Do electron scattering and muonic x-ray data really require a central depression in the charge distribution of208Pb?

It is well known that for reflections corresponding to large interplanar spacings (i.e., sin θ/λ small), the electron scattering amplitude, f, is sensitive to the ionicity and to the charge distribution around the atoms. We have used this in order to obtain information about the charge distribution in FeTi, which is a candidate for storage of hydrogen. Our goal is to study the changes in electron distribution in the presence of hydrogen, and also the ionicity of hydrogen in metals, but so far our study has been limited to pure FeTi. FeTi has the CsCl structure and thus Fe and Ti scatter with a phase difference of π into the 100-ref lections. Because Fe (Z = 26) is higher in the periodic system than Ti (Z = 22), an immediate “guess” would be that Fe has a larger scattering amplitude than Ti. However, relativistic Hartree-Fock calculations show that the opposite is the case for the 100-reflection. An explanation for this may be sought in the stronger localization of the d-electrons of the first row transition elements when moving to the right in the periodic table. The tabulated difference between fTi (100) and ffe (100) is small, however, and based on the values of the scattering amplitude for isolated atoms, the kinematical intensity of the 100-reflection is only 5.10-4 of the intensity of the 200-reflection.

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Chemical modification of the bacterial wall to determine sites of metal deposition

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100082613 ◽

1978 ◽

Vol 36 (2) ◽

pp. 350-351

Author(s):

T. J. Beveridge

Keyword(s):

Electron Scattering ◽

Metal Salt ◽

Metal Deposition ◽

Suitable Model ◽

X Ray Diffraction ◽

Subtilis Cell ◽

Thin Sections ◽

X Ray ◽

Section Data ◽

Metal Impregnation

The Bacillus subtilis cell wall provides a protective sacculus about the vital constituents of the bacterium and consists of a collection of anionic hetero- and homopolymers which are mainly polysaccharidic. We recently demonstrated that unfixed walls were able to trap and retain substantial amounts of metal when suspended in aqueous metal salt solutions. These walls were briefly mixed with low concentration metal solutions (5mM for 10 min at 22°C), were well washed with deionized distilled water, and the quantity of metal uptake (atomic absorption and X-ray fluorescence), the type of staining response (electron scattering profile of thin-sections), and the crystallinity of the deposition product (X-ray diffraction of embedded specimens) determined.Since most biological material possesses little electron scattering ability electron microscopists have been forced to depend on heavy metal impregnation of the specimen before obtaining thin-section data. Our experience with these walls suggested that they may provide a suitable model system with which to study the sites of reaction for this metal deposition.

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Artefacts in the x-ray microanalysis of frozen-hydrated biological samples

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100127724 ◽

1987 ◽

Vol 45 ◽

pp. 658-659

Author(s):

Patrick Echlin

Keyword(s):

Electron Scattering ◽

Biological Samples ◽

Fracture Face ◽

Detailed Structure ◽

X Ray ◽

Morphological Appearance ◽

The Poor ◽

Freeze Dried

A number of papers have appeared recently which purport to have carried out x-ray microanalysis on fully frozen hydrated samples. It is important to establish reliable criteria to be certain that a sample is in a fully hydrated state. The morphological appearance of the sample is an obvious parameter because fully hydrated samples lack the detailed structure seen in their freeze dried counterparts. The electron scattering by ice within a frozen-hydrated section and from the surface of a frozen-hydrated fracture face obscures cellular detail. (Fig. 1G and 1H.) However, the morphological appearance alone can be quite deceptive for as Figures 1E and 1F show, parts of frozen-dried samples may also have the poor morphology normally associated with fully hydrated samples. It is only when one examines the x-ray spectra that an assurance can be given that the sample is fully hydrated.

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Extended Core Edge Fine Structure in the E.M.

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s1431927600000489 ◽

1980 ◽

Vol 38 ◽

pp. 120-121

Author(s):

R.D. Leapman

Keyword(s):

Fine Structure ◽

Electron Scattering ◽

Inelastic Electron ◽

High Resolution Imaging ◽

X Ray ◽

Analytical Electron ◽

Analytical Electron Microscope ◽

Core Electrons ◽

Resolution Imaging ◽

X Ray Absorption

Extended X-ray Absorption Fine Structure (EXAFS) analysis makes use of synchrotron radiaion to measure modulations in the absorption coefficient above core edges and hence to obtain information about local atomic environments. EXAFS arises when ejected core electrons are backscattered by surrounding atoms and interfere with the outgoing waves. Recently, interest has also been shown in using inelastic electron scattering1-4. Some advantages of Extended X-ray-edge Energy Loss Fine Structure (EXELFS) are: a) small probes formed by the analytical electron microscope give spectra from μm to nm sized areas, compared with mm diameter areas for the X-ray technique, b) EXELFS can be combined with other techniques such as electron diffraction or high resolution imaging, and c) EXELFS is sensitive to low Z elements with K edges from ˜200 eV to ˜ 3000 eV (B to Cl).

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The phosphatase domains of LAR, CD45, and PTP1B: structural correlations with peptide-based inhibitors1

Biochemistry and Cell Biology ◽

10.1139/o99-069 ◽

2000 ◽

Vol 78 (1) ◽

pp. 39-50 ◽

Cited By ~ 8

Author(s):

Nicholas R Glover ◽

Alan S Tracey

Keyword(s):

Insulin Receptor ◽

Charge Distribution ◽

T Cell Activation ◽

Active Site ◽

Cell Activation ◽

Cell Function ◽

Protein Tyrosine Phosphatases ◽

Surface Topology ◽

X Ray ◽

Protein Tyrosine

PTP1B is a cytosolic protein tyrosine phosphatase that is a regulator of the kinase activity of the insulin receptor; the two protein tyrosine phosphatases LAR and CD45 are receptor type phosphatases crucially important to cell function. LAR also is involved in regulation of the insulin receptor while CD45 is critical for T-cell activation. Although LAR and CD45 are both transmembrane phosphatases, these enzymes manifest their phosphatase activity through a catalytic cytosolic domain. We have utilized X-ray coordinates of related phosphatases (RPTPalpha and RPTPµ) and comparative protein modeling to obtain molecular models of the D1 catalytic domains of CD45 and LAR. The models were tested using established protocols and found to be comparable to low resolution X-ray structures. The structure obtained for LAR was compared with the recently reported X-ray structure. Both the CD45-D1 and LAR-D1 structures were then compared to and contrasted with PTP1B. The active site of pockets of the three enzymes were found to be very uniform in structure and charge distribution. Also, the gross surface topology around the active site was found to be somewhat similar for the 3 phosphatases. However, there were significant differences in surface topology, and, more importantly, large changes in surface charge distribution. The differences between the surface features of these enzymes provide an explanation for the selectivity of inhibition by a number of peptides.

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