Packing of ganglioside-phospholipid monolayers: an x-ray diffraction and reflectivity study

The Interaction between Amyloid Prefibrillar Oligomers of Salmon Calcitonin and a Lipid-Raft Model: Molecular Mechanisms Leading to Membrane Damage, Ca2+-Influx and Neurotoxicity

To investigate the interaction between amyloid assemblies and “lipid-rafts”, we performed functional and structural experiments on salmon calcitonin (sCT) solutions rich in prefibrillar oligomers, proto- and mature-fibers interacting with liposomes made of monosialoganglioside-GM1 (4%), DPPC (48%) and cholesterol (48%). To focus on the role played by electrostatic forces and considering that sCT is positive and GM1 is negative at physiologic pH, we compared results with those relative to GM1-free liposomes while, to assess membrane fluidity effects, with those relative to cholesterol-free liposomes. We investigated functional effects by evaluating Ca²⁺-influx in liposomes and viability of HT22-DIFF neurons. Only neurotoxic solutions rich in unstructured prefibrillar oligomers were able to induce Ca²⁺-influx in the “lipid-rafts” model, suggesting that the two phenomena were correlated. Thus, we investigated protein conformation and membrane modifications occurring during the interaction: circular dichroism showed that “lipid-rafts” fostered the formation of β-structures and energy filtered-transmission electron microscopy that prefibrillar oligomers formed pores, similar to Aβ did. We speculate that electrostatic forces between the positive prefibrillar oligomers and the negative GM1 drive the initial binding while the hydrophobic profile and flexibility of prefibrillar oligomers, together with the membrane fluidity, are responsible for the subsequent pore formation leading to Ca²⁺-influx and neurotoxicity.

Force Field Comparison of GM1 in a DOPC Bilayer Validated with AFM and FRET Experiments

The great physiological relevance of glycolipids is being increasingly recognized, and glycolipid interactions have been shown to be central to cell-cell recognition, neuronal plasticity, protein-ligand recognition, and other important processes. However, detailed molecular-level understanding of these processes remains to be fully resolved. Molecular dynamics simulations could reveal the details of the glycolipid interactions, but the results may be influenced by the choice of the employed force field. Here, we have compared the behavior and properties of GM1, a common, biologically important glycolipid, using the CHARMM36, OPLS, GROMOS, and Amber99-GLYCAM06 (in bilayers comprising SLIPIDS and LIPID14 lipids) force fields in bilayers comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine lipids and compared the results to atomic force microscopy and fluorescence resonance energy transfer experiments. We found discrepancies within the GM1 behavior displayed between the investigated force fields. Based on a direct comparison with complementary experimental results derived from fluorescence and AFM measurements, we recommend using the Amber99-GLYCAM force field in bilayers comprising LIPID14 or SLIPIDS lipids followed by CHARMM36 and OPLS force fields in simulations. The GROMOS force field is not recommended for reproducing the properties of the GM1 head group.

Enhanced Ordering in Monolayers Containing Glycosphingolipids: Impact of Carbohydrate Structure

The influence of carbohydrate structure on the ordering of glycosphingolipids (GSLs) and surrounding phospholipids was investigated in monolayers at the air-water interface. Binary mixtures composed of GSLs, chosen to span a range of carbohydrate complexity, and zwitterionic dipalmitoylphosphatidylcholine phospholipid, were studied. X-ray reflectivity was used to measure the out-of-plane structure of the monolayers and characterize the extension and conformation of the GSL carbohydrates. Using synchrotron grazing incidence x-ray diffraction, the in-plane packing of the lipid acyl chains and the area per molecule within ordered domains were characterized at different mole ratios of the two components. Our findings indicate that GSL-containing mixtures, regardless of the carbohydrate size, enhance the ordering of the surrounding lipids, resulting in a larger fraction of ordered phase of the monolayer and greater dimensions of the ordered domains. Reduction of the averaged area per molecule within the ordered domains was also observed but only in the cases where there was a size mismatch between the phospholipid headgroups and GSL components, suggesting that the condensation mechanism involves the relief of steric interactions between headgroups in mixtures.

Newly Synthesized Lipid-Porphyrin Conjugates: Evaluation of Their Self-Assembling Properties, Their Miscibility with Phospholipids and Their Photodynamic Activity In Vitro

Lipid-porphyrin conjugates are considered nowadays as promising building blocks for the conception of supramolecular structures with multifunctional properties, required for efficient cancer therapy by photodynamic therapy (PDT). The synthesis of two new lipid-porphyrin conjugates coupling pheophorbide-a (Pheo-a), a photosensitizer derived from chlorophyll-a, to either chemically modified lyso-phosphatidylcholine (PhLPC) or egg lyso-sphingomyelin (PhLSM) is reported. The impact of the lipid backbone of these conjugates on their self-assembling properties, as well as on their physicochemical properties, including interfacial behavior at the air/buffer interface, fluorescence and absorption properties, thermotropic behavior, and incorporation rate in the membrane of liposomes were studied. Finally, their photodynamic activity was evaluated on esophageal squamous cell carcinoma (ESCC) and normal esophageal squamous epithelium cell lines. The liposome-like vesicles resulting from self-assembly of the pure conjugates were unstable and turned into aggregates with undefined structure within few days. However, both lipid-porphyrin conjugates could be efficiently incorporated in lipid vesicles, with higher loading rates than unconjugated Pheo-a. Interestingly, phototoxicity tests of free and liposome-incorporated lipid-porphyrin conjugates demonstrated a better selectivity in vitro to esophageal squamous cell carcinoma relative to normal cells.

Monosialoganglioside-GM1 triggers binding of the amyloid-protein salmon calcitonin to a Langmuir membrane model mimicking the occurrence of lipid-rafts

GM1 ganglioside is known to be involved in the amyloid-associated diseases and it is a crucial factor for the assembly of amyloid proteins on lipid-rafts, which are lipid structures located on the synaptic plasma membranes. Due to its slow aggregation rate, we employed salmon calcitonin (sCT) as a suitable probe representative of amyloid proteins, to study the interaction between this class of proteins and a membrane model. Here, we prepared a neuronal membrane model by depositing onto mica two Langmuir-Blodgett films in liquid-condensed phase: the outer monolayer was characterized by high content of GM1 (50%) and minority parts of cholesterol and POPC (25-25%), while the inner one by plain POPC. To deeply investigate the interaction of sCT with this model and the role-played by GM1, we prepared the outer leaflet adding sCT at a concentration such that the number of proteins equals that of GM1. Atomic Force Microscopy revealed the occurrence of two distinct kinds of flat surfaces, with globular aggregates localized exclusively on top of the highest one. To unravel the nature of the interaction, we studied by ζ-potential technique liposomes composed as the outer leaflet of the model. Results demonstrated that an electrostatic interaction sCT-GM1 occurred. Finally, to investigate the interaction thermodynamics between sCT and the outer leaflet, Langmuir films as the outer monolayer and containing increasing content of sCT were studied by compression isotherms and Brewster Angle Microscopy experiments. Based on the all body of results we propose an interaction model where GM1 plays a pivotal role.

X-ray reflectivity and grazing incidence diffraction studies of interaction between human adhesion/growth-regulatory galectin-1 and DPPE-GM1 lipid monolayer at an air/water interface

The specific interaction of ganglioside GM1 with the homodimeric (prototype) endogenous lectin galectin-1 triggers growth regulation in tumor and activated effector T cells. This proven biorelevance directed interest to studying association of the lectin to a model surface, i.e. a 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine/ganglioside GM1 (80 : 20 mol%) monolayer, at a bioeffective concentration. Surface expansion by the lectin insertion was detected at a surface pressure of 20 mN/m. On combining the methods of grazing incidence X-ray diffraction and X-ray reflectivity, a transient decrease in lipid-ordered phase of the monolayer was observed. The measured electron density distribution indicated that galectin-1 is oriented with its long axis in the surface plane, ideal for cis-crosslinking. The data reveal a conspicuous difference to the way the pentameric lectin part of the cholera toxin, another GM1-specific lectin, is bound to the monolayer. They also encourage further efforts to monitor effects of structurally different members of the galectin family such as the functionally antagonistic chimera-type galectin-3.

Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy

Local surface charge density of lipid membranes influences membrane–protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values.

Surface charges on lipid bilayers deeply influence the way proteins interact with cellular membranes, yet their precise quantification has proven challenging. Here, the authors report on a quantitative method to map and evaluate surface charge densities under physiological conditions.

Carbohydrate conformation and lipid condensation in monolayers containing glycosphingolipid Gb3: influence of acyl chain structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3's influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3's capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment's impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.

Impact of Lipid Oxidization on Vertical Structures and Electrostatics of Phospholipid Monolayers Revealed by Combination of Specular X-ray Reflectivity and Grazing-Incidence X-ray Fluorescence

The influence of phospholipid oxidization of floating monolayers on the structure perpendicular to the global plane and on the density profiles of ions near the lipid monolayer has been investigated by a combination of grazing incidence X-ray fluorescence (GIXF) and specular X-ray reflectivity (XRR). Systematic variation of the composition of the floating monolayers unravels changes in the thickness, roughness and electron density of the lipid monolayers as a function of molar fraction of oxidized phospholipids. Simultaneous GIXF measurements enable one to qualitatively determine the element-specific density profiles of monovalent (K(+) or Cs(+)) and divalent ions (Ca(2+)) in the vicinity of the interface in the presence and absence of two types of oxidized phospholipids (PazePC and PoxnoPC) with high spatial accuracy (±5 Å). We found the condensation of Ca(2+) near carboxylated PazePC was more pronounced compared to PoxnoPC with an aldehyde group. In contrast, the condensation of monovalent ions could hardly be detected even for pure oxidized phospholipid monolayers. Moreover, pure phospholipid monolayers exhibited almost no ion specific condensation near the interface. The quantitative studies with well-defined floating monolayers revealed how the elevation of lipid oxidization level alters the structures and functions of cell membranes.

Effect of protein-encapsulation on thermal structural stability of liposome composed of glycosphingolipid/cholesterol/phospholipid

We have studied the thermal structural stability of liposomes encapsulating proteins by using synchrotron radiation small- and wide-angle X-ray scattering (SR-SWAXS). Liposomes are known to be effective drug-delivery systems (DDSs) because they can reduce drug toxicity due to biodegradability and biocompatibility and can offer promising carriers of various types of drugs. However, in spite of numerous studies of liposomes, physicochemical characteristics of liposomes entrapping proteins are rarely known. The liposome studied is characterized by the lipid composition (mixture of acidic glycosphingolipid (ganglioside)/cholesterol/phospholipid). Gangliosides are one of the major constituents of so-called lipid rafts playing the role of a platform of cell-signaling. We have found that the encapsulation of proteins elevates the thermal transition temperature of the liposome membrane and suppresses the deformation of its shape. The present results suggest that not only membrane proteins, but also water-soluble proteins affect liposome stability through the revalence between osmotic pressure and membrane elasticity. In addition, we have found the presence of the size-effect depending on the molar content of gangliosides in the liposome, indicating the ability of ganglioside molecule controlling both the size and effective surface charge of the liposome. The present results would have significance from two different points of view. One is the confinement effect of proteins within a limited space like cell, and the other is a stability of a new type of DDS using gangliosides. Due to the intrinsic properties, gangliosides are expected to be promising agents for targeting and long-circulation properties of liposomal DDSs.

Rheology and simulation of 2-dimensional clathrin protein network assembly

Clathrin is a three-legged protein complex that assembles into lattice structures on the cell membrane and transforms into fullerene-like cages during endocytosis. This dynamic structural flexibility makes clathrin an attractive building block for guided assembly. The assembly dynamics and the mechanical properties of clathrin protein lattices are studied using rheological measurements and theoretical modelling in an effort to better understand two dynamic processes: protein adsorption to the interface and assembly into a network. We find that percolation models for protein network formation are insufficient to describe clathrin network formation, but with Monte Carlo simulations we can describe the dynamics of network formation very well. Insights from this work can be used to design new bio-inspired nano-assembly systems.

Lipid monolayers and adsorbed polyelectrolytes with different degrees of polymerization

Polystyrene sulfonate (PSS) of different molecular weight M(w) is adsorbed to oppositely charged DODAB monolayers from dilute solutions (0.01 mmol/L). PSS adsorbs flatly in a lamellar manner, as is shown by X-ray reflectivity and grazing incidence diffraction (exception: PSS with M(w) below 7 kDa adsorbs flatly disordered to the liquid expanded phase). The surface coverage and the separation of the PSS chains are independent of PSS M(w). On monolayer compression, the surface charge density increases by a factor of 2, and the separation of the PSS chains decreases by the same factor. Isotherms show that on increase of PSS M(w) the transition pressure of the LE/LC (liquid expanded/liquid condensed) phase transition decreases. When the contour length exceeds the persistence length (21 nm), the transition pressure is low and constant. For low-M(w) PSS (<7 kDa) the LE/LC transition of the lipids and the disordered/ordered transition of adsorbed PSS occur simultaneously, leading to a maximum in the contour length dependence of the transition enthalpy. These findings show that lipid monolayers at the air/water interface are a suitable model substrate with adjustable surface charge density to study the equilibrium conformation of adsorbed polyelectrolytes as well as their interactions with a model membrane.

Grazing incidence X-ray diffraction studies of condensed double-chain phospholipid monolayers formed at the soft air/water interface

The use of highly brilliant synchrotron light sources in the middle of the 1980s for X-ray diffraction has revolutionized the research of condensed monolayers. Since then, monolayers gained popularity as convenient quasi two-dimensional model systems widely used in biophysics and material science. This review focuses on structures observed in one-component phospholipid monolayers used as simplified two-dimensional models of biological membranes. In a monolayer system the phase transitions can be easily triggered at constant temperature by increasing the packing density of the lipids by compression. Simultaneously the monolayer structure changes are followed in situ by grazing incidence X-ray diffraction. Competing interactions between the different parts of the molecule are responsible for the different monolayer structures. These forces can be modified by chemical variations of the hydrophobic chain region, of the hydrophilic head group region or of the interfacial region between chains and head groups. Modifications of monolayer structures triggered by changes of the chemical structure of double-chain phospholipids are highlighted in this paper.

Cholesterol as a factor regulating the influence of natural (PAF and lysoPAF) vs synthetic (ED) ether lipids on model lipid membranes

In this work we have performed a comparative study on the effect of antineoplastic ether lipid-edelfosine (ED), its natural analogs - Platelet Activating Factor (PAF) and its precursor (lyso-PAF), both lacking anticancer properties, on cholesterol/phosphatidylcholine (Chol/PC) monolayers, serving as model membranes. Since all the above ether lipids are membrane active, it can be expected that their effect on membranes may differentiate their biological activity. Our investigations were aimed at studying potential relationship of the effect of ED, PAF and lyso-PAF on model membranes, differing in condensation. We have modified molecular packing of Chol/PC model systems either by increasing the level of sterol in the system or changing the structure of PC, while keeping the same sterol content. Additionally, we have performed a detailed comparison of the miscibility of ED, PAF and lyso-PAF with various membrane lipids. The collected data evidenced that all the investigated ether lipids influence Chol/PC films in the same way; however, in a different magnitude. Moreover, the interactions of ED, PAF and lyso-PAF with model membranes were the strongest at the highest level of sterol in the system. A thorough analysis of the obtained results has proved that the effect of the investigated ether lipids on membranes is not dependent on the condensation of the system, but it is strongly determined by the concentration of cholesterol. Since ED was found to interact with model membranes stronger than PAF and lyso-PAF, we have suggested that this fact may contribute to differences in cytotoxicity of these compounds.

Physical interactions of fish protamine and antisepsis peptide drugs with bacterial membranes revealed by combination of specular x-ray reflectivity and grazing-incidence x-ray fluorescence

As a defined model of outer membranes of gram negative bacteria, we investigated the interaction of monolayers of lipopolysacchrides from Salmonella enterica rough strains R90 (LPS Ra) with natural and synthetic peptides. The fine structures perpendicular to the membrane plane and the ion distribution near the interface were determined by specular x-ray reflectivity (XRR) and grazing-incidence x-ray fluorescence (GIXF) in the presence and absence of divalent cations. The unique combination of XRR and GIXF allows for the quantitative identification of different modes of interactions in a high spatial resolution, which cannot be assessed by other experimental methods. Natural fish protamine disrupts the stratified membrane structures in the absence of Ca(2+) ions, while staying away from the membrane surface in the presence of Ca(2+) ions. In contrast, synthetic antisepsis peptide Pep 19-2.5 weakly adsorbs to the membrane and stays near the uncharged sugar units even in the absence of Ca(2+). In the presence of Ca(2+), Pep 19-2.5 can reach the negatively charged inner core without destroying the barrier capability against ions.

Atomic force microscopy studies of a floating-bilayer lipid membrane on a Au(111) surface modified with a hydrophilic monolayer

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

Engineered synthetic virus-like particles and their use in vaccine delivery

Engineered nanoparticles have been designed based on the self-assembling properties of synthetic coiled-coil lipopeptide building blocks. The presence of an isoleucine zipper within the lipopeptide together with the aggregating effects of an N-terminal lipid drives formation of 20-25 nm nanoparticles in solution. Biophysical studies support a model in which the lipid is buried in the centre of the nanoparticle, with 20-30 trimeric helical coiled-coil bundles radiating out into solution. A promiscuous T-helper epitope and a synthetic B-cell epitope mimetic derived from the circumsporozoite protein of Plasmodium falciparum have been linked to each lipopeptide chain, with the result that 60-90 copies of each antigen are displayed over the surface of the nanoparticle. These nanoparticles elicit strong humoral immune responses in mice and rabbits, including antibodies able to cross-react with the parasite, thereby, supporting the potential value of this delivery system in synthetic vaccine design.

Crystalline domain structure and cholesterol crystal nucleation in single hydrated DPPC:cholesterol:POPC bilayers

Grazing incidence X-ray diffraction measurements were performed on single hydrated bilayers and monolayers of DPPC:Cholesterol:POPC at varying concentrations. There are substantial differences in the phase and structure behavior of the crystalline domains formed within the bilayers relative to the corresponding monolayers, due to interactions between the opposing leaflets. Depending on the lipid composition, these interactions led to phase separation, changes in molecular tilt angle, or formation of cholesterol crystals. In monolayers, DPPC and cholesterol form a single crystalline phase at all compositions studied. In bilayers, a second crystalline phase appears when cholesterol levels are increased: domains of cholesterol and DPPC form monolayer thick crystals where each of the lipid leaflets diffracts independently, whereas excess cholesterol forms cholesterol bilayer thick crystals at a DPPC:Chol ratio < 46:54 +/- 2 mol %. The nucleation of the cholesterol crystals occurs at concentrations relevant to the actual cell plasma membrane composition.

Ordering transitions in micrometer-thick films of nematic liquid crystals driven by self-assembly of ganglioside GM1

We report an investigation of the self-assembly of the monosialoganglioside (GM(1)) at interfaces formed between aqueous solutions of 10 microM GM(1) (at 25 degrees C) and micrometer-thick films of the nematic liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl (5CB). We observe the process of spontaneous transfer of GM(1) onto the interfaces to be accompanied by continuous ordering transitions within the micrometer-thick films of the LC. At saturation coverage, the GM(1) orders the LC in an orientation that is perpendicular to the interface, an orientation that is similar to that caused by phospholipids such as dilauroylphosphatidylcholine (DLPC). This result suggests an interaction between the LC and GM(1) that is dominated by the hydrophobic tails of the GM(1). Relative to DLPC, however, we observe the dynamics of the LC ordering transition driven by GM(1) to be slow (2 h for DLPC versus 100 h for GM(1)). To provide insight into the origins of the slow dynamics of the GM(1)-induced ordering transition in the LC, we performed two additional measurements. First, we quantified the time-dependent adsorption of GM(1) at the LC interface by using fluorescently-labeled GM(1). Second, we used the Langmuir-Schaefer method to transfer preorganized monolayers of GM(1) from an air-water interface to the aqueous-LC interface. Results obtained from these two experiments are consistent with a physical picture in which the final stages of spontaneous adsorption/ordering of GM(1) at the aqueous-LC interface dictate the dynamics of the LC ordering transition. This rate limiting process underlying the ordering transition was substantially accelerated by heating the system above the phase transition temperature of GM(1)(26 degrees C), suggesting that the phase state of the GM(1) micellar aggregates in bulk solution strongly influences the kinetics of the final stages of ordering/adsorption of GM(1) at the LC interface. Overall, these results and others presented in this manuscript reveal that it is possible to decorate interfaces of a nematic LC with GM(1), and that the assembly of GM(1) at these interfaces impacts the dynamic and equilibrium ordering of the LC.

Langmur monolayers of cerebroside with different head groups originated from sea cucumber: binary systems with dipalmitoylphosphatidylcholine (DPPC)

Surface properties (Langmuir monolayer) of two different cerebrosides which are extracted from the sea cucumber (Bohadschia argus) were investigated. A main difference in chemical structure of cerebroside between BAC-2a and BAC-4 is their head groups (glucose and galactose, respectively). Furthermore, miscibility and interaction between dipalmitoylphosphatidylcholine (DPPC) and cerebrosides (BAC-2a and BAC-4) in the monolayer have been systematically examined. The surface pressure (pi)-area (A), the surface potential (Delta V)-A, and the dipole moment (mu(perpendicular))-A isotherms for monolayers of DPPC, cerebrosides, and their binary combinations have been measured using the Wilhelmy method and the ionizing electrode method. BAC-4 forms a stable liquid-expanded (LE) monolayer, whereas BAC-2a has a first-order phase transition from the LE phase to the liquid-condensed (LC) state on 0.15 M NaCl at 298.2 K. The fundamental properties for each cerebroside monolayer were elucidated in terms of the surface dipole moment based on the three-layer model [R.J. Demchak, T. Fort Jr., J. Colloid Interface Sci. 46 (1974) 191-202] for both cerebrosides and the apparent molar quantity change (Delta s(gamma), Delta h(gamma), and Delta u(gamma)) for BAC-2a. In addition, their miscibility with DPPC was examined by the variation of the molecular areas and the surface potentials as a function of cerebroside mole fractions, the additivity rule. The miscibility was also confirmed by constructing the two-dimensional phase diagrams. The phase diagrams for the both binary systems were of negative azeotropic type. That is, the two-component DPPC/BAC-2a and DPPC/BAC-4 monolayers are miscible. Furthermore, the Joos equation for the analysis of the collapse pressure of binary monolayers allowed calculation of the interaction parameter and the interaction energy between the DPPC and cerebroside monolayers. The miscibility in the monolayer state was also confirmed by the morphological observation with Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM).

Electrochemical and PM-IRRAS a glycolipid-containing biomimetic membrane prepared using Langmuir-Blodgett/Langmuir-Schaefer deposition

Differential capacitance, chronocoulometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of a DMPC + cholesterol + GM 1 (60:30:10 mol %) bilayer supported at a Au(111) electrode surface prepared using combined Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) deposition. The electrochemical measurements indicate that the incorporation of ganglioside GM 1 into the membrane significantly improves the quality of the bilayer, reflected in the very low capacitance value of approximately 0.8 microF cm (-2). An analysis of the infrared data suggests that the incorporation of the glycolipid into the membrane changes both the orientation of the lipid acyl chains in the membrane and the hydration of the membrane, particularly with respect to the interfacial region of the lipids.

Lipid membrane templates the ordering and induces the fibrillogenesis of Alzheimer's disease amyloid-beta peptide

The lipid membrane has been shown to mediate the fibrillogenesis and toxicity of Alzheimer's disease (AD) amyloid-beta (Abeta) peptide. Electrostatic interactions between Abeta40 and the phospholipid headgroup have been found to control the association and insertion of monomeric Abeta into lipid monolayers, where Abeta exhibited enhanced interactions with charged lipids compared with zwitterionic lipids. To elucidate the molecular-scale structural details of Abeta-membrane association, we have used complementary X-ray and neutron scattering techniques (grazing-incidence X-ray diffraction, X-ray reflectivity, and neutron reflectivity) in this study to investigate in situ the association of Abeta with lipid monolayers composed of either the anionic lipid 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG), the zwitterionic lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), or the cationic lipid 1,2-dipalmitoyl 3-trimethylammonium propane (DPTAP) at the air-buffer interface. We found that the anionic lipid DPPG uniquely induced crystalline ordering of Abeta at the membrane surface that closely mimicked the beta-sheet structure in fibrils, revealing an intriguing templated ordering effect of DPPG on Abeta. Furthermore, incubating Abeta with lipid vesicles containing the anionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) induced the formation of amyloid fibrils, confirming that the templated ordering of Abeta at the membrane surface seeded fibril formation. This study provides a detailed molecular-scale characterization of the early structural fluctuation and assembly events that may trigger the misfolding and aggregation of Abeta in vivo. Our results implicate that the adsorption of Abeta to anionic lipids, which could become exposed to the outer membrane leaflet by cell injury, may serve as an in vivo mechanism of templated-aggregation and drive the pathogenesis of AD.

Integration of ganglioside GT1b receptor into DPPE and DPPC phospholipid monolayers: an X-ray reflectivity and grazing-incidence diffraction study

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivity, the in-plane and out-of-plane structures of mixed-ganglioside GT(1b)-phospholipid monolayers were investigated at the air-liquid interface and compared with monolayers of the pure components. The receptor GT(1b) is involved in the binding of lectins and toxins, including botulinum neurotoxin, to cell membranes. Monolayers composed of 20 mol % ganglioside GT(1b), the phospholipid dipalmitoyl phosphatidylethanolamine (DPPE), and the phospholipid dipalmitoyl phosphatidylcholine (DPPC) were studied in the gel phase at 23 degrees C and at surface pressures of 20 and 40 mN/m, and at pH 7.4 and 5. Under these conditions, the two components did not phase-separate, and no evidence of domain formation was observed. The x-ray scattering measurements revealed that GT(1b) was intercalated within the host DPPE/DPPC monolayers, and slightly expanded DPPE but condensed the DPPC matrix. The oligosaccharide headgroups extended normally from the monolayer surfaces into the subphase. This study demonstrated that these monolayers can serve as platforms for investigating toxin membrane binding and penetration.

Asymmetric structural features in single supported lipid bilayers containing cholesterol and GM1 resolved with synchrotron X-Ray reflectivity

The cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor G(M1) affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10 A thicker and have a higher acyl-chain electron density (rho(chain) = 0.33 e(-)/A(3)) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (rho(chain) = 0.28 e(-)/A(3)). Incorporating G(M1) into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4-9 A. The structural effect of G(M1) is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl-sn-glycero-3-phosphocholine, where the distal chains show a high electron density (rho(chain) = 0.33 e(-)/A(3)) and the lipid diffusion constant is reduced by approximately 50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer.

Part I: an x-ray scattering study of cholera toxin penetration and induced phase transformations in lipid membranes

Cholera toxin is a highly efficient biotoxin, which is frequently used as a tool to investigate protein-membrane interactions and as a reporter for membrane rafts. Cholera toxin binds selectively to gangliosides with highest affinity to GM(1). However, the mechanism by which cholera toxin crosses the membrane remains unresolved. Using x-ray reflectivity and grazing incidence diffraction, we have been able to monitor the binding and penetration of cholera toxin into a model lipid monolayer containing the receptor GM(1) at the air-water interface. Very high toxin coverage was obtained allowing precise measurements of how toxin binding alters lipid packing. Grazing incidence x-ray diffraction revealed the coexistence of two monolayer phases after toxin binding. The first was identical to the monolayer before toxin binding. In regions where toxin was bound, a second membrane phase exhibited a decrease in order as evidenced by a larger area per molecule and tilt angle with concomitant thinning of the monolayer. These results demonstrate that cholera toxin binding induces the formation of structurally distinct, less ordered domains in gel phases. Furthermore, the largest decrease in lateral order to the monolayer occurred at low pH, supporting a low endosomal pH in the infection pathway. Surprisingly, at pH = 8 toxin penetration by the binding portion of the toxin, the B(5) pentamer, was also observed.

Condensing and fluidizing effects of ganglioside GM1 on phospholipid films

Mixed monolayers of the ganglioside G(M1) and the lipid dipalmitoylphosphatidlycholine (DPPC) at air-water and solid-air interfaces were investigated using various biophysical techniques to ascertain the location and phase behavior of the ganglioside molecules in a mixed membrane. The effects induced by G(M1) on the mean molecular area of the binary mixtures and the phase behavior of DPPC were followed for G(M1) concentrations ranging from 5 to 70 mol %. Surface pressure isotherms and fluorescence microscopy imaging of domain formation indicate that at low concentrations of G(M1) (<25 mol %), the monolayer becomes continually more condensed than DPPC upon further addition of ganglioside. At higher G(M1) concentrations (>25 mol %), the mixed monolayer becomes more expanded or fluid-like. After deposition onto a solid substrate, atomic force microscopy imaging of these lipid monolayers showed that G(M1) and DPPC pack cooperatively in the condensed phase domain to form geometrically packed complexes that are more ordered than either individual component as evidenced by a more extended total height of the complex arising from a well-packed hydrocarbon tail region. Grazing incidence x-ray diffraction on the DPPC/G(M1) binary mixture provides evidence that ordering can emerge when two otherwise fluid components are mixed together. The addition of G(M1) to DPPC gives rise to a unit cell that differs from that of a pure DPPC monolayer. To determine the region of the G(M1) molecule that interacts with the DPPC molecule and causes condensation and subsequent expansion of the monolayer, surface pressure isotherms were obtained with molecules modeling the backbone or headgroup portions of the G(M1) molecule. The observed concentration-dependent condensing and fluidizing effects are specific to the rigid, sugar headgroup portion of the G(M1) molecule.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Biofunctionalized Surfactant Mesophases as Polyvalent Inhibitors of Cholera Toxin

The cubic lyotropic mesophase composed of the ganglioside G(M1) and the synthetic surfactant phytantriol has been employed as a scaffold to prepare a polyvalent inhibitor of cholera toxin (CT). Surfactant mixtures containing up to 20% (w/w) G(M1)/phytantriol afforded a hydrated insoluble gel-like material, which retained an inverse cubic phase (Q) structure in excess water and at 22 degrees C, as confirmed using small-angle X-ray scattering. The G(M1)-functionalized mesophases bind up to 98.8% of CT from solution containing 100 ng/mL of CT with a dissociation constant (K(d)) of 67-73 nM. The estimated IC50 values for the mesophase systems were 0.1-0.27 microM. The inhibitory effect of the mesophases may be enhanced through the high internal surface area of the inverse cubic phase in addition to the optimal self-orientation of G(M1) ligand to match the distribution of binding sites on the toxin surface. As a result, polyvalent inhibitor materials manufactured using these mesophase structures do not require precise control of ligand distribution, which offers advantages with respect to complexity of synthesis and formulation when compared to more rigid and conformationally restricted materials. This approach provides a route to a unique class of polyvalent inhibitors, which should be broadly applicable to a range of bacterial and plant toxins.

Characterization of the conformational and orientational dynamics of ganglioside GM1 in a dipalmitoylphosphatidylcholine bilayer by molecular dynamics simulations

The structure and dynamics of a single GM1 (Gal5-beta1,3-GalNAc4-beta1,4-(NeuAc3-alpha2,3)-Gal2-beta1,4-Glc1-beta1,1-Cer) embedded in a DPPC bilayer have been studied by MD simulations. Eleven simulations, each of 10 ns productive run, were performed with different initial conformations of GM1. Simulations of GM1-Os in water and of a DPPC bilayer were also performed to delineate the effects of the bilayer and GM1 on the conformational and orientational dynamics of each other. The conformation of the GM1 headgroup observed in the simulations is in agreement with those reported in literature; but the headgroup is restricted when embedded in the bilayer. NeuAc3 is the outermost saccharide towards the water phase. Glc1 and Gal2 prefer a parallel, and NeuAc3, GalNac4 and Gal5 prefer a perpendicular, orientation with respect to the bilayer normal. The overall characteristics of the bilayer are not affected by the presence of GM1; however, GM1 does influence the DPPC molecules in its immediate vicinity. The implications of these observations on the specific recognition and binding of GM1 embedded in a lipid bilayer by exogenous proteins as well as proteins embedded in lipids have been discussed.

Thermodynamic description of the interactions between lipids in ternary Langmuir monolayers: the study of cholesterol distribution in membranes

The aim of this work was to get insight into cholesterol distribution between two leaflets of a phospholipids bilayer. In this order, the thermodynamic analysis of the interactions between membrane lipids in binary (cholesterol/phospholipid) and ternary (phospholipid/ phospholipid/cholesterol) mixed Langmuir monolayers has been performed. For our investigation, phosphatidylcholine and phosphatidylethanolamine, which are the main types of phospholipids determining the distribution of cholesterol in membrane leaflets, were chosen and mixed in proportions corresponding to their molar ratios in the inner and outer layers of the natural human erythrocyte membrane. Into these mixed systems, various amount of cholesterol were incorporated. It has been found that despite strong differences in the phospholipid composition of both investigated ternary mixed systems, the influence of cholesterol is very similar, which indicates that cholesterol is symmetrically distributed between the inner and outer leaflets of the human erythrocytes membrane.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Insertion selectivity of antimicrobial peptide protegrin-1 into lipid monolayers: effect of head group electrostatics and tail group packing

The ability to selectively target the harmful microbial membrane over that of the host cell is one of the most important characteristics of the antimicrobial peptides (AMPs). This selectivity strongly depends on the chemical and structural properties of the lipids that make up the cell membrane. A systematic study of the initial membrane selectivity of protegrin-1 (PG-1), a beta-sheet AMP, was performed using Langmuir monolayers. Constant pressure insertion assay was used to quantify the amount of PG-1 insertion and fluorescence microscopy was employed to observe the effect of PG-1 on lipid ordering. Charge and packing properties of the monolayer were altered by using lipids with different head groups, substituting saturated with unsaturated lipid tail group(s) and incorporating spacer molecules. PG-1 inserted most readily into anionic films composed of phosphatidylglycerol (PG) and lipid A, consistent with its high selectivity for microbial membranes. It also discriminated between zwitteranionic phospholipids, inserting more readily into phosphatidylcholine (PC) monolayers than those composed of phosphatidylethanolamine, potentially explaining why PG-1 is hemolytic for PC-rich human erythrocytes and not for the PE-rich erythrocytes of ruminants. Increased packing density of the monolayer by increased surface pressure, increased tail group saturation or incorporation of dihydrocholesterol diminishes the insertion of PG-1. Fluorescence microscopy shows that lipid packing is disordered upon PG-1 insertion. However, the presence of PG-1 can still affect lipid morphology even with no observed PG-1 insertion. These results show the important role that lipid composition of the cell membrane plays in the activity of AMPs.

Behavior of GM3 ganglioside in lipid monolayers mimicking rafts or fluid phase in membranes

We studied the interaction of GM3 ganglioside with sphingomyelin (SM) and palmitoyl-oleoyl-phosphatidylcholine (POPC) in Langmuir monolayers mimicking, respectively, raft and fluid phase of a cellular membrane, by surface pressure measurements and fluorescence microscopy. No difference was observed in the behavior of SM-GM3 and POPC-GM3 monolayers. In both cases, a GM3 threshold concentration has been underlined between 20 and 40 mol%. Below this threshold, SM-GM3 and POPC-GM3 monolayers behave ideally, suggesting that GM3 and host lipid would form separated domains. On the contrary, above the threshold, a condensation of monolayers is observed. This could be due to a partial solubilisation of GM3 in host lipid, leading to a change in orientation of GM3 molecules at the air-water interface.

Microstructural analysis of the effects of incorporation of myelin basic protein in phospholipid layers

We report an X-ray reflectivity study on the effects of adsorption of myelin basic protein (MBP) on Langmuir monolayers and on deposited Langmuir-Schaefer multilayers of the phospholipid dipalmitoyl phosphatidylglycerol (DPPG). We provide for the first time, direct microscopic evidence on the destructuring effects of MBP leading to plasticity of the DPPG layers supporting commonly accepted models of the stabilizing role of MBP in the myelin membrane. We also show how protein adsorption onto the layer is determined both by electrostatic and nonspecific hydrophobic interactions.

Cerebroside Langmuir monolayers originated from the echinoderms: II. Binary systems of cerebrosides and steroids

Two-component Langmuir monolayers formed on a subphase of 0.5M sodium chloride solution were investigated for two different cerebrosides (LMC-1 and LMC-2) with steroids of cholesterol (Ch) and cholesteryl sodium sulfate (Ch-S); i.e. LMC-1/Ch, LMC-1/Ch-S, LMC-2/Ch, and LMC-2/Ch-S were examined in terms of surface pressure (pi), the surface potential (DeltaV) and the dipole moment (mu( perpendicular)) as a function of surface area (A) by employing the Langmuir method, the ionizing electrode method, and the fluorescence microscopy. Surface potentials (DeltaV) of steroids were analyzed using the three-layer model proposed by Demchak and Fort. The miscibility of cerebrosides and steroids in the insoluble monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the steroid molar fraction (X(steroid)) based upon the additivity rule. From the A-X(steroid) and DeltaV(m)-X(steroid) plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the different surface pressures. The PMA and APSP with the mole fraction were discussed for the miscible system. Judging from the two-dimensional phase diagrams, they can be classified into two types. The first is a completely immiscible type; the combination of cerebrosides with cholesterol. The second is a negative azeotropic type, where cerebrosides and cholesteryl sodium sulfate are completely miscible both in the expanded state and in the condensed state. In addition, a regular surface mixture (the Joos equation for the analysis of the collapse pressure of two-component monolayers) allowed calculation of the interaction parameter (xi) and the interaction energy (-Delta epsilon) between the cerebrosides and Ch-S. The miscibility of cerebroside and steroid components in the monolayer state was also supported by fluorescence microscopy.

Structure of cholesterol/ceramide monolayer mixtures: implications to the molecular organization of lipid rafts

The structure of monolayers of cholesterol/ceramide mixtures was investigated using grazing incidence x-ray diffraction, immunofluorescence, and atomic force microscopy techniques. Grazing incidence x-ray diffraction measurements showed the existence of a crystalline mixed phase of the two components within a range of compositions of cholesterol/ceramide between 100:0 and 67:33. The mixed phase coexists with the ceramide crystalline phase in the range of compositions between 50:50 and 30:70; between 30:70 and 0:100 only the highly crystalline phase of ceramide was detected. The latter was determined and modeled. Immunolabeling was performed with an antibody specific to the cholesterol monohydrate crystalline arrangement. The antibody recognizes crystalline cholesterol monolayers, but does not interact with crystalline ceramide. Immunofluorescence and atomic force microscopy data show that in uncompressed ceramide monolayers, the highly crystalline phase coexists with a disordered loosely packed phase. In contrast, no disordered phase coexists with the new crystalline mixed phase. We conclude that the new mixed phase represents a stable homogeneous arrangement of cholesterol with ceramide. As ceramide incorporates the lipid backbone common to all sphingolipids, this arrangement may be relevant to the understanding of the molecular organization of lipid rafts.

Cerebroside Langmuir monolayers originated from the echinoderms

The surface pressure (pi)-area (A), the surface potential (DeltaV)-A and the dipole moment (mu( perpendicular))-A isotherms were obtained for two-component monolayers of two different cerebrosides (LMC-1 and LMC-2) with phospholipids of dipalmitoylphosphatidylcholine (DPPC) and with dipalmitoylphosphatidylethanolamine (DPPE) on a subphase of 0.5 M sodium chloride solution as a function of phospholipid compositions by employing the Langmuir method, the ionizing electrode method, and the fluorescence microscopy. Surface potentials (DeltaV) of pure components were analyzed using the three-layer model proposed by Demchak and Fort. The contributions of the hydrophilic saccharide group and the head group to the vertical component of the dipole moment (mu( perpendicular)) were estimated. The miscibility of cerebroside and phospholipid in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the phospholipid molar fraction (X(phospholipid)), using the additivity rule. From the A-X(phospholipid) and DeltaV(m)-X(phospholipid) plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible system. Judging from the two-dimensional phase diagrams, these can be classified into two types. The first is a positive azeotropic type; the combinations of cerebrosides with DPPC are miscible with each other. The second is a completely immiscible type: the combination of cerebrosides with DPPE. Furthermore, a regular surface mixture, for which the Joos equation was used for the analysis of the collapse pressure of two-component monolayers, allowed calculation of the interaction parameter (xi) and the interaction energy (-Delta epsilon) between the cerebrosides and DPPC component. The miscibility of cerebroside and phospholipid components in the monolayer state was also supported by fluorescence microscopy.

Oligomer-to-polymer transition in short ethylene glycol chains connected to mobile hydrophobic anchors

We studied the structure of short ethylene glycol (EG) chains with N repeating units (EGN, N = 3, 6, 9, 12, and 15) connected to hydrophobic dihexadecyl chains by means of a combination of differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS/WAXS). These synthetic amphiphiles dispersed in water form planar lamellar stacks and hexagonal cylinders confining the EG chains to restricted geometries. Owing to the self-assembly of the anchoring points, the lateral density of EG chains in planar lamella can be quantitatively controlled. Furthermore, the chain-melting phase transition of the anchors enables us to "switch" the intermolecular distance reversibly. SAXS/WAXS results suggest that the shorter EG chains (N = 3, 6, and 9) assume a helical conformation in stacks of planar lamella. When the EG chains are further elongated (N = 12 and 15), the lamellar periodicities cannot be explained by a linear extrapolation of shorter oligomers, but can be interpreted well as polymer brushes following the scaling theorem. Such rich phase behaviors of EGN molecules can be used as a simple model of oligo/poly-saccharide chains on cell surfaces, which act not only as flexible repellers between neighboring cells but also as stable spacers for functional ligands.