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Showing papers on "Resolution (electron density) published in 1981"




Journal ArticleDOI
TL;DR: In this paper, the phases for the x-ray data for crystals of azurin from P. aeruginosa have been refined and extended from 3 A to 2.7 A resolution by a method based on the direct space averaging of the electron density of the four molecules in the asymmetric unit.
Abstract: The phases for the x-ray data for crystals of azurin from P. aeruginosa have been refined and extended from 3 A to 2.7 A resolution by a method based on the direct space averaging of the electron density of the four molecules in the asymmetric unit. The electron density in the present map is much improved over the earlier one, and most of the side-chains are now evident. In the detailed fitting of a model to the present map, an alternative interpretation, conforming to the plastocyanin model and reversing the chain directions in strands one and two of the original azurin model, was found to better represent certain features of the present map. The surface of the molecule closest to the copper consists of an extended region of invariant or semiconserved hydrophobic residues. There is not a region of conserved charge that is not paired with a compensating charge.

229 citations


Journal ArticleDOI
TL;DR: In this article, the resolution of an ultrasonic pulse-echo imaging system is determined by the dimensions of the resolution cell, which is characterized by a granular pattern, or "speckle", which varies from place to place in the tissue; it can be explained in terms of the coherent formation of the echo from many small scatterers within the resolution cells.

133 citations



Journal ArticleDOI
TL;DR: A treatment of errors in electron imaging has been developed that makes possible the systematic combination of data from several image areas for phase determination by multiple isomorphous replacement of the purple membrane.

99 citations




Journal ArticleDOI
TL;DR: In this article, a Fourier transform spectrometer has been used to obtain rotational levels belonging to the vibrational states (000, (020), (020, (100), and (001) from three spectra of D 2 16 O between 2170 and 3090 cm −1.

81 citations



Journal ArticleDOI
J.R. Fryer1
01 Jan 1981-Carbon
TL;DR: In this article, three high-resolution electron microscopes operating at 100, 120 and 500 KV were used to examine carbon carbons, polymer based carbon and partially graphitized carbon.

Journal ArticleDOI
TL;DR: In this article, the depth resolution in compositional profiles of layers on rough substrates, analysed by ion sputtering with Auger electron spectroscopy or other surface analytical techniques, is evaluated in detail to define the precise effect of the substrate.

Journal ArticleDOI
TL;DR: An electron energy filter of the Castaing-Henry type in a high resolution transmission electron microscope was tested for sensitivity and spatial resolution at a specific electron energy loss in energy selected images of a murine leukaemia virus as mentioned in this paper.
Abstract: SUMMARY An electron energy filter of the Castaing-Henry type in a high resolution transmission electron microscope was tested for sensitivity and spatial resolution at a specific electron energy loss in energy selected images of a murine leukaemia virus. Electron spectroscopic images of phosphorus within the virus membrane bilayer demonstrated a best spatial resolution between 0·3 and 0·5 nm and a sensitivity of 2 × 10−21 g.

Journal Article
TL;DR: In this article, a generalization of Duflo's sum formula for interrelating induced ideals has been given, and the main result is a resolution for certain primitive quotients of the enveloping algebra U (g) of complex semisimple Lie algebra.
Abstract: Let 9 be a complex semisimple Lie algebra. In ([8], Prop. 12) Duflo gave a remarkable sum formula interrelating induced ideals. The main result of this paper provides a natural generalization of this formula and more precisely gives a resolution for certain primitive quotients of the enveloping algebra U (g). The proof has three distinct steps. One, the extension of the Bernstein-Gelfand-Gelfand (in short, B.G.G.) resolution of a simple finite dimensional U(g) module to certain simple highest weight modules. Two, the description of the so-called t-finite part of the space of homomorphisms of any one Verma module to any other. Three, the proof of exactness of a certain functor. The last can be viewed as a non-trivial generalization of the fact that a Verma module with dominant highest weight is projective in the so-called C category. A by-product gives some results on a problem of Kostant relating U (g) to the t-finite part of the space of endomorphisms of a simple highest weight module.

Journal ArticleDOI
01 Jan 1981
TL;DR: The resolution obtained with biological structures is disappointing when compared with the resolving power of modern electron microscopes of between 0.2 and 0.5 nm, and it is thus not the performance of the electron microscope that is the limiting factor, but the damage caused during specimen preparation and irradiation by the electron beam.
Abstract: In the investigation of biological ultrastructure by electron microscopy the most significant observation to date is a 1 nm repeat in the purple membrane of Halobacterium halobium (73). With a high degree of certainty, the spacing can be ascribed to the inter a-helical separation within bacteriorhodopsin molecules. This resolution is only made possi­ ble because of the unique properties of the purple membrane and is due in particular to its two-dimensional crystallinity, which results in a repetition of the structural data. For a nonperiodic object, the meaning­ ful resolution often lies between 3 and 5 nm, as illustrated by comparing two models that have been proposed independently for the structure of ribosomes (45, 82). More frequently, it is in even larger structures that uncertainties become apparent. For example, there are good arguments for questioning the validity of the unit membrane as it is seen in most ultrathin sections (62), and electron microscopy has not yet been able to settle the question of whether chromatin fibres are 10, 20 or 30 nm in diameter. In short, the resolution obtained with biological structures is disappointing when compared with the resolving power of modern electron microscopes of between 0.2 and 0.5 nm. It is thus not the performance of the electron microscope that is the limiting factor, but the damage caused during specimen preparation and irradiation by the electron beam (4). Cryoelectron microscopy has long been considered as a possible avenue to overcome both of these limitations. On the one hand it was anticipated that the beam would have a less damaging effect on cooled specimens (i.e. cryostabilization), whereas on the other hand, freezing


Journal ArticleDOI
TL;DR: In this paper, the atomic number dependence of electron backscattering can be used as the basis of a microanalysis technique and the operating procedures and condition for quantitative measurements of specimen atomic number are outlined and an expression relating the accuracy of composition to atomic number sensitivity has been derived.
Abstract: SUMMARY The atomic number dependence of electron backscattering can be used as the basis of a microanalysis technique. The operating procedures and condition for quantitative measurements of specimen atomic number are outlined and an expression relating the accuracy of composition to the atomic number sensitivity has been derived. Some measurements of the spatial resolution of backscattered electron microanalysis are also presented and compared with the resolution of X-ray microanalysis. Although the range of application of this technique is limited, where it can be applied it has the following advantages: (i) higher spatial resolution than X-ray microanalysis for bulk specimens; (ii) very rapid measurement; (iii) can be applied to compounds of low atomic number elements, (e.g. borides, carbides, nitrides, etc.); (iv) specimen preparation is often relatively straightforward.

Journal ArticleDOI
TL;DR: In this article, it is demonstrated how to obtain the temperature and weight/length curves from quasi-isothermal thermogravimetric and dilatometric studies of thermal decompositions.

Journal ArticleDOI
TL;DR: In this article, the silicon-sapphire interface of CVD silicon on a (1102) sapphire substrate has been studied by high-resolution transmission electron microscopy.
Abstract: The silicon‐sapphire interface of CVD silicon on a (1102) sapphire substrate has been studied by high‐resolution transmission electron microscopy. Cross‐section images of the interface are presented where the silicon and sapphire lattices are directly resolved. The images show that the interface is planar and abrupt to the limit of resolution (less than 3 A). Defect anisotropy is evident and can be linked to tilt of the [100] direction of the silicon layer with respect to the normal to the substrate.


Journal ArticleDOI
TL;DR: In this article, the dependence of the depth resolution on the contribution of a Gaussian crater shape and the finite width of the Gaussian excitation beam was considered for Auger electron spectroscopy depth profiling.
Abstract: The dependence of the depth resolution on the contribution of a Gaussian crater shape and the finite width of a Gaussian excitation beam (and/or Gaussian acceptance function) is considered. The results show that for Auger electron spectroscopy depth profiling, the contribution of the beam shapes to the depth resolution can, in most cases, be neglected. With X-ray photoelectron spectroscopy and secondary ion mass spectroscopy depth profiling, care has to be taken to obtain a well resolved depth profile.




Journal ArticleDOI
P Platz, J Ramette, E Belin, C Bonnelle, A Gabriel 
TL;DR: In this paper, a Johann-type reflection spectrometer with a wavelength resolution of 15000 at 0.229 nm (CrKalpha) is described and the diffracting crystal is a (310)-quartz plate with 50*30 mm2 total and useful area, bent to a radius of 1380 mm.
Abstract: A Johann-type reflection spectrometer with a wavelength resolution of 15000 at 0.229 nm (CrKalpha ) is described. The diffracting crystal is a (310)-quartz plate with 50*30 mm2 total and useful area, bent to a radius of 1380 mm. The use of a novel, simple crystal bending device and of a position-sensitive proportional counter makes this instrument interesting also for other domains of X-ray spectroscopy. Results of extended calculations on diffraction properties of quartz as a function of diffraction plane index and wavelength are also presented.

Journal ArticleDOI
TL;DR: The phases were derived from a model resulting from the joint refinement of x-ray and neutron data at 2.0-A and 2.8-A resolution by using neutron diffraction data obtained from a single crystal of RNase A, and histidine-12 is clearly hydrogen bonded to the carbonyl oxygen of threonine-45 and to the oxygen of the inorganic phosphate.
Abstract: Difference Fourier maps have been calculated at 2.8-A resolution by using neutron diffraction data obtained from a single crystal of RNase A. The phases were derived from a model resulting from the joint refinement of x-ray and neutron data at 2.0-A and 2.8-A resolution, respectively. The orientation of histidine-48 assumed during the refinement of the x-ray model at 2.5 A was confirmed, whereas the other three histidines had to be rotated around C beta--C gamma bonds in order to agree with the neutron difference Fourier maps. In the final model, histidine-12 is clearly hydrogen bonded to the carbonyl oxygen of threonine-45 and to the oxygen of the inorganic phosphate, and histidine-119 is bonded to another oxygen of the phosphate and to the oxygen OD1 of aspartic acid-121.

Journal ArticleDOI
TL;DR: In this paper, a study of a photoelectron x-ray microscope is made, where the usual detector is replaced by a thin photocathode, and the emitted secondary photoelectrons are then imaged by a suitable emission electron microscope.
Abstract: Synchrotron radiation has recently contributed to the development of high‐resolution x‐ray microscopy either for biological investigations or for chemical microanalysis. Contact microscopy for this end appears to be particularly useful, but is mainly limited by the detectors that are used. In order to overcome these limitations—i.e., to obtain real time imaging and quantitative information—a study of a photoelectron x‐ray microscope is made. Contact configuration is used, but the usual detector is replaced by a thin photocathode. The emitted secondary photoelectrons are then imaged by a suitable emission electron microscope. Problems of photoemission, resolution, and D.Q.E. limitation are discussed. The particular nature of electron lens aberrations, due to photoemission characteristics, is taken into account. The conclusions are that resolutions down to 500 A can be obtained in certain conditions. A preliminary low‐ magnification experiment is described and results are shown.

Journal ArticleDOI
TL;DR: A platinum-containing derivative of bacteriorhodopsin has been prepared by treating purple membranes with glycyl-L-methionatoplatinum and low-dose electron diffraction was used to identify Pt binding sites.
Abstract: A platinum-containing derivative of bacteriorhodopsin has been prepared by treating purple membranes with glycyl-L-methionatoplatinum. Low-dose electron diffraction was used to identify Pt binding sites in the 5.6 A resolution reconstruction of the bacteriorhodopsin unit cell in projection. This is a necessary first step in the use of the Pt derivative for identifying the parts of the amino acid sequence corresponding to the α helices in the bacteriorhodopsin structure and for obtaining phases for reflections out to 3.5 A resolution by the method of heavy-atom isomorphous replacement. The largest peak in a Fourier difference map between platinum-labeled and native purple membrane is larger than the spurious features expected to arise from errors in measurements of diffraction intensities.

Journal ArticleDOI
TL;DR: In this paper, a modified scanning Auger microprobe (SAM) is used to perform spatially resolved work function measurements using patches of different work functions giving rise to different onsets of secondary electron emission.