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100 kV is investigated as the operating voltage for single-particle electron cryomicroscopy (cryoEM). Reducing the electron energy from the current standard of 300 or 200 keV offers both cost savings and potentially improved imaging. The latter follows from recent measurements of radiation damage to biological specimens by high-energy electrons, which show that at lower energies there is an increased amount of information available per unit damage. For frozen hydrated specimens around 300 Å in thickness, the predicted optimal electron energy for imaging is 100 keV. Currently available electron cryomicroscopes in the 100–120 keV range are not optimized for cryoEM as they lack both the spatially coherent illumination needed for the high defocus used in cryoEM and imaging detectors optimized for 100 keV electrons. To demonstrate the potential of imaging at 100 kV, the voltage of a standard, commercial 200 kV field-emission gun (FEG) microscope was reduced to 100 kV and a side-entry cryoholder was used. As high-efficiency, large-area cameras are not currently available for 100 keV electrons, a commercial hybrid pixel camera designed for X-ray detection was attached to the camera chamber and was used for low-dose data collection. Using this configuration, five single-particle specimens were imaged: hepatitis B virus capsid, bacterial 70S ribosome, catalase, DNA protection during starvation protein and haemoglobin, ranging in size from 4.5 MDa to 64 kDa with corresponding diameters from 320 to 72 Å. These five data sets were used to reconstruct 3D structures with resolutions between 8.4 and 3.4 Å. Based on this work, the practical advantages and current technological limitations to single-particle cryoEM at 100 keV are considered. These results are also discussed in the context of future microscope development towards the goal of rapid, simple and widely available structure determination of any purified biological specimen.

The Earth's mantle is isotopically heterogeneous on length scales ranging from centimetres to more than 10 4 kilometres 1 , 2 . This heterogeneity originates from partial melt extraction and plate tectonic recycling, whereas stirring during mantle convection tends to reduce it. Here we show that mid-ocean ridge basalts from 2,000 km along the southeast Indian ridge (SEIR) display a bimodal hafnium isotopic distribution. This bimodality reveals the presence of ancient compositional striations (streaks) in the Indian Ocean upper mantle. The number density of the streaks is described by a Poisson distribution, with an average thickness of ∼40 km. Such a distribution is anticipated for a well-stirred upper mantle, in which heterogeneity is continually introduced by plate tectonic recycling, and redistributed by viscous stretching and convective refolding.

The recent success of electron cryomicroscopy in biology has drawn the attention of the materials science community, which is starting to employ similar techniques for imaging a wide variety of nonbiological specimens. This article reviews the theory and practical implications of radiation damage in electron microscopy, and then considers how electron cryomicroscopy techniques may be applied to other radiation-sensitive specimens of interest to materials scientists. We also discuss aspects of radiation damage that warrant further study as instrumentation technology advances and consider new methods that might be useful in the future.

This article analyzes the provisions of IDEA '97 as they pertain to students in private schools. More specifically, the article reviews implementing regulations and relevant case law, issues concerning the extent to which services must be provided and the manner in which they are provided, and the quality of services that are provided. Finally, the article examines guidelines for implementing the new provisions and suggestions for how to avoid violating the establishment clause when one is providing services to students who attend sectarian schools.

The Individuals with Disabilities Education Act (IDEA) requires public school districts to provide special education and related services to students with disabilities enrolled in private schools. When the private school is a parochial school this issue may be complicated because of the First Amendment's separation of church and state doctrine. Until recently, providing most on-site special education services was prohibited. A recent U.S. Supreme Court decision lifted the ban, and the 1997 amendments to IDEA have helped to clarify a school district's obligation for providing services to parochial school students. In this article the authors summarize and analyze the major court decisions concerned with the provision of special education and related services to parochial school students and discuss their implications for providing on-site special education services at parochial schools. In particular, two questions are addressed: Must private school students with disabilities be provided with the same level of services as their public school peers? Are school districts required to provide parochial school students with on-site special education services?

This article examines the different approaches taken in the United States (US) and South Africa with regard to the best interests of students/learners. The US has a tradition of invoking a best interest standard for certain kinds of decisions involving children, but that tradition has generally been limited to divorce decrees, while the emphasis in school-related matters has been on rights of parents grounded in the US Constitution and in federal and state statutes. On the other hand, South Africa has enshrined the best interest of the child in its Constitution and it is of paramount importance in all matters pertaining to the child. How these two approaches differ and what the future of the parent-child relationship should be in influencing decision-making regarding learners at schools is the object of this article. The article also considers the relevant US federal constitutional and state statutory interpretations as they differ from the South African constitutional language. This article asks whether the long US constitutional tradition of framing children's best interests in terms of parental decision-making on behalf of their children needs to be reconsidered. Conversely, in South Africa the most important question is whether courts could very well expand the concept of a parent, while at the same time considering whether the best interest of the child standard is a fundamental right or a rule of construction.