Extended X‐ray absorption fine structure (EXAFS) investigations of model compounds for zinc enzymes

Abstract

A test of the ability of extended X‐ray absorption fine structure (EXAFS) to determine structural information with specific reference to zinc sites in enzymes has been made. X‐ray absorption spectra of 18 compounds of zinc have been measured and the nearest‐neighbour scattering has been interpreted using a Fourier transform and an ab initio technique. Empirical Zn‐N, Zn‐O, Zn‐S and Zn‐Cl amplitude and phase functions have been extracted from Zn(C 3 H 4 N 2 ) 4 (ClO 4 ) 2 , ZnO, Zn(S 2 COC 2 H 5 ) 2 and [N(CH 3 ) 4 ] 2 [ZnCl 4 ], respectively and tabulated as a function of the wavevector with respect to 9660.0 eV X‐ray energy. These amplitude and phase functions were then tested with respect to the other 14 compounds. For a single species of atoms in the first coordination shell the interatomic distances can be established to ± 0.5 pm (± 5 × 10 −3 Å) whilst when mixed shells exist errors in distances are ±4 pm (± 40 × 10 −3 Å). Coordination numbers are given to ±16% for the single species case and ±25% for the mixed coordination case. Using the theoretical amplitude and phase functions of McKale et al. [(1988) J. Am. Chem. Soc. 110 , 3763–3768] the deduced distances are systematically too small by an average of 0.6 pm (6 × 10 −3 Å). The errors in the coordination numbers are 18%.