Metal ion-mediated substrate-assisted catalysis in type II restriction endonucleases

The 2.15-A resolution cocrystal structure of EcoRV endonuclease mutant T93A complexed with DNA and Ca2+ ions reveals two divalent metals bound in one of the active sites. One of these metals is ligated through an inner-sphere water molecule to the phosphate group located 3' to the scissile phosphate. A second inner-sphere water on this metal is positioned approximately in-line for attack on the scissile phosphate. This structure corroborates the observation that the pro-SP phosphoryl oxygen on the adjacent 3' phosphate cannot be modified without severe loss of catalytic efficiency. The structural equivalence of key groups, conserved in the active sites of EcoRV, EcoRI, PvuII, and BamHI endonucleases, suggests that ligation of a catalytic divalent metal ion to this phosphate may occur in many type II restriction enzymes. Together with previous cocrystal structures, these data allow construction of a detailed model for the pretransition state configuration in EcoRV. This model features three divalent metal ions per active site and invokes assistance in the bond-making step by a conserved lysine, which stabilizes the attacking hydroxide ion nucleophile.     

Observation of alpha particle loss from JET plasmas during ion cyclotron resonance frequency heating using a thin foil Faraday cup detector array

The loss of MeV alpha particles from JET plasmas has been measured with a set of thin foil Faraday cup detectors during third harmonic heating of helium neutral beam ions. Tail temperatures of ～ 2?MeV have been observed, with radial scrape off lengths of a few centimeters. Operational experience from this system indicates that such detectors are potentially feasible for future large tokamaks, but careful attention to screening rf and MHD induced noise is essential.     

Mg sheet metal forming: Lessons learned from deep drawing Li and Y solid-solution alloys

The sheet formability of current magnesium alloys at ambient temperatures is poor; however, the formability at moderately elevated temperatures can be excellent. Cylindrical cup drawing tests are used to compare the warm forming characteristics of conventional alloy AZ31B with alloys containing lithium oryttrium solid solutions. While both types of experimental alloy can have better room-temperature ductility (ε_f∼25–30%) than AZ31B, only the lithium alloy has comparable or better deep-drawing capacity. The results are discussed in terms of the sheet anisotropy. Particular attention is drawn to the fact that magnesium alloys exhibit poor bending ductility due to their anisotropy and mechanical twinning-induced tension-compression strength asymmetry.     

Atomic force microscopy of collagen structure in bone and dentine revealed by osteoclastic resorption

Mineralised tissues such as bone consist of two material phases: collagen protein fibrils, secreted by osteoblasts, form model structures for subsequent deposition of mineral, calcium hydroxyapatite. Collagen and mineral are removed in a three-dimensional manner by osteoclasts during bone turnover in skeletal growth or repair. Bone active drugs have recently been developed for skeletal diseases, and there is revived interest in changes in the structure of mineralised tissues seen in disease and upon treatment. The resolution of atomic force microscopy and use of unmodified samples has enabled us to image bone and dentine collagen exposed by the natural process of cellular dissolution of mineralised matrix. The morphology of bone and dentine has been analysed when fully mineralised and after osteoclast-mediated bone resorption, and compared with results from other microscopy techniques. Banded type I collagen, with 66.5+/-1.4 nm axial D-periodicity and 62.2+/-7.0 nm diameter, has been identified within resorption lacunae in bone and 69.4+/-4.3 nm axial D-periodicity and 140.6+/-12.4 nm diameter in dentine substrates formed by human and rabbit osteoclasts, respectively. This observation suggests a route by which the material and morphological properties of bone collagen can be analysed in situ, compared with collagen from non-skeletal sites, and contrasted in diseases of medical importance, such as osteoporosis, where skeletal tissue is mechanically weakened.     

Slip-System-Related Dislocation Study from <Emphasis Type="Italic">In-Situ</Emphasis> Neutron Measurements

A combined experimental/computational approach is employed to study slip-system-related dislocation-substructure formation during uniaxial tension of a single-phase, face-centered-cubic (fcc), nickel-based alloy. In-situ neutron-diffraction measurements were conducted to monitor the peak-intensity, peak-position, and peak-broadening evolution during a displacement-controlled, monotonic-tension experiment at room temperature. The measured lattice-strain evolution and the macrostress/macrostrain curves were used to obtain the material parameters required for simulating the texture development by a visco-plastic self-consistent (VPSC) model. The simulated texture compared favorably with experimentally-determined texture results over a range of 0 to 30 pct engineering strain. The grain-orientation-dependent input into the Debye-intensity ring was considered. Grains favorably oriented relative to the two detector banks in the geometry of the neutron experiment were indicated. For the favorably oriented grains, the simulated slip-system activity was used to calculate the slip-system-dependent, dislocation-contrast factor. The combination of the calculated contrast factor with the experimentally-measured peak broadening allows the assessment of the parameters of the dislocation arrangement within the specifically oriented grains, which has a quantitative agreement with the transmission-electron-microscopy results. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.

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Nanomechanical detection of antibiotic-mucopeptide binding in a model for superbug drug resistance

The alarming growth of the antibiotic-resistant superbugs methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) is driving the development of new technologies to investigate antibiotics and their modes of action. We report the label-free detection of vancomycin binding to bacterial cell wall precursor analogues (mucopeptides) on cantilever arrays, with 10 nM sensitivity and at clinically relevant concentrations in blood serum. Differential measurements have quantified binding constants for vancomycin-sensitive and vancomycin-resistant mucopeptide analogues. Moreover, by systematically modifying the mucopeptide density we gain new insights into the origin of surface stress. We propose that stress is a product of a local chemical binding factor and a geometrical factor describing the mechanical connectivity of regions activated by local binding in terms of a percolation process. Our findings place BioMEMS devices in a new class of percolative systems. The percolation concept will underpin the design of devices and coatings to significantly lower the drug detection limit and may also have an impact on our understanding of antibiotic drug action in bacteria.     

A heuristic technique for CTIS image reconstruction

An iterative method is presented for computed tomography imaging spectrometer (CTIS) image reconstruction in the presence of both photon noise in the image and postdetection Gaussian system noise. The new algorithm, which assumes the transfer matrix of the system has a particular structure, is evaluated experimentally with the result that it is significantly better, for larger problems, than both the multiplicative algebraic reconstruction technique (MART) and the mixed-expectation image-reconstruction technique (MERT) with respect to accuracy and computation time.     

Reductions in the social anxiety of women associated with group membership: Distraction, anonymity, security, or diffusion of evaluation?

This study examined whether social anxiety is diminished among women in the company of a group and, if so, whether it is associated with perceptions of anonymity, being distracted, feelings of security, or an expectation that any evaluation will be diffused across the members of the group. Two social scenarios were presented to 61 female undergraduate students: a physique-salient and a general social situation. For the former situation, self-presentational anxiety was less pronounced in a group of female friends and a mixed group of male and female friends than when alone or in a group of male friends. For the general social situation, in comparison with being alone, being in a group of female friends, being in a group of male friends, and being in a mixed group of male and female friends were associated with less social anxiety. Diffusion of evaluation and perceptions of security were most strongly associated with the reduction in social anxiety. (PsycINFO Database Record (c) 2010 APA, all rights reserved)