheral and integral proteins. This model considers the lipid bilayer as a two-dimensional fluid in which lipids and proteins are free to diffuse. As a direct consequence, both types of molecules would be expected to be randomly distributed within the membrane. In fact, evidences are accumulating to indicate the occurrence of both a transverse and lateral regionalization of membranes which can be described in terms of micro- and macrodomains, including the two leaflets of the lipid bilayer. The nature of the interactions responsible for the formation of domains, the way they develop and the time- and space-scale over which they exist represent today as many challenging problems in membranology. In this report, we will first consider some of the basic observations which point to the role of proteins in the transverse and lateral regionalization of membranes. Then, we will discuss some of the possible mechanisms which, in particular in terms of lipid/protein interactions, can explain lateral heterogenities in membranes and which have the merit of providing a thermodynamic support to the existence of lipid domains in membranes.

Hydrophobic mismatch and long-range protein/lipid interactions in bacteriorhodopsin/phosphatidylcholine vesicles

Mismatch between the hydrophobic thicknesses of transmembrane proteins and the supporting lipid bilayer and its consequences on the lateral organization of lipids have been investigated with bacteriorhodopsin and phosphatidylcholine species with a variety of acyl-chain lengths. The purple membrane, from the bacterium Halobacterium halobium, was used and reconstituted with dilauroyl-(Lau2GroPCho), dimyristoyl- (Myr2GroPCho), dipalmitoyl- (Pam2GroPCho) and distearoyl- (Ste2GroPCho) glycerophosphocholine. The phase behaviour of the lipids was investigated at different temperatures and different protein/lipid molar ratios, by analyzing the fluorescence excitation spectra of the 1-acyl-2-[8-(2-anthroyl)-octanoyl]-sn-glycero-3-phosphocholine probe, and by measuring the fluorescence depolarization of the 1,6-diphenyl-1,3,5-hexatriene probe. Data obtained with 1-acyl-2-[8-(2-anthroyl)-octanoyl]-sn-glycero-3-phosphocholine shows that bacteriorhodopsin produced positive or negative shifts in the phase transition temperature of the host lipids depending on the strength and sign of the mismatch between the lipid and protein hydrophobic thicknesses and also on the protein concentration and aggregation state in the lipid bilayer. In the region of high protein concentration (bacteriorhodopsin/phosphatidylcholine molar ratios approximately 1:50) and despite the presence of the endogenous lipids, bacteriorhodopsin (hydrophobic length dP approximately 3.0-3.1 nm) brought about a large upward shift in the phase-transition temperature of Lau2GroPCho (delta T approximately 40 K, mean hydrophobic thickness d approximately 2.4 nm), and to a lesser extent of Myr2GroPCho (delta T approximately 23 K, d approximately 2.8 nm), accounting for a strong rigidifying effect of the protein on these short-chain lipids. Bacteriorhodopsin had no influence on the phase properties of Pam2GroPCho (delta T approximately 0 K, d approximately 3.2 nm), a lipid whose mean hydrophobic thickness is similar to that of the protein. In contrast, the transition temperature of Ste2GroPCho was decreased (delta T approximately -13 K, d approximately 3.7 nm), indicating a fluidifying effect of the protein on this long-chain lipid. Similar effects on the lipid acyl-chain order were observed in the region of high-protein dilution (bacteriorhodopsin/phosphatidylcholine molar ratios < 1:500). In this region and for Lau2GroPCho, both the spectroscopic data and circular-dichroism spectra indicated that the protein was in the monomeric form. Phase diagrams, in temperature versus bacteriorhodopsin concentration, were constructed for Lau2GroPCho and Ste2GroPCho. On account of microscopic theoretical models and of the relative values of dP and d, these diagrams indicate a preference of the protein for those lipid molecules which are in the gel-ordered state in Lau2GroPCho but in the liquid disordered state in Ste2GroPCho.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid enrichment and selectivity of integral membrane proteins in two-component lipid bilayers

A model recently used to study lipid-protein interactions in one-component lipid bilayers (Sperotto and Mouritsen, 1991 a, b) has been extended in order to include two different lipid species characterized by different acyl-chain lengths. The model, which is a statistical mechanical lattice model, assumes that hydrophobic matching between lipid-bilayer hydrophobic thickness and hydrophobic length of the integral protein is an important aspect of the interactions. By means of Monte Carlo simulation techniques, the lateral distribution of the two lipid species near the hydrophobic protein-lipid interface in the fluid phase of the bilayer has been derived. The results indicate that there is a very structured and heterogeneous distribution of the two lipid species near the protein and that the protein-lipid interface is enriched in one of the lipid species. Out of equilibrium, the concentration profiles of the two lipid species away from the protein interface are found to develop a long-range oscillatory behavior. Such dynamic membrane heterogeneity may be of relevance for determining the physical factors involved in lipid specificity of protein function.

Fluorescence resonance energy transfer measurements on cell surfaces. A spectroscopic tool for determining protein interactions

The interaction of cell surface components may influence several events during the process of transmembrane signalling. Receptor clustering, conformational changes and altered molecular interactions often play essential roles in the final outcome of ligand receptor interactions. Fluorescence resonance energy transfer (FRET) is an excellent tool which can be used to determine distance relationships and supramolecular structure on cell surfaces. This paper reviews the theoretical basis of fluorescence resonance energy transfer, its spectrofluorometric and flow cytometric applications, and provides a critical evaluation of the methods. Finally, examples are given to illustrate the use of the method of fluorescence resonance energy transfer in solving biological problems.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

The physical state of the erythrocyte membrane in myotonic dystrophy

The molecular pathology of myotonic dystrophy is believed to be expressed at the plasma membrane level. Previous assessments of membrane fluidity, a marker of the biochemical state of the membrane, have yielded conflicting results. In this study, erythrocyte membrane fluidity was reevaluated using highly sensitive fluorescence probe techniques. Steady-state anisotropy was measured with diphenylhexatriene (DPH), trimethylaminophenyl-hexatriene (TMA-DPH) and phenylhexatrienylphenylpropionic acid, probing different regions of the membrane. In the patients, significantly increased steady-state anisotropy was obtained with DPH, probing the hydrophobic core of the membrane, while slightly reduced anisotropy was found with TMA-DPH. The dynamic properties of the membrane lipids were further examined by means of time-resolved measurements with DPH. The excited state decay kinetics could best be described by a bi-exponential decay model. A large redistribution of the probe populations and a reduction of the average order parameter were found in the patients indicating a less ordered or more fluid lipid matrix. These perturbations might be induced by a protein abnormality and altered protein-lipid interaction within the erythrocyte membrane.

The interaction of prostaglandins with human serum lipoproteins

Using high density and low density lipoproteins (HDL and LDL) labeled with fluorescent analogues of phosphatidylcholine or sphingomyelin it was found that low amounts (10(-12) M) of prostaglandins E1 and F2 alpha induced different structural rearrangements of the lipoprotein surface, whereas prostaglandins E2 and F1 alpha had no effect. The effects of prostaglandin E1 on HDL were largely paralleled by those of this prostaglandin on synthetic recombinants prepared from pure apolipoprotein A1, phospholipids and cholesterol and were demonstrated to be caused by prostaglandin-apolipoprotein interaction. The interaction resembled that of a ligand with a specific receptor protein because it was specific, reversible, concentration and temperature dependent and saturable. However the retaining capacity of HDL or LDL for prostaglandin E1 as determined by equilibrium dialysis was very low and a single prostaglandin E1 molecule was able to induce structural changes in large numbers of discrete lipoprotein particles. To explain this remarkable fact a non-equilibrium model of ligand-receptor interaction is proposed. According to that model in open systems characterized by weak ligand-receptor binding, high diffusion rate of the ligand and long relaxation times which exceed the interval between two successive receptor occupations, the ligand-induced changes will accumulate, resulting in transformation of the system into a new state which may be far away from equilibrium. It is emphasized that the low mobility of lipids constituting the environment of the receptor protein plays a critical role in this type of signal amplification. It was further demonstrated that the PGE1-induced changes of the lipoprotein surface resulted in an enhancement of LDL-to-HDL transfer of cholesterol esters and phosphatidylcholine especially in the presence of serum lipid transfer proteins. The acceleration of the interlipoprotein transfer caused by prostaglandin E1 in turn increases the rate of cholesterol esterification in serum. It is suggested that in such a way prostaglandin E1 may influence the homeostasis of cholesterol.

Luminescence spectroscopic approaches in studying cell surface dynamics

The major elements of membranes, such as proteins, lipids and polysaccharides, are in dynamic interaction with each other (Albertset al.1983). Protein diffusion in the lipid matrix of the membrane, the lipid diffusion and dynamic domain formation below and above their transition temperature from gel to fluid state, have many functional implications. This type of behaviour of membranes is often summarized in one frequently used word membrane fluidity (coined by Shinitzky & Henkart, 1979). The dynamic behaviour of the cell membrane includes rotational, translational and segmental movements of membrane elements (or their domain-like associations) in the plane of, and perpendicular to the membrane. The ever changing proximity relationships form a dynamic pattern of lipids, proteins and saccharide moieties and are usually described as ‘cell-surface dynamics’ (Damjanovichet al.1981). The knowledge about the above defined behaviour originates from experiments performed mostly on cytoplasmic membranes of eukaryotic cells. Nevertheless numerous data are available also on the mitochondrial and nuclear membranes, as well as endo (sarco-)plasmic reticulum (Martonosi, 1982; Slater, 1981; Siekevitz, 1981).

Differential scanning calorimetry and 2H NMR studies of the phase behavior of gramicidin-phosphatidylcholine mixtures

The extents of two-phase coexistence in the phase diagrams of mixtures of gramicidin with 1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine (DPPC-d62) and with 1,2-bis(perdeuteriomyristoyl)-sn-glycero-3-phosphocholine (DMPC-d54) mixtures have been explored with differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H NMR). For both systems, increased gramicidin content causes a decrease in transition enthalpy and a broadening of the peak in excess heat capacity at the transition. In DMPC-d54-based mixtures, the broadening is roughly symmetric about the pure lipid transition temperature. Addition of gramicidin to DPPC-d62 extends the excess heat capacity peak on the low-temperature side, resulting in a slightly asymmetric scan. Deuterium NMR spectra showing a superposition of gel and liquid-crystalline components, observed for both mixtures, indicate the presence of two-phase coexistence. For the DPPC-d62-based mixtures, two-phase coexistence is restricted to an approximately 2 degrees C temperature range below the pure transition temperature. For DMPC-d54-based mixtures, the region of two-phase coexistence is even narrower. For both mixtures, beyond a gramicidin mole fraction of 2%, distinct gel and liquid-crystal contributions to the spectra cannot be distinguished. Along with the broad featureless nature of the DSC scan in this region, this is taken to indicate that the transition has been replaced by a continuous phase change. These results are consistent with the existence of a closed two-phase region having a critical concentration of gramicidin below 2 mol%.

The 'delta pH'-probe 9-aminoacridine: response time, binding behaviour and dimerization at the membrane

The fluorescence quenching of 9-aminoacridine (9-AA) after imposition of a transmembrane pH gradient (inside acidic) in liposomes has been investigated for a number of different lipid systems. The initial fluorescence decrease after a rapid pH jump, induced in the extravesicular medium by a stopped-flow mixing technique, was ascribed to a response of 9-AA to the imposed pH gradient and not to changes in the vesicular system itself. Time constants for this fluorescence quenching are in the range of several hundred milliseconds at 25 degrees C. Fluorescence recovery which should be correlated to the dissipation of the pH gradient occurs in the 100 s time range and is 10-30-times faster than the delta pH decay monitored with the entrapped hydrophilic pH-indicator dye pyranine. The quenching was severely hindered below the lipid phase transition of dipalmitoylphosphatidylglycerol. No delta pH-induced quenching was obtained in lipid vesicles containing only zwitterionic, net uncharged phosphatidylcholine headgroups. For the occurrence of quenching, the presence of negatively charged headgroups, i.e. phosphatidylglycerol or phosphatidylserine, was necessary. The extent of quenching, at a specific pH difference applied, had a cooperative dependency (Hill coefficient approximately 2) on the number of negative headgroups in the membrane and on the concentration of unquenched (unbound) 9-AA molecules. The concentration of quenched 9-AA molecules was furthermore proportional to the number of dimer-excimer complexes of 9-AA which are formed during the quenching process.

Effects of temperature and benzyl alcohol on the structure and adenylate cyclase activity of plasma membranes from bovine adrenal cortex

Adenylate cyclase activation by corticotropin (ACTH), fluoride and forskolin was studied as a function of membrane structure in plasma membranes from bovine adrenal cortex. The composition of these membranes was characterized by a very low cholesterol and sphingomyelin content and a high protein content. The fluorescent probes 1,6-diphenylhexa-1,3,5-triene (DPH) and a cationic analogue 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) were, respectively, used to probe the hydrophobic and polar head regions of the bilayer. When both probes were embedded either in the plasma membranes or in liposomes obtained from their lipid extracts, they exhibited lifetime heterogeneity, and in terms of the order parameter S, hindered motion. Under all the experimental conditions tested, S was higher for TMA-DPH than for DPH but both S values decreased linearly with temperature within the range of 10 to 40 degrees C, in the plasma membranes and the liposomes. This indicated the absence of lipid phase transition and phase separation. Addition to the membranes of up to 100 mM benzyl alcohol at 20 degrees C also resulted in a linear decrease in S values. Membrane perturbations by temperature changes or benzyl alcohol treatment made it possible to distinguish between the characteristics of adenylate cyclase activation with each of the three effectors used. Linear Arrhenius plots showed that when adenylate cyclase activity was stimulated by forskolin or NaF, the activation energy was similar (70 kJ.mol-1). Fluidification of the membrane with benzyl alcohol concentrations of up to 100 mM at 12 or 24 degrees C produced a linear decrease in the forskolin-stimulated activity, that led to its inhibition by 50%. By contrast, NaF stabilized adenylate cyclase activity against the perturbations induced by benzyl alcohol at both temperatures. In the presence of ACTH, biphasic Arrhenius plots were characterized by a well-defined break at 18 degrees C, which shifted at 12.5 degrees C in the presence of 40 mM benzyl alcohol. These plots suggested that ACTH-sensitive adenylate cyclase exists in two different states. This hypothesis was supported by the striking difference in the effects of benzyl alcohol perturbation when experiments were performed below and above the break temperature. The present results are consistent with the possibility that clusters of ACTH receptors form in the membrane as a function of temperature and/or lipid phase fluidity.(ABSTRACT TRUNCATED AT 400 WORDS)

Monte Carlo calculations of order parameter profiles in models of lipid-protein interactions in bilayers

The Monte Carlo method has been utilized to calculate lipid chain order parameters in model monomolecular layers (half-bilayers) containing several different model polypeptides. The systems all consist of a periodic array of identical cells, each containing 35 hydrocarbon chains and 1 "perturbant" (a small model polypeptide or protein). The lipid chains are each 10 CH2 subunits long, have one end constrained to lie in the bilayer plane, and interact via van der Waals forces between all subunits. The chains also interact with the perturbant via van der Waals forces. With standard Monte Carlo procedures order parameter profiles are calculated for chains that are close to the perturbant and for the nonneighboring chains. In order to examine a wide range of possibilities, several different model polypeptides are considered: (i) a rigid smooth cylinder, (ii) a cylinder with identical side chains at alpha-helical positions, (iii) a cylinder with nonidentical side chains at alpha-helical positions, and (iv) a cylinder identical with (ii) but which only extends about halfway through the monolayer. Although results differ for the different systems studied, in all cases only slight conformational differences between the bulk chains and the chains that are nearest the perturbants are found, and it is not possible to characterize the boundary chains as "more ordered" or "less ordered" than the nonboundary chains.