Search PubMed⌕ Search

Biomedical subjects

Erik Johnson

Publications and source records attributed to Erik Johnson.

7 recordsLinked to original sources
PubMed2006

Beamline X29: a novel undulator source for X-ray crystallography.

A high-flux insertion device and beamline for macromolecular crystallography has been built at the National Synchrotron Light Source (NSLS) that employs a mini-gap undulator source developed by the NSLS. The mini-gap undulator at beamline X29 is a hybrid-magnet device of period 12.5 mm operating at proven gaps of 3.3-10 mm. The beamline provides hard X-rays for macromolecular crystallography experiments from the second and third harmonics over an energy range of 5-15 keV. The X-ray optics is designed to deliver intense and highly collimated X-rays. Horizontal focusing is achieved by a cryogenically cooled sagittally focusing double-crystal monochromator with approximately 4.1:1 demagnification. A vertical focusing mirror downstream of the monochromator is used for harmonic rejection and vertical focusing. The experimental station hosts an Area Detector Systems Quantum 315 CCD detector with 2.2 s readout time between exposures and Crystal Logic goniostat for crystal rotation and detector positioning. An auto-mounter crystal changer has been installed to facilitate the high-throughput data collection required by the major users, which includes structural genomics projects and the Macromolecular Crystallography Research Resource mail-in program. X29 is 10(3) times brighter than any existing bending-magnet beamline at NSLS with an actual flux of 2.5 x 10(11) photons s(-1) through a 0.12 mm square aperture at 11.271 keV.

Crystallography, X-Ray↗
PubMed2004

Nanoscale lead and noble gas inclusions in aluminum: structures and properties.

Transmission electron microscopy has been used for structural and physical characterization of nanoscale inclusions of lead and noble gases in aluminum. When the inclusion sizes approach nanoscale dimensions, many of their properties are seen to deviate from similar properties in bulk and in most cases the deviations will increase as the inclusion sizes decrease. Binary alloys of lead and noble gases with aluminum are characterized by extremely low mutual solubilities and inclusions will, therefore, exist as practically pure components embedded in the aluminum matrix. Furthermore, the thermal vacancy mobility in aluminum at and above room temperature is sufficiently high to accommodate volume strains associated with the inclusions thus leading to virtually strain free crystals. The inclusions grow in parallel cube alignment with the aluminum matrix and have a cuboctahedral shape, which reflects directly the anisotropy of the interfacial energies. Inclusions in grain boundaries can have single crystalline or bicrystalline morphology that can be explained from a generalized Wulff analysis such as the xi-vector construction. The inclusions have been found to display a variety of nanoscale features such as high Laplace pressure, size-dependent superheating during melting, deviations from the Wulff shape displaying magic size effects, a shape dependence of edge energy, and so on. All these effects have been observed and monitored by TEM using conventional imaging conditions and high-resolution conditions in combination with in-situ analysis at elevated temperatures.

Algorithms↗
PubMed2003

17beta-Estradiol extends ischemic thresholds and exerts neuroprotective effects in cerebral subcortex against transient focal cerebral ischemia in rats.

Neuroprotective effects of estrogens are demonstrated consistently in the cerebral cortex, but not in subcortical areas. In the present study, transient middle cerebral artery occlusions (MCAO) were induced for various duration, and protective effects of estrogen treatment on the cerebral cortex and subcortex were evaluated. MCAO was induced for 30, 40 or 60 min in ovariectomized rats. Animals were treated with 17beta-estradiol (E2) or vehicle (OVX) 2 h before MCAO and sacrificed 24 h after the indicated duration of MCAO. Ischemic lesion was evaluated by 2,3,5-triphenyltetrazolium chloride staining, hematoxylin and eosin staining, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. E2 treatment reduced the magnitude and delayed the appearance of the total ischemic lesion area and largely prevented TUNEL staining in the cortex. In the subcortex, E2 treatment prevented the ischemic lesion in the 30-min group, reduced lesion area in the 40-min group, but had no effect on ischemic lesion area in the 60-min group. E2 treatment significantly decreased apoptotic cell number in the subcortical area at 30 and 40 min, but not at 60 min of MCAO. This study demonstrated that estrogen treatment can protect the cerebral subcortex in a severity-dependent manner, suggesting that the lack of protective effects of estrogen treatment in the subcortex is not due to the lack of estrogen receptors. Further, this study indicates that estrogens could be used as a neuroprotectant to prolong the therapeutic window of thrombolysis and prolong the time of cerebral circulation intervention for neurosurgical procedure.

Animals↗
PubMed2003

The role of telencephalic NMDA receptors in avoidance learning in goldfish (Carassius auratus).

Studies with goldfish (Carassius auratus) have suggested that N-methyl-D-aspartate (NMDA) receptors are concentrated most densely in the telencephalon, a simple structure homologous to the limbic structure of higher vertebrates. The present study investigated the amnestic effects of microinjections of the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) to the goldfish telencephalon on avoidance conditioning. Results showed that microinjections of D-AP5 before training impaired avoidance learning at doses that did not impair performance processes. High-performance liquid chromatography measurements showed that D-AP5 was detected only in the telencephalon following microinjections. Thus, D-AP5 impaired avoidance learning through its interaction with telencephalic NMDA receptors in goldfish. Furthermore, microinjections of D-AP5 to the goldfish telencephalon immediately following training did not impair memory consolidation of avoidance conditioning.

Animals↗