Showing papers on "Resolution (electron density) published in 1990"
TL;DR: A complete atomic model for bacteriorhodopsin between amino acid residues 8 and 225 has been built and suggests that pK changes in the Schiff base must act as the means by which light energy is converted into proton pumping pressure in the channel.
2,772 citations
TL;DR: In this article, the reflectance spectra of minerals are studied as a function of spectral resolution in the range from 0.2 to 3.0 microns, and selected absorption bands were studied at resolving powers as high as 2240.
Abstract: The reflectance spectra of minerals are studied as a function of spectral resolution in the range from 0.2 to 3.0 microns. Selected absorption bands were studied at resolving powers as high as 2240. At resolving powers of approximately 1000, many OH-bearing minerals show diagnostic sharp absorptions at the resolution limit. At low resolution, some minerals may not be distinguishable, but as the resolution is increased, most can be easily identified. As the resolution is increased, many minerals show fine structure, particularly in the OH-stretching overtone region near 1.4 micron. The fine structure can enhance the ability to discriminate between minerals, and in some cases the fine structure can be used to determine elemental composition.
1,349 citations
TL;DR: A Bloch-wave analysis shows how a signal dependent on the electron intensity at the atom sites can be used to form an incoherent image of a crystal structure that can be predicted and interpreted intuitively.
Abstract: A Bloch-wave analysis shows how a signal dependent on the electron intensity at the atom sites can be used to form an incoherent image of a crystal structure. To a good approximation the image is given by a convolution of a compositionally sensitive object function with an appropriate resolution function, and as such can be predicted and interpreted intuitively. Information on a scale below the resolution limit can be interpreted by deconvolution.
554 citations
TL;DR: In this article, an azimuthal-integrated resolution function for small-angle scattering in pinhole geometry is derived, which can be applied to scattering from a material with a circular symmetric scattering cross section.
Abstract: Analytical expression for the resolution function for small-angle scattering in pinhole geometry are derived. The contributions to the resolution function due to wavelength spread, finite collimation and detector resolution are determined separately using Gaussian functions to approximate the contributions. A general resolution function is derived which is the result of the combined effect of the three contributions. An azimuthal-integrated resolution function, which can be applied to scattering from a material with a circular symmetric scattering cross section, is calculated. This resolution function contains in addition a contribution from the averaging procedure itself. The analytical results are compared with the results of computer simulations. The comparison shows that Gaussian functions give a good description of the resolution function and that the widths agree with those calculated by the analytical expressions. The resolution function is applied in the analysis of two experimental examples: neutron scattering from latex particles [Wignall, Christen & Ramakrishnan (1988). J. Appl. Cryst. 21, 438–451] and neutron scattering from lamellar structures of bilayer lipid membranes (Mortensen, Pfeiffer, Sackmann & Knoll, unpublished). The analytical expressions for the resolution function allow a least-squares analysis to be performed and excellent agreement between experimental and theoretical scattering patterns are obtained.
418 citations
TL;DR: The structure of the icosahedral bacteriophage MS2 has been determined by X-ray crystallography and the coat protein has no structural similarity to that of any other known RNA virus.
Abstract: The structure of the icosahedral bacteriophage MS2 has been determined to 3.3 A resolution by X-ray crystallography. The phase determination involved both molecular replacement at low resolution using a known structure and heavy-atom substitution. The coat protein has no structural similarity to that of any other known RNA virus.
384 citations
TL;DR: In this paper, the optimum configuration for a neutron time-of-flight diffractometer to study amorphous solids is discussed and it is concluded that measurements should be performed at relatively low scattering angles using a cold moderator.
Abstract: The optimum configuration for a neutron time-of-flight diffractometer to study amorphous solids is discussed and it is concluded that measurements should be performed at relatively low scattering angles using a cold moderator. Good reciprocal space resolution is also important. Time-of-flight data for vitreous silica are combined with twin-axis data from part II (P.A.V. Johnson et al., J. Non-Cryst. Solids 58 (1983) 109) to give an accurate composite interference function and excellent real space resolution on Fourier transformation. The maximum scattering vector, Q max , is 45.2 A −1 . A peak fit to the resulting correlation function yields a mean SiO bond length of 1.608±0.004 A .
266 citations
TL;DR: In this paper, the first experiments with a new ESCA instrument with monochromatic X-ray excitation were presented, and the measurements were selected to assess the performance of the instrument in terms of energy and spatial resolution, information rate and surface sensitivity.
243 citations
IBM1
TL;DR: A theory of atomic resolution of scanning tunneling microscopy in terms of actual tip states, for example, d z 2 tip states on tungsten tips is presented, which shows clear contradiction to the resolution limits previously predicted.
Abstract: Scanning tunneling microscopy has repeatedly resolved individual atoms on a number of metal surfaces with atomic distances 2.5--3 \AA{}. This is in sharp contradiction to the resolution limits previously predicted, 6--9 \AA{}. We present a theory of such atomic resolution in terms of actual tip states, for example, ${\mathit{d}}_{\mathit{z}}^{2}$ tip states on tungsten tips. Quantitative interpretation of the observed images is obtained with no adjustable parameters. We predict that to achieve atomic resolution, the tip material should be either a d-band metal or certain semiconductor.
189 citations
TL;DR: Atomic-resolution imaging of duplex DNA is described and the potential of the STM for characterization of large biomolecular structures is demonstrated, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.
Abstract: THE scanning tunnelling microscope (STM) has been used to visualize DNA under water, under oil and in air. Images of single-stranded DNA have shown that submolecular resolution is possible. Here we describe atomic-resolution imaging of duplex DNA. Topographic STM images of uncoated duplex DNA on a graphite substrate obtained in ultra-high vacuum are presented that show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles show excellent correlation with atomic contours of the van der Waals surface of A-form DNA derived from X-ray crystallography. A comparison of variations in the barrier to quantum mechanical tunnelling (barrier-height) with atomic-scale topography shows correlation over the phosphate-sugar backbone but anticorrelation over the base pairs. This relationship may be due to the different chemical characteristics of parts of the molecule. Further investigation of this phenomenon should lead to a better understanding of the physics of imaging adsorbates with the STM and may prove useful in sequencing DNA. The improved resolution compared with previously published STM images of DNA may be attributable to ultra-high vacuum, high data-pixel density, slow scan rate, a fortuitously clean and sharp tip and/or a relatively dilute and extremely clean sample solution. This work demonstrates the potential of the STM for characterization of large biomolecular structures, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.
177 citations
TL;DR: In this article, high-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments have been performed on R1 and R> 1 illite/smectite (I/S) samples that from X-ray powder diffraction (XRD) experiments appear to contain well-ordered layer sequences.
Abstract: High-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments have been performed on R1 and R> 1 illite/smectite (I/S) samples that from X-ray powder diffraction (XRD) experiments appear to contain well-ordered layer sequences. The HRTEM images confirmed earlier computer image simulations, which suggested that periodicities due to I/S ordering can be imaged in TEM instruments of moderate resolution. The experiments also confirmed that in instruments of this sort, the strongest contrast arising from the compositional difference between I and S layers occurs under rather unusual imaging conditions of strong oveffocus. Some selected-area electron diffraction (SAD) patterns showed additional diffraction spots consistent with R1 and R3 ordering. SAD patterns and cross-fringes arising in HRTEM images from non-00l reciprocal lattice rows indicated that the stacking vectors of most adjacent 2:1 layers were not randomly oriented with respect to each other. Thus, the I/S was not fully turbostratic, but instead consisted of very thin, coherently stacked crystallites that extended across the fundamental particles postulated by Nadeau and coworkers. S/(I + S) ratios were determined for about seventy HRTEM images obtained and interpreted by three different TEM operators. These ratios were consistent with those obtained from standard XRD procedures, suggesting that results obtained by XRD can be used to infer the initial structural state of mixed-layer I/S prior to treatment of samples for XRD experiments. The HRTEM experiments thus demonstrated that the two specimens examined consisted of ordered I/S existing as small crystals, most of which contained more layers than the fundamental particles of Nadeau and coworkers. The non-turbostratic stacking suggests an energetic interaction between the individual fundamental particles, leading to at least two alternative thermodynamic descriptions of these materials. Although the I/S crystals in the present experiments probably were disaggregated into fundamental particles during sample preparation for XRD, the I/S crystals appear to have separated only along the smectite interlayers. If the term “fundamental particle” is to be used for primary, untreated I/S, its original definition should be modified to include not only free particles, but also those that occur as layers within small crystals. It further should be recognized that these particles can interact thermodynamically and crystallographically with their neighbors.
122 citations
TL;DR: In this article, it was shown that diamagnetic susceptibility inhomogeneity can degrade this resolution and examples of image reconstruction for simple geometries are given, however, this degradation increases with the size of the polarizing field and the optimal resolution cannot be restored by simply increasing the gradient strength unless appropriate echo summation methods are used.
TL;DR: It is concluded that, with the present accuracy of data collection and the present magnitude of delta F/F available for the derivatives, the phasing power is too small and the phases that are obtained are not sufficiently accurate to provide a reliably interpretable map.
TL;DR: A density map has been calculated by combining Fourier amplitudes from electron diffraction patterns with phases from images, and a specific arrangement for the ten alpha-helices that have been suggested as spanning the bilayer is proposed.
TL;DR: Three‐dimensional imaging of biological objects becomes possible throughocal scanning laser microscopy because the lateral resolution is better than the axial resolution and, thus, the microscope provides orientation‐dependent images.
Abstract: SUMMARY
Confocal scanning laser microscopy (CSLM) provides optical sectioning of a fluorescent sample and improved resolution with respect to conventional optical microscopy. As a result, three-dimensional (3-D) imaging of biological objects becomes possible. A difficulty is that the lateral resolution is better than the axial resolution and, thus, the microscope provides orientation-dependent images.
However, a theoretical investigation of the process of image formation in CSLM shows that it must be possible to improve the resolution obtained in practice. We present two methods for achieving such a result in the case of 3-D fluorescent objects.
The first method applies to conventional CSLM, where the image is detected only on the optical axis for any scanning position. Since the resulting 3-D image is the convolution of the object with the impulse-response function of the instrument, the problem of image restoration is a deconvolution problem and is affected by numerical instability. A short introduction to the linear methods developed for obtaining stable solutions of these problems (the so-called regularization theory of ill-posed problems) is given and an application to a real image is discussed.
The second method applies to a new version of CSLM proposed in recent years. In such a case the full image must be measured by a suitable array of detectors. For each scanning position the data are not single numbers but vectors. Then, in order to recover the object, one must solve a Fredholm integral equation of the first kind. A method for the solution of this equation is presented and the possibility of achieving super-resolution is demonstrated. More precisely, we show that it is possible to improve by about a factor of 2 the resolution of conventional CSLM both in the lateral and axial directions.
TL;DR: The structure of PhoE porin in projection normal to the membrane plane has been determined to a resolution of about 3.5 A by electron crystallographic techniques using high resolution images and electron diffraction patterns to obtain the structure factor phase information and the more accurate values of the amplitude.
TL;DR: In this article, an energy-filtered transmission electron microscope (EFTEM) was used to filter out inelastic electron-plasmon scattering process, which resulted in a dramatic decrease in defocusing values.
TL;DR: In this article, the reliability of scanning tunneling microscopy (STM) images of mesoscopically rough surfaces was discussed, and the specific structure of these images represented a convolution between the real surface topography and the shape of the tip.
Abstract: This paper discusses the reliability of scanning tunneling microscopy (STM) images of mesoscopically rough surfaces. The specific structure of these images represents a convolution between the real surface topography and the shape of the tip. In order to interpret these images quantitatively, the line scans of steep and high steps can be used to obtain an image of the tip itself. This image shows tip radii ranging typically from 5 to 15 nm and cone angles of about 30° over a length of 80 nm, and can in turn be used to recognize the limits of STM resolution on a rough surface: High‐resolution transmission electron microscopy cross‐section images of Au island films on a Au‐Nb double layer are convoluted with the experimentally observed tip shape; the resulting line scans correspond very well with STM graphs of the same samples. Finally an overall criterion for the resolution of the STM on such surfaces is proposed.
15 Dec 1990-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: An NMR microscope with an operating frequency of 500 MHz has been constructed in this article, which has a best in-plane resolution of 4.5 pm and a slice thickness of 100 pm.
Abstract: An NMR microscope with an operating frequency of 500 MHz has been constructed. Using this instrument images showing cellular detail in plant systems have been generated. These have a best in-plane resolution of 4.5 pm and a slice thickness of 100 pm. Many features of these images can be interpreted as the effects of local magnetic susceptibility differences inside the sample.
TL;DR: The structure of cytochrome oxidase from beef heart mitochondria has been analysed by cryo-electron microscopy of vesicle crystals of the space group p22(1)2(1), and the structures described here are consistent with one another and allow an interpretation at higher resolution than from previous work.
TL;DR: In this paper, the conditions générales d'utilisation (http://www.numdam.org/conditions) are defined, i.e., toute utilisation commerciale ou impression systématique is constitutive d'une infraction pénale.
Abstract: © Annales de l’institut Fourier, 1990, tous droits réservés. L’accès aux archives de la revue « Annales de l’institut Fourier » (http://annalif.ujf-grenoble.fr/) implique l’accord avec les conditions générales d’utilisation (http://www.numdam.org/conditions). Toute utilisation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright.
TL;DR: Magnetic force microscopy (MFM) images of rapidly quenched FeNdB are presented in this article, where the magnetic domain structure as observed by MFM consists of elongated, polyhedral-shaped domains imaged by measuring forces smaller than 10−9 N at distances ranging from 20 to more than 200 nm.
Abstract: Magnetic force microscopy (MFM) images of rapidly quenched FeNdB are presented. The magnetic domain structure as observed by MFM consists of elongated, polyhedral‐shaped domains imaged by measuring forces smaller than 10−9 N at distances ranging from 20 to more than 200 nm. The domain transition regions, which are quite sharp and well defined, often show a double‐peaked structure with a peak‐to‐peak distance of 10 nm. At force sensor tip‐to‐sample separations larger than 90 nm only a single peak is observable. It has to be assumed that the relevant effective magnetic volume of the force sensing tip is considerably smaller than the geometric dimensions as determined by scanning electron microscopy in order to understand this 10‐nm resolution.
TL;DR: The photon scanning tunneling microscope (PSTM) has been operated in spectroscopic mode to both image the topography and analyze stress features on a microindented, chromium-implanted sapphire surface, and the topographic resolution observed is appreciably finer than the diffraction limit.
Abstract: The photon scanning tunneling microscope (PSTM) has been operated in spectroscopic mode to both image the topography and analyze stress features on a microindented, chromium-implanted sapphire surface. Light originating primarily from the evanescent field generated by an internally reflected beam, with some contribution from scattering by roughness features and radiation from fluorescence and luminescence, was coupled into the probe tip. The topographic resolution observed (\ensuremath{\sim}50 nm) is appreciably finer than the diffraction limit. Stresses of 3 kbar around microindents were measured by monitoring shifts in the photoluminescence peaks. A discussion of the spectroscopic resolution obtainable with this ``analytical PSTM'' and applications are presented.
TL;DR: In this article, a high-resolution photoelectron spectrometer designed for core level studies of gas phase volatile and involatile compounds using synchrotron radiation is briefly described.
TL;DR: In this paper, an electron energy loss spectrometer coupled with an analytical electron microscope having a field emission source was used for compositional analysis of thin nanoscale native oxide films formed on silicon wafer surfaces.
Abstract: Compositional analysis of thin nanoscale native oxide films formed on {001} silicon wafer surfaces at room temperature was done with an electron energy loss spectrometer coupled to an analytical electron microscope having a field emission source, with better than 4 nm spatial resolution. The electron energy loss spectra show a shift in the threshold onset energy of the Si–L edge of the native oxide from ∼99 eV loss corresponding to pure elemental silicon to ∼105 eV loss, and elemental analysis using the ionization regions of the core loss edges showed the composition to be SiO, within a few percent. Microdiffraction and high resolution electron microscopy (HREM) results showed that the native oxide was completely amorphous, and did not contain detectable nanocrystals. The native oxide can be removed from the Si surface by heating in UHV for a short time at 1000 °C. However, this procedure resulted in the formation of small amounts of a crystalline phase on the Si wafer surface, which was shown to be β-SiC by the same methods.
TL;DR: The electron microscopy of a crystalline assembly of an alpha-helical coiled-coil protein extracted from the ootheca of the praying mantis is described, showing tightly packed molecules some 400 A long lying with their long axes in the plane of the projection.
TL;DR: These systems do not require preliminary reduction to a normal form and, in the first order case, intermingle resolution steps with Skolemization steps.
Abstract: We present non-clausal resolution systems for propositional modal logics whose Kripke models do not involve symmetry, and for first order versions whose Kripke models do not involve constant domains. We give systems for K, T , K4 and S4; other logics are also possible. Our systems do not require preliminary reduction to a normal form and, in the first order case, intermingle resolution steps with Skolemization steps.
TL;DR: Spectral resolution of two-component systems by multiway analysis of excitation-emission-frequency arrays is compared with single-matrix analysis of steady-state excitationemission matrices for both real and computer-simulated systems as discussed by the authors.
Abstract: Spectral resolution of two-component systems by multiway analysis of excitation-emission-frequency arrays is compared with single-matrix analysis of steady-state excitation-emission matrices for both real and computer-simulated systems. Fluorescence lifetime selectivity can improve resolution, especially for the minor component in mixtures in which the two components have unequal contributions to the total intensity. However, in some cases the lifetime difference between two fluorophores is insufficient to aid resolution.
TL;DR: In this article, the best conditions for atomic imaging of 3Al2O3-2SiO2 mullite were discussed relative to the atomic resolution microscope at Berkeley, where multibeam images have been produced at a resolution better than 0.19 nm, with a firm transfer of information on the oxygen sublattice.
Abstract: The crystallographic structure of 3Al2O3-2SiO2 mullite has been studied by means of high-resolution electron microscopy. The best conditions for atomic imaging of this compound are discussed relative to the atomic resolution microscope at Berkeley. [001] multibeam images have been produced at a resolution better than 0.19 nm, allowing the cation sublattice to be directly imaged, with a firm transfer of information on the oxygen sublattice. Under optimal conditions of defocus setting and thickness, typical contrasts have been detected and consistently interpreted in terms of the presence of oxygen vacancies along the defective atomic columns. These observations are discussed in relation to the currently accepted model for the average structure of mullite.