Direct visualization of surface charge in aqueous solution

Molecular interactions of hesperidin with DMPC/cholesterol bilayers

Since cell membranes are complex systems, the use of model lipid bilayers is quite important for the study of their interactions with bioactive molecules. Mammalian cell membranes require cholesterol (CHOL) for their structure and function. For this reason, the mixtures of phospholipid and cholesterol are necessary to use in model membrane studies to better simulate the real systems. In the present study, we investigated the effect of the incorporation of hesperidin in model membranes consisting of dimyristoylphosphatidylcholine (DMPC) and CHOL by using differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM). ATR-FTIR results demonstrated that hesperidin increases the fluidity of the DMPC/CHOL binary system. DSC findings indicated that the presence of 5 mol% hesperidin induces a broadening of the main phase transition consisting of three overlapping components. AFM experiments showed that hesperidin increases the thickness of DMPC/CHOL lipid bilayer model membranes. In addition to experimental results, molecular docking studies were conducted with hesperidin and human lanosterol synthase (LS), which is an enzyme found in the final step of cholesterol synthesis, to characterize hesperidin's interactions with its surrounding via its hydroxyl and oxygen groups. Then, hesperidin's ADME/Tox (absorption, distribution, metabolism, excretion and toxicity) profile was computed to see the potential impact on living system. In conclusion, considering the data obtained from experimental studies, this work ensures molecular insights in the interaction between a flavonoid, as an antioxidant drug model, and lipids mimicking those found in mammalian membranes. Moreover, computational studies demonstrated that hesperidin may be a great potential for use as a therapeutic agent for hypercholesterolemia due to its antioxidant property.

Macrocyclic Peptide-Conjugated Tip for Fast and Selective Molecular Recognition Imaging by High-Speed Atomic Force Microscopy

Fast and selective recognition of molecules at the nanometer scale without labeling is a much desired but still challenging goal to achieve. Here, we show the use of high-speed atomic force microscopy (HS-AFM) for real-time and real-space recognition of unlabeled membrane receptors using tips conjugated with small synthetic macrocyclic peptides. The single-molecule recognition method is validated by experiments on the human hepatocyte growth factor receptor (hMET), which selectively binds to the macrocyclic peptide aMD4. By testing and comparing aMD4 synthesized with linkers of different lengths and rigidities, we maximize the interaction between the functionalized tip and hMET added to both a mica surface and supported lipid bilayers. Phase contrast imaging by HS-AFM enables us to discriminate nonlabeled hMET against the murine MET homologue, which does not bind to aMD4. Moreover, using ligands and linkers of small size, we achieve minimal deterioration of the spatial resolution in simultaneous topographic imaging. The versatility of macrocyclic peptides in detecting unlimited types of membrane receptors with high selectivity and the fast imaging by HS-AFM broaden the range of future applications of this method for molecular recognition without labeling.

Bioactive Cellulose Nanocrystal-Poly(ε-Caprolactone) Nanocomposites for Bone Tissue Engineering Applications

3D-printed bone scaffolds hold great promise for the individualized treatment of critical-size bone defects. Among the resorbable polymers available for use as 3D-printable scaffold materials, poly(ε-caprolactone) (PCL) has many benefits. However, its relatively low stiffness and lack of bioactivity limit its use in load-bearing bone scaffolds. This study tests the hypothesis that surface-oxidized cellulose nanocrystals (SO-CNCs), decorated with carboxyl groups, can act as multi-functional scaffold additives that (1) improve the mechanical properties of PCL and (2) induce biomineral formation upon PCL resorption. To this end, an in vitro biomineralization study was performed to assess the ability of SO-CNCs to induce the formation of calcium phosphate minerals. In addition, PCL nanocomposites containing different amounts of SO-CNCs (1, 2, 3, 5, and 10 wt%) were prepared using melt compounding extrusion and characterized in terms of Young's modulus, ultimate tensile strength, crystallinity, thermal transitions, and water contact angle. Neither sulfuric acid-hydrolyzed CNCs (SH-CNCs) nor SO-CNCs were toxic to MC3T3 preosteoblasts during a 24 h exposure at concentrations ranging from 0.25 to 3.0 mg/mL. SO-CNCs were more effective at inducing mineral formation than SH-CNCs in simulated body fluid (1x). An SO-CNC content of 10 wt% in the PCL matrix caused a more than 2-fold increase in Young's modulus (stiffness) and a more than 60% increase in ultimate tensile strength. The matrix glass transition and melting temperatures were not affected by the SO-CNCs but the crystallization temperature increased by about 5.5°C upon addition of 10 wt% SO-CNCs, the matrix crystallinity decreased from about 43 to about 40%, and the water contact angle decreased from 87 to 82.6°. The abilities of SO-CNCs to induce calcium phosphate mineral formation and increase the Young's modulus of PCL render them attractive for applications as multi-functional nanoscale additives in PCL-based bone scaffolds.

Imaging and Force Spectroscopy of Single Transmembrane Proteins with the Atomic Force Microscope

The atomic force microscope (AFM) has opened avenues and provided opportunities to investigate biological soft matter and processes ranging from nanometer (nm) to millimeter (mm). The high temporal (millisecond) and spatial (nanometer) resolutions of the AFM are suited for studying many biological processes in their native conditions. The AFM cantilever-aptly termed as a "lab on a tip"-can be used as an imaging tool as well as a handle to manipulate single bonds and proteins. Recent examples have convincingly established AFM as a tool to study the mechanical properties and monitor processes of single proteins and cells with high sensitivity, thus affording insight into important mechanistic details. This chapter specifically focuses on practical and analytical protocols of single-molecule AFM methodologies related to high-resolution imaging and single-molecule force spectroscopy of transmembrane proteins in a lipid bilayer (reconstituted or native). Both these techniques are operator oriented, and require specialized working knowledge of the instrument, theory and practical skills.

Atomic force microscopy and spectroscopy to probe single membrane proteins in lipid bilayers

The atomic force microscope (AFM) has opened vast avenues hitherto inaccessible to the biological scientist. The high temporal (millisecond) and spatial (nanometer) resolutions of the AFM are suited for studying many biological processes in their native conditions. The AFM cantilever stylus is aptly termed as a "lab on a tip" owing to its versatility as an imaging tool as well as a handle to manipulate single bonds and proteins. Recent examples assert that the AFM can be used to study the mechanical properties and monitor processes of single proteins and single cells, thus affording insight into important mechanistic details. This chapter specifically focuses on practical and analytical protocols of single-molecule AFM methodologies related to high-resolution imaging and single-molecule force spectroscopy of membrane proteins. Both these techniques are operator oriented, and require specialized working knowledge of the instrument, theoretical, and practical skills.

Application of atomic force microscopy in bacterial research

The atomic force microscope (AFM) has evolved from an imaging device into a multifunctional and powerful toolkit for probing the nanostructures and surface components on the exterior of bacterial cells. Currently, the area of application spans a broad range of interesting fields from materials sciences, in which AFM has been used to deposit patterns of thiol-functionalized molecules onto gold substrates, to biological sciences, in which AFM has been employed to study the undesirable bacterial adhesion to implants and catheters or the essential bacterial adhesion to contaminated soil or aquifers. The unique attribute of AFM is the ability to image bacterial surface features, to measure interaction forces of functionalized probes with these features, and to manipulate these features, for example, by measuring elongation forces under physiological conditions and at high lateral resolution (<1 A). The first imaging studies showed the morphology of various biomolecules followed by rapid progress in visualizing whole bacterial cells. The AFM technique gradually developed into a lab-on-a-tip allowing more quantitative analysis of bacterial samples in aqueous liquids and non-contact modes. Recently, force spectroscopy modes, such as chemical force microscopy, single-cell force spectroscopy, and single-molecule force spectroscopy, have been used to map the spatial arrangement of chemical groups and electrical charges on bacterial surfaces, to measure cell-cell interactions, and to stretch biomolecules. In this review, we present the fascinating options offered by the rapid advances in AFM with emphasizes on bacterial research and provide a background for the exciting research articles to follow.

Dynamic microcantilever sensors for discerning biomolecular interactions

Response of a conductive micromechanical cantilever placed in close proximity to a surface undergoing electrical excitation near the resonance frequency of the cantilever is influenced by the presence of microscopic dielectrics in the gap between the cantilever and the sample surface. The variations of the resonance response of unmodified cantilevers at gap distances below a few hundred nanometers are used to discern biomolecular differences of oligomeric nucleic acids in an array format without the use of extrinsic labels. The resonance response variation paves the way for the development of high throughput detection of biomolecular reactions, such as DNA hybridization reactions or antibody-antigen interactions without the use of external labels, in which the need is only to see the presence or absence of interaction. This dynamic method is simple, does not require immobilizing individual elements on a cantilever array, and is compatible with current generation DNA chips in which DNA spots are deposited in micro- and nanoarray format.

Quantitative membrane electrostatics with the atomic force microscope

The atomic force microscope (AFM) is sensitive to electric double layer interactions in electrolyte solutions, but provides only a qualitative view of interfacial electrostatics. We have fully characterized silicon nitride probe tips and other experimental parameters to allow a quantitative electrostatic analysis by AFM, and we have tested the validity of a simple analytical force expression through numerical simulations. As a test sample, we have measured the effective surface charge density of supported zwitterionic dioleoylphosphatidylcholine membranes with a variable fraction of anionic dioleoylphosphatidylserine. The resulting surface charge density and surface potential values are in quantitative agreement with those predicted by the Gouy-Chapman-Stern model of membrane charge regulation, but only when the numerical analysis is employed. In addition, we demonstrate that the AFM can detect double layer forces at a separation of several screening lengths, and that the probe only perturbs the membrane surface potential by <2%. Finally, we demonstrate 50-nm resolution electrostatic mapping on heterogeneous model membranes with the AFM. This novel combination of capabilities demonstrates that the AFM is a unique and powerful probe of membrane electrostatics.

Probing the double layer: effect of image forces on AFM

Force probes such as AFM tips or laser trap latex beads have a dielectric constant much less than that of the water that they displace. Thus when a probe approaches a charged surface under water it will be repelled simply based upon the image forces, and these can be of nN magnitude.

Watching the components of photosynthetic bacterial membranes and their in situ organisation by atomic force microscopy

The atomic force microscope has developed into a powerful tool in structural biology allowing information to be acquired at submolecular resolution on the protruding structures of membrane proteins. It is now a complementary technique to X-ray crystallography and electron microscopy for structure determination of individual membrane proteins after extraction, purification and reconstitution into lipid bilayers. Moving on from the structures of individual components of biological membranes, atomic force microscopy has recently been demonstrated to be a unique tool to identify in situ the individual components of multi-protein assemblies and to study the supramolecular architecture of these components allowing the efficient performance of a complex biological function. Here, recent atomic force microscopy studies of native membranes of different photosynthetic bacteria with different polypeptide contents are reviewed. Technology, advantages, feasibilities, restrictions and limits of atomic force microscopy for the acquisition of highly resolved images of up to 10 A lateral resolution under native conditions are discussed. From a biological point of view, the new insights contributed by the images are analysed and discussed in the context of the strongly debated organisation of the interconnected network of membrane-associated chlorophyll-protein complexes composing the photosynthetic apparatus in different species of purple bacteria.

Calcium-dependent Open/Closed Conformations and Interfacial Energy Maps of Reconstituted Hemichannels

Using an atomic force microscope, we have studied three-dimensional molecular topography and calcium-sensitive conformational changes of individual hemichannels. Full-length (non-truncated) Cx43 hemichannels (connexons), when reconstituted in lipid bilayer, appear as randomly distributed individual particles and clusters. They show a lack of preferential orientation of insertion into lipid membrane; in a single bilayer, connexons with protrusion of either the extracellular face or the large non-truncated cytoplasmic face are observed. Extracellular domains of these undocked hemichannels are structurally different from hemichannels in the docked gap junctional plaques examined after their exposure by force dissection or chemical dissection. Calcium induced a reversible change in the extracellular pore diameter. Hemichannels imaged in a physiological buffer with 1.8 mm Ca(+2) had the pore diameter of approximately 1.8 nm, consistent with the closed channel conformation. Reducing Ca(+2) concentration to approximately 1.4, 1, and 0 mm, which changes hemichannels from the closed to open conformation, increased the pore diameter to approximately 2.5 nm for approximately 27, 74, and 100% of hemichannels, respectively. Thus, open/close probability of the hemichannel appears to be [Ca(2+)]-dependent. Computational analysis of the atomic force microscopy phase mode imaging reveals a significantly higher interfacial energy for open hemichannels that results from the interactions between the atomic force microscope probe and the hydrophobic domains. Thus, hydrophobic extracellular domains of connexins regulate calcium-dependent conformational changes.

Surface heterogeneity of polystyrene latex particles determined by dynamic force microscopy

Atomic force microscopy (AFM) was employed to characterize the surface chemistry distribution on individual polystyrene latex particles. The particles were obtained by surfactant-free emulsion polymerization and contained hydrophilic quaternary ammonium chloride, sodium sulfonate, or hydroxyethyl groups. The phase shift in dynamic force mode AFM is sensitive to charge/chemical interactions between an oscillating atomic force microscope tip and a sample surface. In this work, the phase imaging technique distinguished phase domains of 50-100 nm on the surfaces of dried latex particles in ambient air. The domains are attributed to the separation of ion-rich and ion-poor components of the polymer on the particle surface.

Temperature dependence of the surface topography in dimyristoylphosphatidylcholine/distearoylphosphatidylcholine multibilayers

Simple lipid binary systems are intensively used to understand the formation of domains in biological membranes. The size of individual domains present in the gel/fluid coexistence region of single supported bilayers, determined by atomic force microscopy (AFM), generally exceeds by two to three orders of magnitude that estimated from multibilayers membranes by indirect spectroscopic methods. In this article, the topography of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) multibilayers, made of two superimposed bilayers supported on mica surface, was studied by AFM in a temperature range from room temperature to 45 degrees C. In the gel/fluid phase coexistence region the size of domains, between approximately 100 nm and a few micrometers, was of the same order for the first bilayer facing the mica and the top bilayer facing the buffer. The gel to fluid phase separation temperature of the first bilayer, however, could be increased by up to 8 degrees C, most likely as a function of the buffer layer thickness that separated it from the mica. Topography of the top bilayer revealed the presence of lipids in ripple phase up to 38-40 degrees C. Above this temperature, a pattern characteristic of the coexistence of fluid and gel domains was observed. These data show that difference in the size of lipid domains given by AFM and spectroscopy can hardly be attributed to the use of multibilayers models in spectroscopy experiments. They also provide a direct evidence for metastable ripple phase transformation into a gel/fluid phase separated structure upon heating.

Protein structural changes induced by their uptake at interfaces

For insertion into lipidic media, most hydrosoluble proteins must cross the lipid-water interface and thus undergo conformational transitions. According to their chemical sequences these transitions may be restricted to changes involving only the tertiary structure, while for other proteins this environment modification will induce drastic changes such as the unfolding of large domains. The structural transitions are mainly governed by the presence of hydrophobic domains and/or by the existence of induced amphipathic properties.

Observing structure, function and assembly of single proteins by AFM

Single molecule experiments provide insight into the individuality of biological macromolecules, their unique function, reaction pathways, trajectories and molecular interactions. The exceptional signal-to-noise ratio of the atomic force microscope allows individual proteins to be imaged under physiologically relevant conditions at a lateral resolution of 0.5-1nm and a vertical resolution of 0.1-0.2nm. Recently, it has become possible to observe single molecule events using this technique. This capability is reviewed on various water-soluble and membrane proteins. Examples of the observation of function, variability, and assembly of single proteins are discussed. Statistical analysis is important to extend conclusions derived from single molecule experiments to protein species. Such approaches allow the classification of protein conformations and movements. Recent developments of probe microscopy techniques allow simultaneous measurement of multiple signals on individual macromolecules, and greatly extend the range of experiments possible for probing biological systems at the molecular level. Biologists exploring molecular mechanisms will benefit from a burgeoning of scanning probe microscopes and of their future combination with molecular biological experiments.

Probing the machinery of intracellular trafficking with the atomic force microscope

Atomic force microscopy has emerged as a powerful tool for characterizing single biological macromolecules, macromolecular assemblies, and whole cells in aqueous buffer, in real time, and at molecular-scale spatial and force resolution. Many of the central elements of intracellular transport are tens to hundreds of nanometers in size and highly dynamic. Thus, atomic force microscopy provides a valuable means of addressing questions of structure and mechanism in intracellular transport. We begin this review of recent efforts to apply atomic force microscopy to problems in intracellular transport by discussing the technical principles behind atomic force microscopy. We then turn to three specific areas in which atomic force microscopy has been applied to problems with direct implications for intracellular trafficking: cytoskeletal structure and dynamics, vesicular transport, and receptor-ligand interactions. In each case, we discuss studies which use both intact cellular elements and reconstituted models. While many technical challenges remain, these studies point to several areas where atomic force microscopy can be used to provide valuable insight into intracellular transport at exquisite spatial and energetic resolution.

Characterization of AC mode scanning ion-conductance microscopy

A scanning ion-conductance microscope (SICM) with a vibrating probe has been recently developed (vSICM). In this system, the amplitude of the AC ionic current is detected by using a lock-in amplifier locked to the vibration frequency of the probe. Such a scheme allows for a better control of the tip position because the AC ionic current is more sensitive to the probe-surface distance than the DC ionic current used previously. In this paper, we demonstrate the utility of this technique to the imaging of topographically rough specimens and high-resolution imaging over selected small areas. We also show that it is possible to record the DC ionic current simultaneously during the scan, which can reveal additional information not apparent in the images obtained with the AC ionic current.

From images to interactions: high-resolution phase imaging in tapping-mode atomic force microscopy

In tapping-mode atomic force microscopy, the phase shift between excitation and response of the cantilever is used as a material-dependent signal complementary to topography. The localization of information in the phase signal is demonstrated with 1.4-nm lateral resolution on purple membrane of Halobacterium salinarum in buffer solution. In a first-order approximation, the phase signal is found to correlate with modulations of the tip oscillation amplitude, induced by topography. Extending the analysis to contributions of the tip-sample interaction area as a second-order approximation, a method is proposed to extract information about the interaction from the phase signal for surfaces with a roughness in the order of the tip radius.

Imaging mixed lipid monolayers by dynamic atomic force microscopy

Phase imaging with tapping mode atomic force microscopy (AFM) and force modulation microscopy were used to probe the mechanical properties of phase-separated lipid monolayers made of a mixture (0.25:0.75) of the surface-active lipopeptide surfactin and of dipalmitoylphosphatidylcholine (DPPC). The pi-A isotherms and the result of a molecular modeling study revealed a loose, 2-D liquid-like organization for the surfactin molecules and a closely packed, 2-D solid-like organization for DPPC molecules. This difference in molecular organization was responsible for a significant contrast in height, tapping mode phase and force modulation amplitude images. Phase imaging at light tapping, i.e., with a ratio of the set-point tapping amplitude with respect to the free amplitude A(sp)/A(0) approximately 0.9, showed larger phase shifts on the solid-like DPPC domains attributed to larger Young's modulus. However, contrast inversion was observed for A(sp)/A(0)<0.7, suggesting that at moderate and hard tapping the image contrast was dominated by the probe-sample contact area. Surprisingly, force modulation amplitude images showed larger stiffness for the liquid-like surfactin domains, suggesting that the contrast was dominated by contact area effects rather than by Young's modulus. These data emphasize the complex nature of the contrast mechanisms of dynamic AFM images recorded on mixed lipid monolayers.

Amyloid-beta peptide assembly: a critical step in fibrillogenesis and membrane disruption

Identifying the mechanisms responsible for the assembly of proteins into higher-order structures is fundamental to structural biology and understanding specific disease pathways. The amyloid-beta (Abeta) peptide is illustrative in this regard as fibrillar deposits of Abeta are characteristic of Alzheimer's disease. Because Abeta includes portions of the extracellular and transmembrane domains of the amyloid precursor protein, it is crucial to understand how this peptide interacts with cell membranes and specifically the role of membrane structure and composition on Abeta assembly and cytotoxicity. We describe the results of a combined circular dichroism spectroscopy, electron microscopy, and in situ tapping mode atomic force microscopy (TMAFM) study of the interaction of soluble monomeric Abeta with planar bilayers of total brain lipid extract. In situ extended-duration TMAFM provided evidence of membrane disruption via fibril growth of initially monomeric Abeta1-40 peptide within the total brain lipid bilayers. In contrast, the truncated Abeta1-28 peptide, which lacks the anchoring transmembrane domain found in Abeta1-40, self-associates within the lipid headgroups but does not undergo fibrillogenesis. These observations suggest that the fibrillogenic properties of Abeta peptide are in part a consequence of membrane composition, peptide sequence, and mode of assembly within the membrane.

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.

Structural studies of a crystalline insulin analog complex with protamine by atomic force microscopy

Crystallographic studies of insulin-protamine complexes, such as neutral protamine Hagedorn (NPH) insulin, have been hampered by high crystal solvent content, small crystal dimensions, and extensive disorder in the protamine molecules. We report herein in situ tapping mode atomic force microscopy (TMAFM) studies of crystalline neutral protamine Lys(B28)Pro(B29) (NPL), a complex of Lys(B28)Pro(B29) insulin, in which the C-terminal prolyl and lysyl residues of human insulin are inverted, and protamine that is used as an intermediate time-action therapy for treating insulin-dependent diabetes. Tapping mode AFM performed at 6 degrees C on bipyramidally tipped tetragonal rod-shaped NPL crystals revealed large micron-sized islands separated by 44-A tall steps. Lattice images obtained by in situ TMAFM phase and height imaging on these islands were consistent with the arrangement of individual insulin-protamine complexes on the P4(1)2(1)2 (110) crystal plane of NPH, based on a low-resolution x-ray diffraction structure of NPH, arguing that the NPH and NPL insulins are isostructural. Superposition of the height and phase images indicated that tip-sample adhesion was larger in the interstices between NPL complexes in the (110) crystal plane than over the individual complexes. These results demonstrate the utility of low-temperature TMAFM height and phase imaging for the structural characterization of biomolecular complexes.

Tapping-mode atomic force microscopy produces faithful high-resolution images of protein surfaces

Compared to contact-mode atomic force microscopy (CMAFM), tapping-mode atomic force microscopy (TMAFM) has the advantage of allowing imaging surfaces of macromolecules, even when they are only weakly attached to the support. In this study, TMAFM is applied to two different regular protein layers whose structures are known to great detail, the purple membrane from Halobacterium salinarum and the hexagonally packed intermediate (HPI) layer from Deinococcus radiodurans, to assess the faithfulness of high-resolution TMAFM images. Topographs exhibited a lateral resolution between 1.1 and 1. 5 nm and a vertical resolution of approximately 0.1 nm. For all protein surfaces, TMAFM and CMAFM topographs were in excellent agreement. TMAFM was capable of imaging the fragile polypeptide loop connecting the transmembrane alpha-helices E and F of bacteriorhodopsin in its native extended conformation. The standard deviation (SD) of averages calculated from TMAFM topographs exhibited an enhanced minimum (between 0.1 and 0.9 nm) that can be assigned to the higher noise of the raw data. However, the SD difference, indicating the flexibility of protein subunits, exhibited an excellent agreement between the two imaging modes. This demonstrates that the recently invented imaging-mode TMAFM has the ability to faithfully record high-resolution images and has sufficient sensitivity to contour individual peptide loops without detectable deformations.

Electrostatically balanced subnanometer imaging of biological specimens by atomic force microscope

To achieve high-resolution topographs of native biological macromolecules in aqueous solution with the atomic force microscope (AFM) interactions between AFM tip and sample need to be considered. Short-range forces produce the submolecular information of high-resolution topographs. In contrast, no significant high-resolution information is provided by the long-range electrostatic double-layer force. However, this force can be adjusted by pH and electrolytes to distribute the force applied to the AFM tip over a large sample area. As demonstrated on fragile biological samples, adjustment of the electrolyte solution results in a local reduction of both vertical and lateral forces between the AFM tip and proteinous substructures. Under such electrostatically balanced conditions, the deformation of the native protein is minimized and the sample surface can be reproducibly contoured at a lateral resolution of 0.6 nm.