The set of elements which strongly interact with proteins, acting as cofactors or as part of substrate binding/utilisation sites, is relatively small. It includes Mg, Al, Ca, all members of first transition series (excluding Sc, Ti), Br, Se, Mo, W, Cd and Hg. Nevertheless, the number of proteins interacting with a metal is high, about 25% to 30% of the proteins for a typical genome. BioXAS (X-ray absorption spectroscopy applied to biological systems) can play an important role in structural genomics programmes especially targeted on metal binding proteins, as it gives information on the electronic structure of the site being studied and on the neighbouring local structure.

 The first study weekend (held in Orsay, on June 30-July 1, 2001) was dedicated to “Contribution of BioXAS to structural genomics: developments in theory & refinement methods”. This meeting, which was the basis of a special issue of the Journal of Synchrotron Radiation (2003, 10, part 1), confirmed that BioXAS has come of age and can be expected to make significant contribution in the structural genomics effort. Allowing exchanges between theoreticians, biophysicists and structural biologists, the previous study weekend was the starting point of a concerted effort on BioXAS. Through an overview on program developments and scientific results, it contributed to define several lines for theoretical developments relevant to the field.

 In the present meeting, there will be a greater focus on biology. In particular, session 1 will concentrate on specific aspects of metalloproteins in structural genomics programmes (identification of genes coding for metalloproteins, expression, metal incorporation, …). Obviously, we shall continue discussions initiated during the previous study weekend on theory and programs relevant for BioXAS. New topics will be introduced, such as dedicated instrumentation and the recording and treatment of weak signals from highly dilute systems, involving considerations from both theoretical and instrumental aspects in sessions 2 and 4 respectively.

 These topics, targeted on biological aspects of XAS, show the specificity of study weekends with respect to XAFS 12 conference in Malmö that will cover, a few days before, the broad field of X-ray absorption.

 One outcome of the first study weekend was the strengthening of collaborations and initiatives aiming at developing the BioXAS user community. A synchrotron radiation facility from the USA, SSRL-Stanford, joined the initiative and is represented in the steering committee of the second BioXAS study weekend.