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At least 577 records · Page 32Linked to original sources
PubMed1995

Visualization of nucleosomal substructure in native chromatin by atomic force microscopy.

Intact rDNA minichromosomes from Tetrahymena thermophila were isolated as native chromatin and imaged by atomic force microscopy (AFM). AFM measurements of condensed rDNA chromatin were consistent with a 30 nm fiber that frequently (87% of molecules observed) contained stretches of nucleosome cores arranged in a zig-zag conformation. Examination of rDNA chromatin in a dispersed conformation by tapping mode AFM in low humidity resulted in high resolution images of partially dissociated nucleosome cores and associated linker DNA. A majority of these nucleosome cores contained six to eight smaller particles with dimensions consistent with those of individual histones. Many of the nucleosome cores showed a striking resemblance to the wedge (35%), axial (15%), and front (6%) views of the nucleosome histone octamer modeled by Arents et al. [Arents, G., Burlingame, R. W., Wang, B.-C., Love, W.E., & Moudrianakis, E. N. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 10148-10152]. This direct visualization of histone subunits and nucleosomal substructure in native chromatin illustrates the potential use of AFM to localize individual proteins in condensed cellular chromatin.

Animals↗
PubMed2005

Atomic force microscopy analysis of normal and photoablated porcine corneas.

We showed the capabilities and accuracy of atomic force microscopy (AFM) techniques for imaging and analyzing the corneal epithelium and the photoablated corneal stroma. Eight normal porcine corneas, half of which were ablated using a scanning-spot excimer laser, were examined. All the corneas were imaged in balanced salt solution after fixation in glutaraldehyde. In the normal untreated corneas we observed the epithelial surface showing the typical polygonal cells and presenting numerous microprojections. The superficial epithelial cells were classified in three types as a result of the anterior-surface roughness measurements. AFM images of the photoablated corneal specimens showed undulations and granule-like features on the ablated stromal surface, specific to 193-nm ArF laser irradiation. Nevertheless, the quantitative analysis confirmed the precision of excimer laser surgery in removing sub-micrometric amounts of tissue. AFM showed to be a high-resolved imaging tool for the scanning of both native as well as photoablated corneal specimens. Also, this technique permits precise topographic analysis of the corneal plane, in the nanometric scale, of which smoothness is an important physical characteristic and necessary to achieve an optimal optical quality of the eye.

Animals↗
PubMed2005

Electrical characterization of silicon tips using conducting atomic force microscopy.

The electrical properties of n-doped Si tips have been characterized in conducting atomic force microscopy under various conditions. Si tips with SiO2 layer on them present complex electric properties: which include a larger positive threshold bias, which is different from that of its doped semiconductor material. Silicon tips after removing their SiO2 layer had smaller positive threshold bias; such bias varied with the loading force: smaller loading forces corresponding to larger positive threshold biases, and it remained constant at lower levels for larger loading forces. Humidity of experiments influenced the threshold bias: lower relative humidities (<25%) and larger loading forces were in favor of getting stable threshold bias. The conductance increased remarkably in high relative humidity although it was kept in a narrow range when relative humidity was lower than 40%. Loading force didn't affect the conductance in the examined relative humidity conditions. One advantage of bare silicon tips over commercial conducting ones is that they smaller radius than gold-coated tips; this is in more favor of reaching single molecular electronics.

Electric Conductivity↗
PubMed2005

Molecular electron transfer of protein junctions characterised by conducting atomic force microscopy.

The transport characteristics of the blue copper metalloprotein, azurin, have been characterised by conducting atomic force microscopy (C-AFM) at molecular level. Tunnel junctions have been constructed by sandwiching chemisorbed protein molecules between a conducting AFM tip and a planar conducting substrate. Asymmetric current curves with respect to the polarity of the bias (I-V) have been observed. The modulation of I-V behaviour with compressional force has been examined and is described by a modified Simmons model within which both tunnel distance (protein dimensions) and tunnel barrier are modulated. The modified Simmons formula, which considered unequal Fermi level shifts on two electrodes as being responsible for the asymmetric I-V curves, accurately describes the behaviour observed.

Azurin↗
PubMed2005

Role of silver ions in destabilization of intermolecular adhesion forces measured by atomic force microscopy in Staphylococcus epidermidis biofilms.

In this paper, we report on the potential use of atomic force microscopy (AFM) as a tool to measure the intermolecular forces in biofilm structures and to study the effect of silver ions on sessile Staphylococcus epidermidis cell viability and stability. We propose a strategy of destabilizing the biofilm matrix by reducing the intermolecular forces within the extracellular polymeric substances (EPSs) using a low concentration (50 ppb) of silver ions. Our AFM studies on the intermolecular forces within the EPSs of S. epidermidis RP62A and S.epidermidis 1457 biofilms suggest that the silver ions can destabilize the biofilm matrix by binding to electron donor groups of the biological molecules. This leads to reductions in the number of binding sites for hydrogen bonds and electrostatic and hydrophobic interactions and, hence, the destabilization of the biofilm structure.

Anti-Bacterial Agents↗
PubMed2005

Atom inlays performed at room temperature using atomic force microscopy.

The ability to manipulate single atoms and molecules laterally for creating artificial structures on surfaces is driving us closer to the ultimate limit of two-dimensional nanoengineering. However, experiments involving this level of manipulation have been performed only at cryogenic temperatures. Scanning tunnelling microscopy has proved, so far, to be a unique tool with all the necessary capabilities for laterally pushing, pulling or sliding single atoms and molecules, and arranging them on a surface at will. Here we demonstrate, for the first time, that it is possible to perform well-controlled lateral manipulations of single atoms using near-contact atomic force microscopy even at room temperature. We report the creation of 'atom inlays', that is, artificial atomic patterns formed from a few embedded atoms in the plane of a surface. At room temperature, such atomic structures remain stable on the surface for relatively long periods of time.

Journal Article↗
PubMed1998

Manipulation and molecular resolution of a phosphatidylcholine-supported planar bilayer by atomic force microscopy.

The morphology of supported planar bilayers has been investigated below phase transition temperature by atomic force microscopy in contact and tapping mode. The bilayers were formed by the vesicle-spreading technique. In contact mode at low scanning forces of about 1 nN true molecular resolution could be achieved for supported phosphatidylcholine bilayers. The resolution was confirmed by experiments that captured the location, average area of individual lipid headgroups and the manipulation of the bilayer surface. Repeated scanning in contact mode shifted the random topology of the surface consecutively to a striped pattern. Height profiles of defect-containing bilayers were analyzed. The shape of the defects became smooth by repeated scanning. The height profiles allowed the estimation of the indentation of the tip into the surface-adsorbed membrane. In tapping mode a disordered pattern of headgroups became visible. Our morphological data at molecular resolution suggest that the native arrangement of the choline head-groups is disordered, free of large packing defects and becomes ordered in Schallamach waves by scanning in contact mode.

Lipid Bilayers↗
PubMed2006

Crystallization of poly(L-lactid acid) monitored by dielectric relaxation spectroscopy and atomic force microscopy.

An investigation was carried out on the crystallization process of poly(l-lactid acid) by dielectric relaxation spectroscopy and atomic force microscopy. Experimental results were generated by dielectric relaxation spectroscopy over a wide range of frequency and temperature in both the wholly amorphous state and during crystallization. The variation of the average relaxation time was studied during crystallization at 80 degrees C and the temperature dependence of this relaxation time for wholly amorphous and crystallized samples was analysed. This behaviour was modelled by Havriliak-Negami and Vogel-Fulcher equations. The sensitivity of the segmental dynamics to the degree of crystallinity was analysed, taking into account the relaxing segments and the thickness of the amorphous layer between lamellae. The morphologies obtained during crystallization processes at 80, 130 and 150 degrees C were monitored by atomic force microscopy at both the lamellar level and by analysing the multilayered superstructures formed. Hedrites, intermediate structures between single lamellar crystals and mature spherulites, were found to appear at the highest temperatures, whereas no evidence of hedrites was found at 80 degrees C, the spherulites seemed to be constructed from a framework of individual dominant lamellae that splay apart and branch. Complementary to the atomic force microscopy study, the evolution of the obtained morphologies was also followed by optical microscopy. Supporting evidence about the thermal behaviour of the polymers was obtained with differential scanning calorimetry.

Journal Article↗
PubMed1998

Imaging two-dimensional arrays of soluble proteins by atomic force microscopy in contact mode using a sharp supertip.

A sharp tip with high aspect ratio is required for imaging biological macromolecules by atomic force microscopy (AFM). A tip with the end radius of curvature less than 3 nm has been reproducibly fabricated by means of electron beam deposition (EBD) in a field-emission scanning electron microscope. Two-dimensional protein arrays of ferritin and catalase, prepared at air/water interface and transferred onto silicon wafer, could be imaged both in air and in water by AFM using this sharp EBD-tip in contact mode. The negative staining preparation conventionally used in the transmission electron microscopy of protein was applied and shown to be quite effective in fixing the protein arrays for the AFM imaging in air. Individual molecules of ferritin and catalase were visible in the two-dimensional arrays. Also, imaging in water of these protein arrays presented molecular images clearer than in air, due probably to the absence of the adhesion force and the resulting weak lateral force during scanning. These images convince us of the capability of this supertip for AFM studies of biological molecules under aqueous conditions.

Air↗
PubMed2005

Membrane thinning due to antimicrobial peptide binding: an atomic force microscopy study of MSI-78 in lipid bilayers.

The interaction of an antimicrobial peptide, MSI-78, with phospholipid bilayers has been investigated using atomic force microscopy, circular dichroism, and nuclear magnetic resonance (NMR). Binding of amphipathic peptide helices with their helical axis parallel to the membrane surface leads to membrane thinning. Atomic force microscopy of supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers in the presence of MSI-78 provides images of the membrane thinning process at a high spatial resolution. This data reveals that the membrane thickness is not reduced uniformly over the entire bilayer area. Instead, peptide binding leads to the formation of distinct domains where the bilayer thickness is reduced by 1.1 +/- 0.2 nm. The data is interpreted using a previously published geometric model for the structure of the peptide-lipid domains. In this model, the peptides reside at the hydrophilic-hydrophobic boundary in the lipid headgroup region, which leads to an increased distance between lipid headgroups. This picture is consistent with concentration-dependent 31P and 2H NMR spectra of MSI-78 in mechanically aligned DMPC bilayers. Furthermore, 2H NMR experiments on DMPC-d54 multilamellar vesicles indicate that the acyl chains of DMPC are highly disordered in the presence of the peptide as is to be expected for the proposed structure of the peptide-lipid assembly.

Antimicrobial Cationic Peptides↗
PubMed1995

Direct measurement of hydrogen bonding in DNA nucleotide bases by atomic force microscopy.

We have used self-assembled purines and pyrimidines on planar gold surfaces and on gold-coated atomic force microscope (AFM) tips to directly probe intermolecular hydrogen bonds. Electron spectroscopy for chemical analysis (ESCA) and thermal programmed desorption (TPD) measurements of the molecular layers suggested monolayer coverage and a desorption energy of about 25 kcal/mol. Experiments were performed under water, with all four DNA bases immobilized on AFM tips and flat surfaces. Directional hydrogen-bonding interaction between the tip molecules and the surface molecules could be measured only when opposite base-pair coatings were used. The directional interactions were inhibited by excess nucleotide base in solution. Nondirectional van der Waals forces were present in all other cases. Forces as low as two interacting base pairs have been measured. With coated AFM tips, surface chemistry-sensitive recognition atomic force microscopy can be performed.

Adenine↗
PubMed2003

Atomic force microscopy and nanoindentation characterization of human lamellar bone prepared by microtome sectioning and mechanical polishing technique.

Surface topography, microstructure, and micromechanical properties of human lamellar bone were characterized by atomic force microscopy and nanoindentation. The lamellar bone surfaces were prepared by two different methods: microtome sectioning and mechanical polishing. The lamellar bone surfaces prepared by mechanical polishing revealed that thin lamellae formed depressions approximately 200 nm deep, whereas the surfaces prepared by microtome sectioning were flat. Atomic force microscopy surface topographic images at higher magnification showed differences between thick and thin lamellae in polished samples, but these differences were less pronounced in microtomed samples. Roughness measurements confirmed that there was a significant difference between thick (21.0 nm) and thin lamellae (8.3 nm) in polished samples (p < 0.001). The difference in surface roughness between thick (13.9 nm) and thin lamellae (12.7 nm) in microtomed sample was statistically insignificant (p = 0.74). Higher elastic modulus values were observed for thick lamella in microtomed samples compared with that of thin lamellae, whereas measured elastic modulus differences between thick and thin lamellae in polished samples were found to be statistically insignificant.

Biomechanical Phenomena↗
PubMed1997

Pore size of agarose gels by atomic force microscopy.

The pore size of agarose gel in water at different concentrations was directly measured using atomic force microscopy (AFM). The experiment was specially designed to work under aqueous conditions and allows direct observation of the "unperturbed" gel without invasive treatment. The pore size a as a function of gel concentration C shows a power law dependence a approximately C-gamma, where gamma lies between the prediction of the Ogston model for a random array of straight chains, 0.5, and the value predicted by De Gennes for a network of flexible chains, 0.75. We confirm that gels present a wide pore size distribution and show that it narrows as the concentration increases.

Chemical Phenomena↗