A guide to the crystallographic analysis of icosahedral viruses

Determining the structure of an icosahedral virus crystal by X-ray diffraction follows very much the same course as conventional protein crystallography. The major differences arise from the relatively large sizes of the particles, which significantly affect the data collection process, data processing and management, and later, the refinement of a model. Most of the other differences are due to the high 5 3 2 point group symmetry of icosahedral viruses. This alters dramatically the means by which initial phases are obtained by molecular substitution, extended to higher resolution by electron density averaging and density modification, and the refinement of the structure in the light of high non-crystallographic symmetry. In this review, we attempt to lead the investigator through the various steps involved in solving the structure of a virus crystal. These steps include the purification of viruses, their crystallization, the recording of X-ray diffraction data, and its reduction to structure amplitudes. It further addresses the problems attending phase determination and ultimately the refinement of a model. Finally, we describe the unique properties of virus crystals and the factors that influence their physical and diffraction properties

The Costs of Implementing a Unilateral One-Sided Exchange Rate Target Zone

In the aftermath of the recent financial crisis, the central banks of small open economies such as the Czech National Bank and the Swiss National Bank (SNB) both implemented a unilateral one-sided exchange rate target zone vis-a-vis the euro currency to counteract deflationary pressures. Recently, the SNB abandoned its minimum exchange rate regime of CHF 1.20 per euro, arguing that after having analyzed the costs and benefits of this non-standard exchange rate policy measure, it was no longer sustainable. This paper proposes a model that allows central banks to estimate ex-ante the costs of implementing and maintaining a unilateral one-sided target zone and to monitor these costs during the period where it is enforced. The model also offers central banks a tool to identify the right timing for the discontinuation of a minimum exchange rate regime. An empirical application to the Swiss case shows the actual size of these costs and reveals that these costs would have been substantial without the abandonment of the minimum exchange rate regime, which accords with the official statements of the SNB