Molecular orientation in lipid bilayers

Data evaluation for surface-sensitive label-free methods to obtain real-time kinetic and structural information of thin films: A practical review with related software packages

Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Given the growing number of applications, analysis of such layers and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent for monitoring the formation kinetics, structure and its evolution of thin layers, even at the nanoscale. In this paper, we review existing and commercially available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information during and after formation, and any subsequent adsorption/desorption processes. We outline techniques, some traditional and some novel, based on the principles of optical and mechanical transduction. Our special focus is the current possibilities of combining label-free methods, which is a powerful approach to extend the range of detected and deduced parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and, hence, with layers in a hydrated state. A thorough treamtmaent of the various kinetic and structural quantities obtained from evaluation of the raw label-free data are provided. Such quantities include layer thickness, refractive index, optical anisotropy (and molecular orientation derived therefrom), degree of hydration, viscoelasticity, as well as association and dissociation rate constants and occupied area of subsequently adsorbed species. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own extensive experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed dedicated software packages for data analysis, which are made available to the scientific community alongside this paper.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

New insights into the molecular mechanisms of biomembrane structural changes and interactions by optical biosensor technology

Biomolecular-membrane interactions play a critical role in the regulation of many important biological processes such as protein trafficking, cellular signalling and ion channel formation. Peptide/protein-membrane interactions can also destabilise and damage the membrane which can lead to cell death. Characterisation of the molecular details of these binding-mediated membrane destabilisation processes is therefore central to understanding cellular events such as antimicrobial action, membrane-mediated amyloid aggregation, and apoptotic protein induced mitochondrial membrane permeabilisation. Optical biosensors have provided a unique approach to characterising membrane interactions allowing quantitation of binding events and new insight into the kinetic mechanism of these interactions. One of the most commonly used optical biosensor technologies is surface plasmon resonance (SPR) and there have been an increasing number of studies reporting the use of this technique for investigating biophysical analysis of membrane-mediated events. More recently, a number of new optical biosensors based on waveguide techniques have been developed, allowing membrane structure changes to be measured simultaneously with mass binding measurements. These techniques include dual polarisation interferometry (DPI), plasmon waveguide resonance spectroscopy (PWR) and optical waveguide light mode spectroscopy (OWLS). These techniques have expanded the application of optical biosensors to allow the analysis of membrane structure changes during peptide and protein binding. This review provides a theoretical and practical overview of the application of biosensor technology with a specific focus on DPI, PWR and OWLS to study biomembrane-mediated events and the mechanism of biomembrane disruption. This article is part of a Special Issue entitled: Lipid-protein interactions.

Sample handling in surface sensitive chemical and biological sensing: a practical review of basic fluidics and analyte transport

This paper gives an overview of the advantages and associated caveats of the most common sample handling methods in surface-sensitive chemical and biological sensing. We summarize the basic theoretical and practical considerations one faces when designing and assembling the fluidic part of the sensor devices. The influence of analyte size, the use of closed and flow-through cuvettes, the importance of flow rate, tubing length and diameter, bubble traps, pressure-driven pumping, cuvette dead volumes, and sample injection systems are all discussed. Typical application areas of particular arrangements are also highlighted, such as the monitoring of cellular adhesion, biomolecule adsorption-desorption and ligand-receptor affinity binding. Our work is a practical review in the sense that for every sample handling arrangement considered we present our own experimental data and critically review our experience with the given arrangement. In the experimental part we focus on sample handling in optical waveguide lightmode spectroscopy (OWLS) measurements, but the present study is equally applicable for other biosensing technologies in which an analyte in solution is captured at a surface and its presence is monitored. Explicit attention is given to features that are expected to play an increasingly decisive role in determining the reliability of (bio)chemical sensing measurements, such as analyte transport to the sensor surface; the distorting influence of dead volumes in the fluidic system; and the appropriate sample handling of cell suspensions (e.g. their quasi-simultaneous deposition). At the appropriate places, biological aspects closely related to fluidics (e.g. cellular mechanotransduction, competitive adsorption, blood flow in veins) are also discussed, particularly with regard to their models used in biosensing.

Birefringence of a normal human red blood cell and related optomechanics in an optical trap

A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell's dimple region and the slow axis is along a diameter. We report an average retardation of 3.5 ± 1.5 nm for linearly polarized green light (λ = 546 nm). We also estimate a retardation of 1.87 ± 0.09 nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.

Optical anisotropy of flagellin layers: in situ and label-free measurement of adsorbed protein orientation using OWLS

The surface adsorption of the protein flagellin was followed in situ using optical waveguide lightmode spectroscopy (OWLS). Flagellin did not show significant adsorption on a hydrophilic waveguide, but very rapidly formed a dense monolayer on a hydrophobic (silanized) surface. The homogeneous and isotropic optical layer model, which has hitherto been generally applied in OWLS data interpretation for adsorbed protein films, failed to characterize the flagellin layer, but it could be successfully modeled as an uniaxial thin film. This anisotropic modeling revealed a significant positive birefringence in the layer, suggesting oriented protein adsorption. The adsorbed flagellin orientation was further evidenced by monitoring the surface adsorption of truncated flagellin variants, in which the terminal protein regions or the central (D3) domain were removed. Without the terminal regions the protein adsorption was much slower and the resulting films were significantly less birefringent, implying that intact flagellin adsorbs on the hydrophobic surface via its terminal regions.

Molecular interaction of a new antibacterial polymer with a supported lipid bilayer measured by an in situ label-free optical technique

The interaction of the antibacterial polymer-branched poly(ethylene imine) substituted with quaternary ammonium groups, PEO and alkyl chains, PEI25QI5J5A815-with a solid supported lipid bilayer was investigated using surface sensitive optical waveguide spectroscopy. The analysis of the optogeometrical parameters was extended developing a new composite layer model in which the structural and optical anisotropy of the molecular layers was taken into consideration. Following in situ the change of optical birefringence we were able to determine the composition of the lipid/polymer surface layer as well as the displacement of lipid bilayer by the antibacterial polymer without using additional labeling. Comparative assessment of the data of layer thickness and optical anisotropy helps to reveal the molecular mechanism of antibacterial effect of the polymer investigated.

S-nitrosylation decreases the adsorption of H-Ras in lipid bilayer and changes intrinsic catalytic activity

Structural, chemical, and mutational studies have shown that C-terminal cysteine residues on H-Ras could potentially be oxidized by nitrosylation. For investigating the effect of nitrosylation of Ras molecule on the adsorption of farnesylated H-Ras into lipid layer, experiments with optical waveguide lightmode spectroscopy were used. The analysis of association/dissociation kinetics to planar phospholipids under controlled hydrodynamic conditions has shown that preliminary treatment of protein by S-nitroso-cysteine decreased the adsorption of farnesylated H-Ras. The authors have found that compared with nitrosylated forms, farnesylated H-Ras has more compact configuration, because of the smaller area occupied by protein upon absorption at the membrane. The association rate coefficient for unmodified H-Ras was lower than similar parameter for farnesylated and nitrosylated forms. However, the desorbability, i.e., parameter, which reflects the rate of dissociation of protein from lipids is higher for farnesylated H-Ras. In addition, it was have found that farnesylation of cytoplasmic H-Ras, in contrast to membrane-derived forms, inhibits intrinsic GTPase activity of protein, and preliminary treatment of H-Ras by S-nitroso-cysteine restores the activity to the control level. These data suggest that nitrosylation of H-Ras rearranges the adsorptive potential and intrinsic GTPase activity of H-Ras through modification of C-terminal cysteines of molecule.

The membrane insertion of helical antimicrobial peptides from the N-terminus of Helicobacter pylori ribosomal protein L1

The interaction of two helical antimicrobial peptides, HPA3 and HPA3P with planar supported lipid membranes was quantitatively analysed using two complementary optical biosensors. The peptides are analogues of Hp(2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1 (RpL1). The binding of these two peptide analogues to zwitterionic dimyristoyl-phosphatidylcholine (DMPC) and negatively charged membranes composed of DMPC/dimyristoylphosphatidylglycerol (DMPG) (4:1) was determined using surface plasmon resonance (SPR) and dual polarisation interferometry (DPI). Using SPR analysis, it was shown that the proline substitution in HPA3P resulted in much lower binding for both zwitterionic and anionic membranes than HPA3. Structural changes in the planar DMPC and DMPC/DMPG (4:1) bilayers induced by the binding of both Hp(2-20) analogues were then resolved in real-time with DPI. The overall process of peptide-induced changes in membrane structure was analysed by the real-time changes in bound peptide mass as a function of bilayer birefringence. The insertion of both HPA3 and HPA3P into the supported lipid bilayers resulted in a decrease in birefringence with increasing amounts of bound peptide which reflects a decrease in the order of the bilayer. The binding of HPA3 to each membrane was associated with a higher level of bound peptide and greater membrane lipid disordering and a faster and higher degree of insertion into the membrane than HPA3P. Furthermore, the binding of both HPA3 and HPA3P to negatively charged DMPC/DMPG bilayers also leads to a greater disruption of the lipid ordering. These results demonstrate the geometrical changes in the membrane upon peptide insertion and the extent of membrane structural changes can be obtained quantitatively. Moreover, monitoring the effect of peptides on a structurally characterised bilayer has provided further insight into the role of membrane structure changes in the molecular basis of peptide selectivity and activity and may assist in defining the mode of antimicrobial action.

PAMAM dendrimer interactions with supported lipid bilayers: a kinetic and mechanistic investigation

The interaction kinetics of polyamidoamine (PAMAM) dendrimers with supported lipid bilayers of 1,2-sn-glycero-dimyristoylphosphocholine prepared by the vesicle deposition has been probed by optical waveguide lightmode spectroscopy and atomic force microscopy (AFM). In particular, the influence of PAMAM dendrimer generation (G2, G4, and G6) and concentration (1 to 100 nM) on the levels of adsorption and lipid bilayer removal have been determined as a function of time; hence interaction kinetics and mechanisms have been further elucidated. Dendrimer interaction kinetics with the lipid bilayer are concentration dependent in a complex manner, with net bilayer removal at 1 and 100 nM and net adsorption at 10 nM; these effects are irrespective of dendrimer generation. The pseudo first order rate constant for bilayer removal (at 1 and 100 nM) follows the order G6 > G4 > G2. In contrast, the pseudo first order rate constant for adsorption at 10 nM follows the order G2 > G4 > G6. AFM has confirmed expansion of lipid bilayer defects, hole formation, and adsorption to the bilayer or bilayer defects, and their concentration and generation dependence. These findings have implications when designing dendrimers for specific biopharmaceutical activities, e.g., as drugs, drug delivery vehicles, transfection agents, or antimicrobials.

Quasi-isotropic analysis of anisotropic thin films on optical waveguides

Thin films assembled on a substrate are often anisotropic. Nevertheless, because of experimental limitations, sufficient parameters to characterize the anisotropy, even in the simplest (and perhaps most common) case of uniaxial thin films, which are birefringent, are not usually available. This paper examines the consequences of treating them as isotropic thin films, with particular reference to their characterization via perturbation of the propagation constants (effective refractive indices) of optical waveguides. It is shown that the refractive index and geometrical thickness of a thin film thus calculated are often unrealistic (especially when the thin film is positively birefringent), but the mass per unit area may be quite precise, depending on the sign and magnitude of the birefringence.

Photoinduced transformations in bacteriorhodopsin membrane monitored with optical microcavities

Photoinduced molecular transformations in a self-assembled bacteriorhodopsin (bR) monolayer are monitored by observing shifts in the near-infrared resonant wavelengths of linearly polarized modes circulating in a microsphere cavity. We quantify the molecular polarizability change upon all-trans to 13-cis isomerization and deprotonation of the chromophore retinal ( approximately -57 A(3)) and determine its orientation relative to the bR membrane ( approximately 61 degrees ). Our observations establish optical microcavities as a sensitive off-resonant spectroscopic tool for probing conformations and orientations of molecular self-assemblies and for measuring changes of molecular polarizability at optical frequencies. We provide a general estimate of the sensitivity of the technique and discuss possible applications.

The optics of vertebrate photoreceptors: anisotropy and form birefringence

The optics of vertebrate photoreceptors have been investigated with specific reference to the effect of form birefringence. The complex dielectric tensor of the lamellar-like outer segment structure has been derived, allowing the transverse spectral absorbance to be calculated for different incident polarizations. These results were used to calculate the changes in the cellular dichroic ratio as a function of both the volume occupied by the bilayers and the real and complex parts of the intrinsic birefringence of the bilayers. Physiologically realistic values of these parameters show the cellular dichroic ratio to be greater than the bilayer dichroic ratio by a factor of approximately 1.3. Furthermore, the calculations of spectral absorbance indicate that form birefringence may affect measurements of optical density in transversely orientated outer segments.

Adhesion and clustering of charge isomers of myelin basic protein at model myelin membranes

The association of myelin basic protein charge isomers with the lipid part of the myelin membrane was investigated at the microscopic (molecular) level in a model membrane system, using optical waveguide lightmode spectrometry to determine with high precision the kinetics of association and dissociation to planar phospholipid membranes under controlled hydrodynamic conditions and over a range of protein concentrations. Detailed analysis of the data revealed a rich and intricate behaviour and clearly showed that the membrane protein affinity is characterized by at least four independent parameters: (i) the association rate coefficient characterizing the protein-membrane interaction energy as the protein approaches the fluid-membrane interface; (ii) the protein-membrane adhesion, i.e., the probability that it will remain at the membrane after arrival; (iii) the protein conformation at the membrane; and (iv) the protein's tendency to cluster at the membrane. Some of these parameters varied in characteristic ways as the bulk solution concentration of the protein was varied, giving further clues to the detailed molecular comportment of the protein. The parameters and their characteristic variations with bulk concentration were markedly different for the different isomers. Implications of these results for neurological disorders involving demyelination, such as multiple sclerosis, are discussed.

Optical anisotropy in lipid bilayer membranes: coupled plasmon-waveguide resonance measurements of molecular orientation, polarizability, and shape

The birefringence and linear dichroism of anisotropic thin films such as proteolipid membranes are related to molecular properties such as polarizability, shape, and orientation. Coupled plasmon-waveguide resonance (CPWR) spectroscopy is shown in the present work to provide a convenient means of evaluating these parameters in a single lipid bilayer. This is illustrated by using 1-10 mol % of an acyl chain chromophore-labeled phosphatidylcholine (PC) incorporated into a solid-supported PC bilayer deposited onto a hydrated silica surface. CPWR measurements were made of refractive index and extinction coefficient anisotropies with two exciting light wavelengths, one of which is absorbed by the chromophore and one of which is not. These results were used to calculate longitudinal and transverse molecular polarizabilities, the orientational order parameter and average angle between the longitudinal axis of the lipid molecule and the membrane normal, and the molecular shape factors of the lipid molecules. The values thereby obtained are in excellent agreement with parameters determined by other techniques, and provide a powerful tool for analyzing lipid-protein, protein-protein, and protein-ligand interactions in proteolipid films.

Regulation of the binding of myristoylated alanine-rich C kinase substrate (MARCKS) related protein to lipid bilayer membranes by calmodulin

The effector domain (ED) of MARCKS proteins can associate with calmodulin (CaM) as well as with phospholipids. It is not clear, however, whether a complex between MARCKS proteins and CaM can form at the surface of phospholipid membranes or whether CaM and membranes compete for ED binding. Using two-mode waveguide spectroscopy, we have investigated how CaM regulates the association of MARCKS-related protein (MRP) with planar supported phospholipid bilayer membranes. Bringing a solution containing CaM into contact with membranes on which MRP had previously been deposited results in low-affinity binding of CaM to MRP. A preformed, high-affinity CaM MRP complex in the aqueous phase binds much more slowly than pure MRP to membranes. Similar observations were made when a peptide corresponding to the ED of MRP was used instead of MRP. Hence CaM cannot form a stable complex with MRP once the latter is bound at the membrane surface. CaM can, however, strongly retard the association of MRP with lipid membranes. The most likely interpretation of these results is that CaM and the phospholipid membrane share the same binding region at the ED and that the ED is forced by membrane binding to adopt a conformation unfavorable for CaM binding.

Nonelectrostatic contributions to the binding of MARCKS-related protein to lipid bilayers

The association of various protein constructs of MARCKS-related protein (MRP) lacking the myristoyl moiety or the basic effector domain (ED) or both to neutral and acidic supported planar phospholipid bilayer membranes has been monitored using two-mode optical waveguide spectroscopy. The importance of the myristoyl moiety for interaction with both neutral and acidic membranes is demonstrated but unmyristoylated MRP still binds appreciably to neutral membranes, albeit less than to acidic membranes. Only when both the myristoyl moiety and the ED are excised does the interaction fall to zero in the case of the acidic membranes, with very small residual binding still detectable in the presence of neutral membranes. These results point to the importance of hydrophobic interactions apart from those associated with the myristoyl moiety in the association of MRP with membranes. The ED is well endowed with hydrophobic as well as with basic residues, and the former are chiefly responsible for binding unmyristoylated MRP to neutral membranes: The very small residual attraction between MRP lacking both the myristoyl moiety and the ED is completely outweighed by electrostatic repulsion between the net acidic MRP and the acidic lipid head groups.