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Biomedical subjects

W O Saxton

Publications and source records attributed to W O Saxton.

11 recordsLinked to original sources
PubMed2002

A new method for the determination of the wave aberration function for high resolution TEM 1. Measurement of the symmetric aberrations.

A new method for the accurate determination of the symmetric coefficients of the wave aberration function has been developed. The relative defoci and displacements of images in a focus series are determined from an analysis of the phase correlation function between pairs of images, allowing the restoration of an image wave even when focus and specimen drift are present. Subsequently, the absolute coefficients of both defocus and 2-fold astigmatism are determined with a phase contrast index function. Overall this method allows a very accurate automated aberration determination even for largely crystalline samples with little amorphous contamination. Using experimental images of the complex oxide Nb16W18O94 we have demonstrated the new method and critically compared it with existing diffractogram based aberration determinations. A series of protocols for practical implementation is also given together with a detailed analysis of the accuracy achieved. Finally a focal series restoration of Nb16W18O94 with symmetric aberrations determined automatically using this method is presented.

Journal Article↗
PubMed2002

Cation segregation in Nb16W18O94 using high angle annular dark field scanning transmission electron microscopy and image processing.

We report the characterization of the complex oxide Nb16W18O94 using high angle annular dark field imaging at 200 kV in a scanning transmission electron microscope. The results of this study suggest that the W and Nb cations are not uniformly distributed among the cation columns projected along [001] but that there is preferential segregation of the heavier species to certain column sites. In order to analyse the experimental data obtained, an image processing methodology has been developed which may also find application in locating specific motifs within a generally distorted image field.

Journal Article↗
PubMed1986

Three-dimensional structure of the regular surface layer (HPI layer) of Deinococcus radiodurans.

The low-resolution structure of the regular surface layer of Deinococcus radiodurans has been determined from negatively stained specimens by three-dimensional electron microscopy. The layer has P6 symmetry, a lattice constant of 18 nm and a thickness of 6.5 nm. Three-dimensional reconstruction was performed by a hybrid real space/Fourier space approach that incorporates partial compensation of lattice distortions: The model obtained is discussed in the light of independent information about the surface structure of this layer, derived from metal shadowing and surface relief reconstruction. While agreement is quite satisfactory for the apparently more rigid inner surface, the outer surface shows severe flattening effects. The structure of the HPI layer is compared with other bacterial surface layers using a classification scheme that is outlined in the Appendix.

Bacteria↗
PubMed1986

Three-dimensional structure of the tetragonal surface layer of Sporosarcina ureae.

The three-dimensional structure of the regular surface layer of Sporosarcina ureae has been determined to a resolution of 1.7 nm by electron microscopy and image reconstruction. The S-layer has p4 symmetry, a lattice constant of 12.9 nm, and a minimum thickness of 6.6 nm. The reconstruction reveals a distinct domain structure: a massive core, arms connecting adjacent unit cells, and spurs which make contact at the subsidiary fourfold symmetry axes. In the z-direction the domains appear to be arranged in three planes, creating two entirely different surface reliefs. The S-layer has a complex pattern of pores and gaps that are 2 to 3 nm wide. In addition, the secondary-structure composition has been determined by infrared spectroscopy: about 35% of the polypeptide appears to have a beta-structure conformation.

Bacterial Proteins↗
PubMed1984

Three-dimensional reconstruction of imperfect two-dimensional crystals.

An outline is given of the general methodology of 3D reconstruction on the basis of correlation averages of the 2D projections: this hybrid real space/Fourier space approach substantially alleviates one of the most serious limitations on obtaining high resolution 3D structures, namely crystal distortions. The paper discusses some of the technical problems involved, namely optimisation of tilt increments, a posteriori tilt angle determination, extraction of lattice line data from averaged unit cells, and stain/protein boundary determination. The approach is illustrated by application to a 2D crystal from a bacterial cell envelope.

Bacteria↗
PubMed1982

Mass mapping of a protein complex with the scanning transmission electron microscope.

A mass map of the hexagonally packed intermediate layer (HPI-layer), a regular protein monolayer from the cell envelope of Micrococcus radiodurans, has been obtained by scanning transmission electron microscopy. Samples were freeze-dried within the microscope, and low-dose images were recorded in the dark-field mode directly in digital form and processed by correlation averaging. The averaged projection of the unstained structure--i.e., the mass map--thus calculated shows a resolution to 3-nm period and reveals morphological features consistent with those obtained by negative staining. The mass of individual morphological domains was extracted by using variously the mass map itself or an average from a negatively stained HPI layer to define the domain boundaries. Protrusions as small as 1,300 daltons could be measured reproducibly within the unit cell of 655,000 daltons. The method developed opens an avenue to identify molecular species in situ and to correlate topographic information with biochemical data.

Bacterial Proteins↗
PubMed1982

Interactive image processing with an off-line minicomputer: organization, performance and applications.

A recently established facility for interactive image processing is described, primarily from the point of view of the system's hardware organization and the special purpose software provided to support it. The performance of more general software (the 'Semper' system) in the new environment has been measured carefully, and found to be about 10 times its performance on our previous PDP 8/E computer; particular attention is given to the data management techniques crucial to the system's efficiency. Some current applications are illustrated, including high resolution image interpretation and image averaging of biological crystals.

Computers↗
PubMed1982

The correlation averaging of a regularly arranged bacterial cell envelope protein.

An adaptation of the 'correlation averaging' method is described which allows reliable and almost fully automatic image averaging in the case of near-periodic structures notwithstanding the presence of substantial crystal imperfections; methods for assessing resolution and symmetry without reliance on crystallinity are also discussed. Electron micrographs of negatively stained and rotary shadowed preparations of the HPI-layer protein from the cell envelope of Micrococcus radiodurans have been averaged using the method, and the projected structure is described to a resolution of about 1.9 nm.

Cell Membrane↗
PubMed1977

Motif detection in quantum noise-limited electron micrographs by cross-correlation.

The use of cross-correlation to detect randomly positioned low-dose realizations of a motif in an image field is quantitatively evaluated. For both the bright- and dark-field cases, we derive theoretical expressions for the minimum dose allowing detection in terms of motif size, resolution, and contrast. Model computations on simulated low-dose images of a spherical virus particle give results that agree with our theoretical formulas and demonstrate the feasibility of this approach.

Microscopy, Electron↗