Probing the Coordination and Geometry of Heme-iron Ligands in Hemoproteins by XANES (X-ray Absorption Near Edge Structure)

X-ray Absorption Near Edge Structure (XANES) spectroscopy, is an emerging technique related to EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy. XANES probes the electronic state and the atomic structure of a metal site in biological macromolecules at any physical state of the sample and any environmental condition. For instance, the pH-dependent and temperature-dependent XANES spectra of ferric aquomet-myoglobin in solution have been studied to probe the main Fe-heme conformational changes following its acid-alkaline chemical transition, and the temperature-dependent spin transition of the alkaline form. In these last years, multiple scattering (MS) theory has been shown able to reproduce satisfactorily the experimental XANES data from the edge up to 150-200 eV. The ability to analyse the low-energy part of a x-ray absorption spectrum allows determining structural data that are very difficult to recover by EXAFS, such as bonding angles. Using light triggered systems allows to study trapped intermediate conformations and some aspects of the dynamics of biological complexes. XANES theory has been applied to interpret the dramatic spectroscopical changes in the XANES spectrum of a carbonmonoxy-myoglobin single crystal, following the photolysis of the Fe-CO bond at T=20K. The distance and orientation of the CO molecule from the Fe atom, and the position of the proximal histidine, have been determined at a resolution comparable with that of the more recent X-ray diffraction (XRD) studies. Preliminary applications of the quantitative XANES analysis to hemoproteins in solutions are also presented.