Histidine coordination occurs in many metalloproteins, but analysis of the contributions to the EXAFS of the outer shells of atoms of the imidazole rings has, in the past, proved difficult. An exact method for simulating the raw experimental EXAFS over the complete energy range (k = 2–16 A-1) is reported and applied to the simulation of tetrakis(imidazole)copper(II) nitrate, tetrakis(imidazole)copper(II) perchlorate, and aquatris(imidazole)copper(II) sulfate. It is shown that strong multiple-scattering contributions are present in the EXAFS over an extended range above the absorption edge and these contributions are necessary to fix the third-shell atoms of the imidazole groups at their correct positions. Furthermore, by including multiple scattering in the EXAFS analysis, it is possible to extend the low-energy fitting range to include the XANES region of the spectrum below k = 3, the general shape of this part of the spectrum being well reproduced. In favorable circumstances, the multiple-scattering approach can provide the basis for determining the number of histidine ligands in a mixed-ligand complex and can clearly distinguish between two and four coordinated imidazole groups, although distinction between three and four histidines is probably unrealistic for a metalloprotein site of unknown structure.
ASJC Scopus subject areas
- Colloid and Surface Chemistry