Introductory overview: X-ray absorption spectroscopy and structural genomics Isabella Ascone, a * Roger Fourme b and S. Samar Hasnain c a LURE, Ba Ãtiment 209E, Universite  Paris-Sud, 91898 Orsay CEDEX, France, b Synchrotron SOLEIL, Ba Ãtiment 209H, Universite  Paris-Sud, 91898 Orsay CEDEX, France, and c Daresbury Laboratory, Warrington WA4 4AD, UK. E-mail: isabella.ascone@lure.u-psud.fr A special issue of the journal is presented, dedicated to biological applications of X-ray absorption spectroscopy BioXAS) and examining the role of this technique in post-genomic biology. The issuecon®rmsthatBioXAShascomeofageanditcanbeexpectedto make a signi®cant contribution in the structural genomics effort on metalloproteins, which are estimated to make up about 30% of proteins coded by genomes. Keywords: EXAFS; XANES; BioXAS; metalloproteins; structural genomics. 1. Current status of BioXAS The biological applications of X-ray absorption spectroscopy BioXAS) remain an active area of research at many synchrotrons and has been extensively reviewed see, for example, Hasnain & Hodgson, 1999). This special issue on `BioXAS and Structural Genomics' follows the study weekend meeting on the `Contribution of BioXAS to structural genomics: developments in theory and re®nement methods' that was held at LURE http://www.lure. u-psud.fr/Congres/BioXAS-SWE/) on 30 June±1 July 2001. The conceptofthestudyweekendoriginatedfromtheEuropeanBioXAS community discussion at the second European BioXAS meeting held atOrsayinJuly2000.Thediscussionhighlightedtheneedforaforum where collaborative efforts could be promoted so that the technique can be advanced and accessed by non-experts from the wider biological community. The focus on developments in theory and re®nement methods is one of the clear examples where collaborative efforts are essential in much the same way as has been achieved by the Crystallographic Computing Project Number 4 CCP4) in protein crystallography PX). This issue is likely to prove a resource for beginners as well as experts as it provides for the ®rst time a `comprehensive' docu- mentationofthecurrentstatusoftheBioXAS®eld.Itisdividedinto three sections. The ®rst section consists of contributions on the Paris-Sud initiative, a canonical structural genomics programme Quevillon-Cheruel et al., 2003), the complementary of XAS and X-ray diffraction, experimental requirements of BioXAS Ascone et al., 2003), and CCP4 Winn, 2003), an initiative which may be of interest for XAS developments. The second section is devoted to theoretical aspects of the XAS technique. The ®nal section contains articles on applications of BioXAS to macromolecules and low- molecular-mass complexes for pharmaceutical applications; as such it presents a showcase for the wider biological community. For present purposes, and throughout this issue, a protein-bound) metal site consists of one or more metal ions and all protein side chains and exogenous bridging and terminal ligands that de®ne the metal ion environment. Such sites can be classi®ed into ®ve main basic types with the following functions: structural, storage, electron transfer, anion transport binding and catalysis Holm et al., 1996). `Biological' metals which occur in the ®ve types of sites above include magne- sium, calcium, all members of the ®rst transition series excluding scandium, titanium and chromium) and molybdenum, tungsten, cadmium and mercury. These metals and their ligands constitute prosthetic groups that are usually covalently bound by endogenous ligands provided by amino acid side chains. Protein structure and environment modulate properties such as electronic structure, redox potential, detailed stereochemistry and ligand chemistry in order to achieveaspeci®cfunction.About30%ofproteinscodedbygenomes are metalloproteins. An estimated 30±50% of all enzymes carry a protein-bound metal centre located mostly at the catalytic site. Approximately one-third of all proteins and enzymes puri®ed to apparent homogeneity require metal ions as co-factors for biological function. XAS can provide information on the electronic structure of the metal atom oxidation state, orbital occupancies) for which protein crystallography, even at atomic resolution, is unable to shed light directly. The protein sample can be a crystal, a powder, a slurry or a solution; insoluble proteins e.g. membrane proteins) are not excluded. This ¯exibility is of particular signi®cance in structure± functionstudiesofbiologicalsystemsasmanyofthefunctionalstates can be probed without being constrained by the requirement to obtain a diffraction-quality crystal of the protein in different states. Asalocalmethodfocusedonasmallportionoftheproteinstructure, a limitation of the technique in some sense, the strict chemical and structural homogeneity of the sample required by crystallography maybesomewhatrelaxed.Thus,inastructuralgenomicsprogramme, a signi®cant proportion of the proteins which may not yield to homogenouspreparationscanbeinvestigatedbyBioXAS.Ofcourse, the metalloproteins which, despite their homogeneity and solubility, may not yield to crystallization can again be probed. The experimental requirements for BioXAS are among the most stringent at the synchrotron radiation facilities. This extreme demand arisesfromthefactthattheratiobetweenthenumberofinvestigated metalatomsandotheratomsatomsoftheproteinandthesolvent)is extremely low, in the range 10 4 ±10 5 . This has catalysed the development of sophisticated instrumentation from highly stable high-resolution monochromators to compact energy-resolving ¯uor- escencedetectors.TheBioXASdataareobtainedinthe¯uorescence mode as a rule rather than in the absorption mode as for many materials science samples. The desirable signal-to-noise ratio in ¯uorescencespectraisoftheorderof10 2 forXANESand10 3 ±10 4 for EXAFS. The high dilution of the absorbing species has deleterious effects on the signal-to-noise ratio, and radiation damage has to be carefully taken into account Ascone et al. , 2003). Data analysis is complicated by both complexity and disorder of these structures as well as the high demand placed by the biological question at hand which often necessitates quanti®cation of subtle structural changes upon substrate/ligand binding or redox change. Hence, the method is permanently pushed to its limits, catalysing advances in instru- mentation as well as in analysis and re®nement methods. Thus, for example, the application of XAS to proteins has driven the devel- opment of multi-element energy-dispersive detection systems based on Ge/Si detector Cramer et al., 1988). The importance of multiple- scattering pathways in biological systems was realised early on Perutz et al., 1982; Strange et al. , 1987). Following theoretical advances, modern programs include full multiple-scattering calcula- tions. The current state is reported in theoretical papers of this issue Natoli et al., 2003; Rehr & Ankudinov, 2003; Di Cicco, 2003; Benfatto et al. , 2003; Joly, 2003). The theoretical developments have J. Synchrotron Rad. 2003). 10, 1±3 # 2003 International Union of Crystallography  Printed in Great Britain ± all rights reserved 1 research papers