A biosensor that uses ion-channel switches

Functional tethered membranes

Phospholipid bilayer membranes at the interface between a substrate and an aqueous phase, supported by or tethered to the solid surface via a polymer cushion, a peptide-, protein-, or oligosaccharide-coupling layer have reached a stage at which they are important as a novel model membrane system but also offer potential for practical applications (e.g. for biosensing purposes with membrane-integral receptors). We briefly summarize some of the recent progress made in the structural characterization of the build-up of these rather complex interfacial architectures, in the functionalization of the pure lipid matrix by the reconstitution of proteins, and in the lateral patterning of the membranes as a prerequisite for the construction of membrane chips for massive parallel monitoring of binding events.

Bioartificial organs in the twenty-first century: nanobiological devices

Bioartificial organs involve the design, modification, growth and maintenance of living tissues embedded in natural or synthetic scaffolds to enable them to perform complex biochemical functions, including adaptive control and the replacement of normal living tissues. Future directions in this area will lead to an abandonment of the trial-and-error implant optimization approach and a switch to the rational production of precisely formulated nanobiological devices. This will be accomplished with the help of three major thrusts: (1) use of molecularly manipulated nanostructured biomimetic materials; (2) application of microelectronic and nanoelectronic interfacing for sensing and control; and (3) application of drug delivery and medical nanosystems to induce, maintain, and replace a missing function that cannot be readily substituted with a living cell and to accelerate tissue regeneration. Biomimetics involves employment of microstructures and functional domains of organismal tissue function, correlation of processes and structures with physical and chemical processes, and use of this knowledge base to design and synthesize new materials for health applications. Nanostructured materials should involve biological materials (rather then synthetic ones) because their prefabricated structure is suitable for modular control of devices from existing materials. Nanostructured tools should encompass surface patterned molecular arrays, nanoscale synthetic scaffolding mimicking the cell-extracellular matrix microenvironment, precise positioning of molecules with specific signals to provide microheterogeneity, composites of bioinorganic and organic molecules, molecular layering (coating), and molecular and supramolecular self-assembly and self-organization (template-directed) assembly. The nanoelectronic interface includes electronic or optoelectronic biointerfaced devices based on individual cells, their aggregates and tissues, organelles, and molecules, such as enzyme-based devices, transport and ion-channel membrane proteins, and receptor-ligand structures, including nanostructured semiconductor chips and microfluidic components. Delivery nanosystems encompass both water and lipid core vehicles (for hydrophilic and lipophilic components) of various geometries: liposomes, micelles, nanoparticles, lipid shells (as imaging and contrasting agents), solid nanosuspensions, lipid nanospheres, and coated film surfaces (molecular layering), all for use in delivering drugs, proteins, cell modifiers, and genes. Nanoelectronic interface and delivery nanosystems will be used for sensing, feedback, control, and analysis of function of bioartificial organs.

Stochastic sensors inspired by biology

Sensory systems use a variety of membrane-bound receptors, including responsive ion channels, to discriminate between a multitude of stimuli. Here we describe how engineered membrane pores can be used to make rapid and sensitive biosensors with potential applications that range from the detection of biological warfare agents to pharmaceutical screening. Notably, use of the engineered pores in stochastic sensing, a single-molecule detection technology, reveals the identity of an analyte as well as its concentration.

Design of bioelectronic interfaces by exploiting hinge-bending motions in proteins

We report a flexible strategy for transducing ligand-binding events into electrochemical responses for a wide variety of proteins. The method exploits ligand-mediated hinge-bending motions, intrinsic to the bacterial periplasmic binding protein superfamily, to establish allosterically controlled interactions between electrode surfaces and redox-active, Ru(II)-labeled proteins. This approach allows the development of protein-based bioelectronic interfaces that respond to a diverse set of analytes. Families of these interfaces can be generated either by exploiting natural binding diversity within the superfamily or by reengineering the specificity of individual proteins. These proteins may have numerous medical, environmental, and defense applications.

Conducting gramicidin channel activity in phospholipid monolayers

Potential step amperometry (chronoamperometry) of the Tl(I)/Tl(Hg) electrochemical reduction process has been used to investigate the underlying mechanisms of gramicidin activity in phospholipid monolayers. The experiments were carried out at gramicidin-modified dioleoyl phosphatidylcholine (DOPC)-coated electrodes. Application of a potential step to the coated electrode system results in a current transient that can be divided into two regions. An initial exponential decay of current corresponds to the inactivation of monomer channel conductance and a longer time scale quasi-steady-state represents the diffusion of ions to a bimolecular surface reaction. Concentrations of monomer conducting channels are relatively low, and the results indicate that two or more forms of gramicidin are in equilibrium with each other in the layer. Aromatic/conjugated compounds incorporated into the monolayer increase the reduction current by decreasing the rate of channel inactivation and increasing the stability of the conducting channel. This effect is positively correlated with the degree of the compound's aromaticity. The anomalous influence of alkali metal ions on the reduction current is consistent with the model of gramicidin being speciated in the monolayer in more than one form. The results have implications on the lability of the peptide conformation in biological membranes and its dependence on lipid environment, solution composition, and applied potential.

The power of single channel recording and analysis: its application to ryanodine receptors in lipid bilayers

1. Since the inception of the patch-clamp technique, single-channel recording has made an enormous impact on our understanding of ion channel function and its role in membrane transport and cell physiology. 2. However, the impact of single-channel recording methods on our understanding of intracellular Ca2+ regulation by internal stores is not as broadly recognized. There are several possible reasons for this. 3. First, ion channels in the membranes of intracellular organelles are not directly accessible to patch pipettes, requiring other methods that are not as widely known as the patch-clamp techniques. 4. Second, bulk assays for channel activity have proved successful in advancing our knowledge of Ca2+ handling by intracellular stores. These assays include Ca2+ imaging, ryanodine binding assays and measurements of muscle tension and Ca2+ release and uptake by vesicles that have been isolated from internal stores. 5. The present review describes methods used for single- channel recording and analysis, as applied to the calcium release channels in striated muscle, and details some of the unique contributions that single-channel recording and analysis have made to our current understanding of the release of Ca2+ from the internal stores of muscle. 6. With this in mind, the review focuses on three aspects of channel function and shows how single-channel investigations have led to an improved understanding of physiological processes in muscle. 7. Finally, the review describes some of the latest improvements in membrane technology that will underpin future advances in single-channel recording.

A functional assay for paralytic shellfish toxins that uses recombinant sodium channels

Saxitoxin (STX) and its derivatives are highly toxic natural compounds produced by dinoflagellates commonly present in marine phytoplankton. During algal blooms ("red tides"), shellfish accumulate saxitoxins leading to paralytic shellfish poisoning (PSP) in human consumers. PSP is a consequence of the high-affinity block of voltage-dependent Na channels in neuronal and muscle cells. PSP poses a significant public health threat and an enormous economic challenge to the shellfish industry worldwide. The standard screening method for marine toxins is the mouse mortality bioassay that is ethically problematic, costly and time-consuming. We report here an alternative, functional assay based on electrical recordings in cultured cells stably expressing a PSP target molecule, the STX-sensitive skeletal muscle Na channel. STX-equivalent concentration in the extracts was calibrated by comparison with purified STX, yielding a highly significant correlation (R=0.95; N=30) between electrophysiological determinations and the values obtained by conventional methods. This simple, economical, and reproducible assay obviates the need to sacrifice millions of animals in mandatory paralytic shellfish toxin screening programs.

Sequence-specific detection of individual DNA strands using engineered nanopores

We describe biosensor elements that are capable of identifying individual DNA strands with single-base resolution. Each biosensor element consists of an individual DNA oligonucleotide covalently attached within the lumen of the alpha-hemolysin (alphaHL) pore to form a "DNA-nanopore". The binding of single-stranded DNA (ssDNA) molecules to the tethered DNA strand causes changes in the ionic current flowing through a nanopore. On the basis of DNA duplex lifetimes, the DNA-nanopores are able to discriminate between individual DNA strands up to 30 nucleotides in length differing by a single base substitution. This was exemplified by the detection of a drug resistance-conferring mutation in the reverse transcriptase gene of HIV. In addition, the approach was used to sequence a complete codon in an individual DNA strand tethered to a nanopore.

Conformational and orientation studies of artificial ion channels incorporated into lipid bilayers

The conformational and orientation studies in lipid bilayers of 21 amino acid peptides bearing six crown ethers are reported. The compounds were designed to form artificial ion channels by stacking the crown rings, and were shown to be functional in bilayer membranes. We used Fourier transform infrared spectroscopy and CD spectropolarimetry to study the conformation of the peptides in solution and in lipid bilayers. These studies revealed that hexacrown peptides retain their alpha-helical conformation when incorporated in a lipid bilayer environment. Attenuated total reflectance spectroscopy was used to investigate the orientation of the peptides in a lipid bilayer. Results demonstrated that the peptides are not oriented at a fixed angle in membrane, but rather are in incorporation equilibrium between an active state parallel to the lipid chain and an inactive state adsorbed at the surface of the bilayer. From these results, we propose a model for the channel activity and the gating mechanism of these hexacrown peptides in bilayer membranes.

Simultaneous multianalyte detection with a nanometer-scale pore

It was recently shown that naturally occurring, genetically engineered or chemically modified channels can be used to detect analytes in solution. We demonstrate here that the overall range of analytes that can be detected by single nanometer-scale pores is expanded using a potentially simpler system. Instead of attaching recognition elements to a channel, they are covalently linked to polymers that otherwise thread through a nanometer-scale pore. Because the rate of unbound polymer entering the pore is proportional to its concentration in the bulk, the binding of analyte to the polymer alters the latter's ability to thread through the pore, and the signal that results from individual polymer translocation is unique to the polymer type; the method permits multianalyte detection and quantitation. We demonstrate here that two different proteins can be simultaneously detected with this technique.

Solution- and Solid-Phase Synthesis of Components for Tethered Bilayer Membranes

The synthesis of the novel compound PhCH(2)SS(C(24)H(44)N(4)O(10))(C(20)H(41)) (5) for the preparation of tethered bilayer membranes is described. The compound is the all-amide analogue of the previously reported ester-containing membrane-forming material PhCH(2)SS(C(24)H(40)O(14))(C(20)H(41)) (1). The advanced intermediate (C(20)H(41)) C(16)H(28)N(3)O(8) (17) was prepared from the same starting materials using both solution-phase (13% yield) and solid-phase (81% yield) techniques. Monolayers on gold derived from 5 have been analyzed by ellipsometry and FTIR. The monolayers exhibit thicknesses similar to monolayers derived from 1 and possess H-bonded amide functionality.

Direct biologically based biosensing of dynamic physiological function

Dynamic regulation of biological systems requires real-time assessment of relevant physiological needs. Biosensors, which transduce biological actions or reactions into signals amenable to processing, are well suited for such monitoring. Typically, in vivo biosensors approximate physiological function via the measurement of surrogate signals. The alternative approach presented here would be to use biologically based biosensors for the direct measurement of physiological activity via functional integration of relevant governing inputs. We show that an implanted excitable-tissue biosensor (excitable cardiac tissue) can be used as a real-time, integrated bioprocessor to analyze the complex inputs regulating a dynamic physiological variable (heart rate). This approach offers the potential for long-term biologically tuned quantification of endogenous physiological function.

Detection of multivalent interactions through two-tiered energy transfer

A method based on two-tiered fluorescence resonant energy transfer (FRET) has been developed for selective and sensitive detection of species involved in a multivalent interaction. Pentavalent binding between cholera toxin and ganglioside GM1 is used as a model system to demonstrate the advantage of the two-tiered FRET over one-stage FRET in both conventional fluorimeter and flow cytometer. In the system, three fluorescent probes (namely, fluorescence donor, acceptor, and intermediate) are covalently tagged to receptors, and the intermediate is used to bridge the energy transfer between the donor and acceptor even though the donor's fluorescence spectrum does not overlap with absorption spectrum of the acceptor. One of the most significant improvements of the scheme over one-stage FRET is a dramatic decrease in the background fluorescence of the acceptor fluorescence, which, theoretically and practically, increases the detection sensitivity.

Design and characterization of gramicidin channels with side chain or backbone mutations

Mutations and chemical substitutions of amino acid side chains and backbone atoms have proved vital for understanding the folding, structure and function of gramicidin channels in phospholipid membranes. The channel's pore is lined by peptide backbone groups; their importance for channel structure and function is shown by a single amide-to-ester replacement within the backbone, which greatly reduces the resulting channel conductance and lifetime. The four tryptophans and the intervening leucines together govern the formation and dissociation of conducting channels from single-stranded subunits. Conducting double-stranded gramicidin conformations (channels) occur rarely in membranes--except when the sequence has been altered to permit special arrangements of tryptophans or (infrequently) in unusually thick membranes. The tryptophans anchor the single-stranded channels to the membrane/solution interface, and the indole dipoles promote cation transport through the channels. Removal of any indole dipole reduces ion conductance; whereas 5-fluorination of an indole, which increases its dipole moment, enhances ion conductance. Some sequence changes at the formyl-NH-terminus (in the membrane interior, away from the tryptophans), including fluorination of the formyl-NH-terminal valine, introduce voltage-dependent channel gating. Gramicidin channels are not just static conductors, but also dynamic entities whose structure and function can be manipulated by backbone and side chain modifications.

Miniaturization Technologies for Molecular Diagnostics

Background: Molecular diagnostics devices are becoming smaller. With the advancement of miniaturization technologies, microchip-based systems will soon be available for genetic testing. The purpose of this review is to highlight the underlying principles in miniaturization, the strategies being developed for bioanalysis, and the potential impact on the practice of this rapidly growing medical discipline.

Approach: The author discusses DNA microchips and their practical importation into the clinical laboratory, based on his background in medical device and microchip design and development. His discussion is supported by a body of literature covering both biomedical and electrical engineering and more recent publications in the field of molecular genetics and pathology.

Content: This review is descriptive and intended to outline the technologic and methodologic approaches to the creation of an integrated genetic analysis instrument based on miniature components. The review draws on published scientific evaluations of these devices without regard to the companies involved in their development.

Summary: The intent of this review is that the reader will better understand the variety of technical approaches toward the miniaturization of molecular genetic testing for the clinical laboratory. With insight into the principles underlying the operation of these chips and the integrated systems, the end user can better evaluate the value to the field in terms of making molecular genetics testing simpler, faster, and less expensive.

Fabrication of phospholipid bilayer-coated microchannels for on-chip immunoassays

Herein we describe a new class of microfluidic immunoassays based upon solid supported lipid bilayers. Two-dimensionally fluid bilayer material, which can accommodate multivalent binding between surface-bound ligands and aqueous receptors, was coated on the surface of poly(dimethylsiloxane) microchannels. The bilayers contained dinitrophenyl (DNP)-conjugated lipids for binding with bivalent anti-DNP antibodies. Twelve independent data points of surface coverage versus bulk protein concentration could be made simultaneously by forming a linear array of channels and flowing fluorescently labeled antibodies into them. This enabled an entire binding curve to be obtained in a single experiment. The measured apparent binding constant for the DNP/anti-DNP system was 1.8 microM. The methodology for performing heterogeneous assays developed here not only produces rapid results but also requires much less protein than traditional procedures and eliminates some standard sources of experimental error.

Long-Range Interaction Forces between Polymer-Supported Lipid Bilayer Membranes

Much of the short-range forces and structures of softly supported DMPC bilayers has been described previously. However, one interesting feature of the measured force-distance profile that remained unexplained is the presence of a long-range exponentially decaying repulsive force that is not observed between rigidly supported bilayers on solid mica substrate surfaces. This observation is discussed in detail here based on recent static and dynamic surface force experiments. The repulsive forces in the intermediate distance regime (mica-mica separations from 15 to 40 nm) are shown to be due not to an electrostatic force between the bilayers but to compression (deswelling) of the underlying soft polyelectrolyte layer, which may be thought of as a model cytoskeleton. The experimental data can be fit by simple theoretical models of polymer interactions from which the elastic properties of the polymer layer can be deduced.

Advances in the characterization of supported lipid films with the atomic force microscope

During the past decade, the atomic force microscope (AFM) has become a key technique in biochemistry and biophysics to characterize supported lipid films, as testified by the continuous growth in the number of papers published in the field. The unique capabilities of AFM are: (i) capacity to probe, in real time and in aqueous environment, the surface structure of lipid films; (ii) ability to directly measure physical properties at high spatial resolution; (iii) possibility to modify the film structure and biophysical processes in a controlled way. Such experiments, published up to June 2000, are the focus of the present review. First, we provide a general introduction on the preparation and characterization of supported lipid films as well as on the principles of AFM. The section 'Structural properties' focuses on the various applications of AFM for characterizing the structure of supported lipid films: visualization of molecular structure, formation of structural defects, effect of external agents, formation of supported films, organization of phase-separated films (coexistence region, mixed films) and, finally, the use of supported lipid bilayers for anchoring biomolecules such as DNA, enzymes and crystalline protein arrays. The section 'Physical properties' introduces the principles of force measurements by AFM, interpretation of these measurements and their recent application to supported lipid films and related structures. Finally, we highlight the major achievements brought by the technique and some of the current limitations.

Design of supported membranes tethered via metal-affinity ligand-receptor pairs

Model lipid layers are very promising in investigating the complex network of recognition, transport and signaling processes at membranes. We have developed a novel and generic approach to create supported lipid membranes tethered by metal-affinity binding. By self-assembly we have generated various interfaces that display histidine sequences (6xHis) via polymer spacers. These histidine-functionalized interfaces are designed to allow specific docking and fusion of vesicles containing metal-chelating lipids. By means of surface plasmon resonance and atomic force microscopy we analyzed the formation and subsequently the structure of these solid-supported membranes. Although the affinity constant of single ligand-receptor pairs is only in the micromolar range, very stable immobilization of these membranes was observed. This behavior can be explained by multivalent interactions resembling many features of cell adhesion. The process is highly specific, because vesicle docking and bilayer formation are strictly dependent on the presence of metal-affinity ligand-receptor pairs. The surface accessibility and geometry of these tethered membranes were probed by binding of histidine-tagged polypeptides. The supported membranes show adsorption kinetics and values similar to planar supported monolayers. Using various combinations of metal-chelating and histidine-tagged lipids or thiols these metal-affinity-tethered membranes should make a great impact on probing and eventually understanding the dynamic dialog of reconstituted membrane proteins.

Functionalization of a poly(amidoamine) dendrimer with ferrocenyls and its application to the construction of a reagentless enzyme electrode

Poly(amidoamine) dendrimers having various degrees of modification with the redox-active ferrocenyls were prepared by controlling the molar ratio of ferrocenecarboxaldehyde to amine groups of dendrimers. By alternate layer-by-layer depositions of partial ferrocenyl-tethered dendrimers (Fc-D) with periodate-oxidized glucose oxidase (GOx) on a Au surface, an electrochemically and enzymatically active multilayered assembly of enzyme was constructed. The resulting GOx/Fc-D multilayer-associated electrodes were electrochemically analyzed, and the surface concentration of ferrocenyl groups, active enzyme coverage, and sensitivity were estimated. A 32% dendrimer modification level of surface amines to ferrocenyls was found to be an optimum in terms of enzyme-dendrimer network formation, electrochemical interconnectivity of ferrocenyls, and electrode sensitivity. With the prepared Fc(32%)-tethered dendrimers, mono- and multilayered GOx/Fc-D electrodes were constructed, and their electrochemical and catalytic properties were characterized. The bioelectrocatalytic signals from the multilayered GOx/Fc-D electrodes were shown to be directly correlated to the number of deposited bilayers. From this result, it seems that the electrode sensitivity is directly controllable, and the multilayer-forming strategy with partial ferrocenyl-tethered dendrimers is useful for the construction of reagentless biosensors.

Tethered polymer-supported planar lipid bilayers for reconstitution of integral membrane proteins: silane-polyethyleneglycol-lipid as a cushion and covalent linker

There is increasing interest in supported membranes as models of biological membranes and as a physiological matrix for studying the structure and function of membrane proteins and receptors. A common problem of protein-lipid bilayers that are directly supported on a hydrophilic substrate is nonphysiological interactions of integral membrane proteins with the solid support to the extent that they will not diffuse in the plane of the membrane. To alleviate some of these problems we have developed a new tethered polymer-supported planar lipid bilayer system, which permitted us to reconstitute integral membrane proteins in a laterally mobile form. We have supported lipid bilayers on a newly designed polyethyleneglycol cushion, which provided a soft support and, for increased stability, covalent linkage of the membranes to the supporting quartz or glass substrates. The formation and morphology of the bilayers were followed by total internal reflection and epifluorescence microscopy, and the lateral diffusion of the lipids and proteins in the bilayer was monitored by fluorescence recovery after photobleaching. Uniform bilayers with high lateral lipid diffusion coefficients (0.8-1.2 x 10(-8) cm(2)/s) were observed when the polymer concentration was kept slightly below the mushroom-to-brush transition. Cytochrome b(5) and annexin V were used as first test proteins in this system. When reconstituted in supported bilayers that were directly supported on quartz, both proteins were largely immobile with mobile fractions < 25%. However, two populations of laterally mobile proteins were observed in the polymer-supported bilayers. Approximately 25% of cytochrome b(5) diffused with a diffusion coefficient of approximately 1 x 10(-8) cm(2)/s, and 50-60% diffused with a diffusion coefficient of approximately 2 x 10(-10) cm(2)/s. Similarly, one-third of annexin V diffused with a diffusion coefficient of approximately 3 x 10(-9) cm(2)/s, and two-thirds diffused with a diffusion coefficient of approximately 4 x 10(-10) cm(2)/s. A model for the interaction of these proteins with the underlying polymer is discussed.

Simultaneous stochastic sensing of divalent metal ions

Stochastic sensing is an emerging analytical technique that relies upon single-molecule detection. Transmembrane pores, into which binding sites for analytes have been placed by genetic engineering, have been developed as stochastic sensing elements. Reversible occupation of an engineered binding site modulates the ionic current passing through a pore in a transmembrane potential and thereby provides both the concentration of an analyte and, through a characteristic signature, its identity. Here, we show that the concentrations of two or more divalent metal ions in solution can be determined simultaneously with a single sensor element. Further, the sensor element can be permanently calibrated without a detailed understanding of the kinetics of interaction of the metal ions with the engineered pore.

Functional tethered lipid bilayers

Our strategy to provide the structural basis for the build-up of functional tethered membranes focuses on three approaches: the first one is based on the pre-organization of a monomolecular layer of a lipopolymer at the water/air interface which is then transferred to a solid support. Prior to deposition, the substrate is coated with a layer of benzophenone-derivatized silane molecules that allow for a stable covalent attachment by photo-cross-linking of some of the monomer units of the lipopolymer to the support. An alternative concept realizes a layer-by-layer deposition of the various structural elements: (1) the attachment layer with the reactive sites for the chemical stabilization; (2) a polymer 'cushion' prepared by adsorption and simultaneous or subsequent partial covalent binding to the reactive sites; and (3) a lipid monolayer transferred from the water/air interface, that contains a certain amount of lipids with reactive headgroups which, upon binding to the polymer tether, act as anchor lipids stabilizing the whole monolayer/cushion-composite. And finally, we build peptide-supported monolayers by first (self-) assembling amino acid sequences of various lengths via a SH-group near their N-terminus onto Au substances and use then their COO(-)-terminus to chemically attach phosphatidyl-ethanolamine lipids to form a stable monolayer of lipid-peptide conjugates. All the individual preparation steps and the various resulting (multi-) layers are characterized by surface plasmon spectroscopy, X-ray and neutron-reflectometry, contact angle measurements, IR spectroscopy, fluorescence microscopy, scanning probe microscopies, as well as, electrochemical techniques. For all tethering systems, the final membranes' architecture is obtained by fusing lipid vesicles onto the lipid monolayer. Proteins can be incorporated by either fusing vesicles that are loaded with the respective receptors, pores, or ion pumps via a reconstitution procedure, or via a transfer directly from a micellar solution to the pre-formed lipid bilayer at the solid support by a dialysis step. Two structural/dynamical features of tethered membranes which are considered to be of particular functional relevance, i.e. the degree of water uptake and, hence, the degree of swelling of the polymer support, as well as the lateral mobility of the lipid molecules in the membrane, are tested by surface plasmon optics and by measurements of the fluorescence recovery after photobleaching (FRAP), respectively. The results confirm that the presented preparation protocols yield fluid bilayers that mimic certain relevant properties of biological membranes. The functional characterization of tethered membranes, which is briefly summarized, is based on various electrochemical techniques, in particular, impedance spectroscopy, cyclic voltammetry, and chronoamperometric studies. The results obtained for reconstituted H(+)-ATPase from chloroplasts and E. coli and for cytochrome oxidase (with and without cytochrome c) confirm the incorporation of the proteins in an active form, thus, opening opportunities for novel sensor formats or offering a completely new model membrane system.

Atomic force microscope image contrast mechanisms on supported lipid bilayers

This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.

Ion-induced interfacial dynamics of phospholipid monolayers

Ion-induced interfacial dynamics of phospholipid monolayers were studied by various electrochemical techniques. The lipid monolayers were constructed by using the mercapto derivatives of natural lipids that were self-assembled directly onto gold electrode surfaces in a tails-down fashion. The supported lipid assemblies appeared to act as rather effective electron-tunneling barriers with K3Fe(CN)6 as the redox probe, despite a relatively low surface coverage and/or a disordered surface structure. Upon the stimulation by alkaline-earth ions, the lipid layers appeared to undergo surface reorganization, exposing part of the electrode surface which resulted in the formation of microscopic mass-transfer lipid channels. The dimensions and/or the number of these channels increased with increasing ion concentrations, and this ion-gate effect appeared to be quite selective, with the most pronounced effects observed among the series of alkaline-earth ions with Ca2+.

Protein design: on the threshold of functional properties

The ultimate goal in protein de novo design is the creation of novel macromolecules with tailor-made receptor, sensory, and catalytic functions. Despite considerable progress in understanding basic rules of secondary structure formation and protein stability, the well-known protein folding problem is still far from being solved and, in general, only a limited number of designed proteins are folded uniquely. In this article the state-of-the-art in protein design is demonstrated on some selected examples, indicating that the construction of protein-like macromolecules mimicking some essential features of natural proteins seems to be within reach. Thus, protein design and mimicry has become an interdisciplinary challenge with most intriguing perspectives.

Detection of DNA via an ion channel switch biosensor

Detection of DNA by an ion channel switch biosensor has been demonstrated in a model system, using single-stranded oligonucleotide sequences of 52-84 bases in length. Two different biotinylated probes are bound, via streptavidin, either to the outer region of a gramicidin ion channel dimer or to an immobilized membrane component. The ion channels are switched off upon detection of DNA containing complementary epitopes to these probes, separated by a nonbinding region, at nanomolar levels. The DNA cross-links the ion channel to the immobilized species, preventing ions passing through the channel. Addition of DNase I after the target DNA has been added switches the ion channels on. The DNA response is dependent on the rate of hybridization of the individual probes to their complementary epitopes, as shown by using a single probe against DNA containing a repeat of the complementary epitope. These results were correlated with hybridization rates determined using surface plasmon resonance (BIAcore 2000), and with free energies of dimer formation for the probes.