title: A quantitative XANES analysis of the calcium high-affinity binding site of the purple membrane.
abstract: In this article we report x-ray absorption measurements of Ca(2+)-substituted bacteriorhodopsin. We present a detailed study of the absorption spectrum close to the absorption edge that is very sensitive to the site geometry. We combined ab initio calculations of the x-ray absorption cross section based on a full multiple scattering approach, with a best fit of the experimental data performed by changing the cluster geometry. The Ca(2+)-bacteriorhodopsin environment is composed of six oxygen atoms showing a distorted orthorhombic symmetry, whereas the Ca(2+) in water solution has a regular octahydrated first sphere of coordination. Our results are in good agreement with previous molecular models suggesting that the high-affinity cationic site could be in the proximity of the retinal pocket. Our results provide strong direct evidence of the specific binding site of the metal cation in bacteriorhodopsin.
authors.affiliation: Departament d'Enginyeria Agroalimentària i Biotecnologia, Escola Superior d'Agricultura, Universitat Politècnica de Catalunya, Barcelona, Spain. francesc.sepulcre@upc.es
authors.forename: Francesc
authors.initials: F
authors.lastname: Sepulcre
authors.forename: M Grazia
authors.initials: MG
authors.lastname: Proietti
authors.forename: Maurizio
authors.initials: M
authors.lastname: Benfatto
authors.forename: Stefano
authors.initials: S
authors.lastname: Della Longa
authors.forename: Joaquin
authors.initials: J
authors.lastname: García
authors.forename: Esteve
authors.initials: E
authors.lastname: Padrós
mesh_terms: D015502:Absorptiometry, Photon; D001436:Bacteriorhodopsins / Q000737:chemistry*; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D019617:Halobacterium salinarum / Q000737:chemistry*; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D018717:Purple Membrane / Q000737:chemistry*
chemical_list: D001436:Bacteriorhodopsins; D010100:Oxygen; D002118:Calcium