The beamlines of ELETTRA and their application to structural biology

Protein crystallographers are nowadays regular users of synchrotron radiati on (SR) facilities for several applications. The goal of majority of users is simply to extract more accurate, higher resolution data from existing cr ystals; they use monochromatic radiation and the rotation method, in order to get a complete survey of the reciprocal space in a short time. In fact t he brilliance of SR is essential, due to the weak scattering power of the s amples, and because of their sensibility to radiation damage. Over the last few years, however, a general increase of interest for measurements at mul tiple wavelengths, which exploit the anomalous dispersion for the phase pro blem (multiwavelength anomalous diffraction - MAD), has generated the need of intense tuneable sources. For these applications, the emphasis is on acc urate measurements of the small differences between the intensities of Brag g reflections at various energies across the absorption edge of an element present in the sample. The macromolecular diffraction beamline at ELETTRA, which is now running routinely since spring 1995, has been designed to prov ide a high flux - highly collimated tuneable X-rays source in the spectral range between 4 and 25 keV. The radiation source is the 57-pole wiggler, wh ich delivers a very intense radiation up to 25 keV, and is shared and used simultaneously with the small angle X-ray scattering (SAXS) beamline. The f ront-end filter system has a cut-off energy at about 4 keV. The beamline op tics consists of a pseudo-channel-cut double-crystal monochromator followed by a double focusing toroidal mirror. The tunability and the stability of the monochromator allows the user to perform MAD experiments, and for this purpose, a fluorescence probe for the exact calibration of the absorption e dge is available on-line. The experimental station is based on an imaging p late area detector from MarResearch, with a sensible area of 345 mm in diam eter. A cooled N-2-stream is available to cool the sample crystal in order to reduce the radiation damage. SAXS is an experimental technique used to d erive structural information about supra-molecular assemblies, amorphous ma terials and partly ordered systems (e.g. size and shape of large molecules) . The high-flux SAXS beamline at ELETTRA is mainly intended for time-resolv ed studies on fast structural transitions in the sub-millisecond time regio n in solutions and in partly ordered systems, triggered by external or proc ess parameters, with a SAXS resolution between 10 and 1400 Angstrom in real space. The source is the already mentioned 57-pole and the SAXS beamline a ccepts three discrete energies of its spectrum, namely 5.4, 8 and 16 keV. T he beamline optics consists of a flat double-crystal monochromator and a do uble focusing toroidal mirror. A multi-purpose sample stage, movable along an optical table in order to optimise the sample to detect distance, allows to perform fast time-resolved relaxation studies based on temperature- or pressure-jumps as well as stopped flow experiments. Moreover, the users hav e option to install their own specialised sample surrounding equipment. The optimisation of the beamline with respect to high-flux and consequently hi gh-flux density, allows to perform the following experiments: low contrast solution scattering, grazing incidence surface diffraction, micro-spot scan ning, X-ray fluorescence analysis, time-resolved studies greater than or eq ual to 11 mu s, simultaneous small- and wide-angle measurements on gels, li quid crystals, biopolymers, amorphous materials, muscles.