摘要:

Scanning tunneling microscopy has been used to obtain images of the surface conformations of 4‐n‐octyl‐4’‐cyano‐biphenyl, a room‐temperature smectic liquid crystal. Working in air and using the constant‐current mode, it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported structures have been observed: an overlapping bilayer of layer spacing 37 Å, and a densely packed monolayer of spacing 24 Å. Periodic dislocations in the molecular packing have also been observed. Surface forces and intermolecular interactions are discussed as factors which could lead to these observed structures.

ISSN:0734-2101    

DOI:10.1116/1.576370

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Adsorbed molecules of copper phthalocyanine (CuPc) and liquid crystals on graphite were measured by scanning tunneling microscopy (STM) and spectroscopy (STS) in air. The islands of molecules of CuPc were detected by STM. Tunneling spectra taken simultaneously with STM showed characteristic peaks correlated with local topographic features. In the observation of a nematic liquid crystal on graphite, one reproducible molecular pattern was found, for which a model of molecular ordering is proposed. Ordinary spectroscopy was found to be difficult due to a large polarization current caused by the liquid surrounding the tip. Bias voltage dependent images of the molecules which demonstrate electronic structure effects were obtained.

ISSN:0734-2101    

DOI:10.1116/1.576371

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Two types of surface interactions of the basal plane of graphite with an organic ambient have been investigated with the scanning tunneling microscope: (1) At the bulk interphase between octylcyanobiphenyl and graphite local surface reactions have been observed with a threshold tip bias of −1.7 V and lifetimes of the reaction products on the order of 1 s. At more negative bias between −2.5 and −4 V the reactions become more intense resulting in localized graphite etch. While a crystalline adsorbate phase of octylcyanobiphenyl is very rarely observed on the flat graphite surface, it occurs more readily after the etching procedure, indicating a lack of nucleation sites on the pristine graphite. (2) A monolayer of a mixed cellulose ether (laurylmethyl) transferred onto graphite by the Langmuir–Blodgett technique has been imaged in the tip bias range of +10 mV to +1 V with a resolution of better than 1 nm. From the images one can determine the conformation of the individual polymer molecule. The polymer is found to be extended over several tens of nanometers and characteristic kinks along the chain are observed, indicative of a conformation which results from the adsorption of a twisted ribbon.

ISSN:0734-2101    

DOI:10.1116/1.576980

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The bare surface of the egg‐PC bilayers after being dried at room temperature was observed by the STM and images with angstrom resolution were obtained. Results indicate that the lipid heads form islands of an average of 210 Å in diameter and 40 Å in separation on the bilayers and that each island contains about 170 lipid heads of 7.5 Å in diameter. The size of the lipid heads is correspondent with that calculated from the standard bond distances and the bond angles. The ordered distribution of the lipid heads in some areas may correspond to the ordered arrangement of the lipid heads on the egg‐PC bilayers before being dried. The STM results also show that there exists the ripple phase in the egg‐PC bilayers below its phase transition temperature.

ISSN:0734-2101    

DOI:10.1116/1.576981

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Direct STM observation of native biomolecules has proven feasible. In this study we have used STM to image two filamentous protein components of the intracellular cytoskeleton: microtubules (MT) and intermediate filaments (IF). MT are 25 nm diameter cylinders comprised of 13 ‘‘protofilaments’’ which are linear chains of 8 nm × 4 nm × 4 nm ‘‘dimer’’ subunits. More variable than MT, IF are 10 nm diameter coils comprised of from 4 to 8 ‘‘subfilaments’’ which are chains of ‘‘tetramer’’ subunits. MT were isolated from porcine brain and IF from cell culture by standard techniques. Preparation/stabilization factors included magnesium buffer, 0.8 molar glycerol, and 0.1% glutaraldehyde. Samples were scanned on graphite in a Nanoscope I or II STM (Digital Instruments, Santa Barbara, California). STM images of MT demonstrated flattened 25 nm diameter structures composed of 4 nm wide protofilaments. Processed inverted images showed rows of 8 nm × 4 nm subunits. STM images of IF showed flattened, parallel 10 nm filaments comprised of coiled chains of 6–12 nm tetramer subunits. Thus STM demonstrated internal structure of two different classes of cytoskeletal proteins. STM and related techniques have an important future in biomolecular studies.

ISSN:0734-2101    

DOI:10.1116/1.576982

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Biological macromolecules are considerably denatured when processed for conventional electron microscopy. To test the applicability of scanning tunneling microscopy for these purposes we chose fibrous protein collagen I pipetted and dried on an oriented pyrolytic graphite surface. Survey scans made over 500×500 nm2showed separate and branching ridges of various lengths as well as regions of densely packed material. In scans over 50×50 nm2there were structures with regular protrusions propoused to be generated at a basic interval of 9 nm. The recorded images corresponded well with biochemically measured dimensions of type I collagen cyanogen bromide peptides.

ISSN:0734-2101    

DOI:10.1116/1.576984

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The surface structures of noncoated biological membranes were studied by the scanning tunneling microscopy, including the surfaces of egg PC(phosphatidylcholine) liposomes, soybean PE(phosphatidylethanolamine) nonbilayer lipid tubes and the membranes of liver ascites cells.

ISSN:0734-2101    

DOI:10.1116/1.576985

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

We have used STM to study a range of biological molecules. The necessity of immobilizing the molecules on the surface was recognized and achieved by the formation of ordered layers or by the addition of chemical groups which strongly adsorbed to the graphite substrate. Clear images of the helical structure of the synthetic polypeptide poly γ‐benzyl‐L‐glutamate were obtained. Images of protein molecules isolated from wheat were found to be consistent with the rare β‐spiral conformation. The binding of the mutagen ethidium bromide to DNA was also studied and images of the cyclic glucan β‐cyclodextrin, tosylated to improve adsorption of the molecule to graphite, were also obtained. Although the contrast mechanism is unclear in some of these examples, these results suggest that STM can be applied successfully to a wide range of biomolecules.

ISSN:0734-2101    

DOI:10.1116/1.576986

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The capability of the scanning tunneling microscope (STM) to produce atomic‐resolution images makes it a powerful tool in both basic and applied science. The main application of STM has been for the analysis of structural features and electronic properties of a wide spectrum of metals and semiconductors, while only a few biological items have been investigated. With a STM operating in air at ambient pressure, we present images showing the molecular structure of naked DNA. High‐resolution topographic images of linear DNA fragments reveal structures with a periodicity of ∼38±6 Å and a width of 28±4 Å, consistent with the molecular dimensions of the helical repeat of double‐stranded DNA. In addition, the supreme resolving power of the STM permits visualization of the major and minor grooves.

ISSN:0734-2101    

DOI:10.1116/1.576987

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

One of the primary goals of scanning tunneling microscopy (STM) work in biology is to obtain high‐resolution images of biological molecules under conditions that resemble thoseinvivo. To accomplish this goal two main difficulties need to be overcome. (1) Problems caused by the low conductivity of biological material, and (2) depositing the sample in a controlled and reliable way on a flat, inert conducting substrate. The first problem can be solved by coating the sample with a thin metal layer, at some cost in resolution. This allows the second problem, controllable depositing the sample, to be approached systematically, independent of difficulties caused by low conductivity. Using this approach, two different kinds of viruses have been imaged, bacteriophage T7 and fd. STM images of intact T7 and fd bacteriophages, and lysed T7 phages have been obtained in a straightforward and routine way. To obtain high‐resolution images it is necessary to use uncoated samples. The conductivity problems cannot be avoided in this case, but they can be minimized by using thin samples. We have been able to obtain STM images of uncoated DNA molecules with about 10 Å resolution, in a variety of packing arrangements and conformations. One conclusion resulting from these experiments is that the STM tip often makes contact with uncoated molecules and sweeps them aside rather than imaging them. The mechanism of conduction through the DNA, and methods by which the tip contact problem can be overcome are discussed.

ISSN:0734-2101    

DOI:10.1116/1.576988

出版商:American Vacuum Society    

年代:1990

数据来源: AIP