X-ray Absorption Spectroscopy of Iron-Tyrosinate Proteins

The iron K-edge absorption features of a series of 28 synthetic iron compounds and a total of 11 different complexes of ovotransferrin, catechol 1,2-dioxygenase, and protocatechuate 3,4-dioxygenase have been compared to determine the coordination number of the protein complexes. The intensity of the 1s to 3d preedge transition measured for the synthetic compounds varies inversely with coordination number. The normalized preedge peak areas averaged 8 units for six-coordinate compounds, 16 units for five-coordinate compounds, and 24 units for four-coordinate compounds. An extended-Hückel molecular orbital calculation showed a good correlation of the preedge peak areas with the total amount of iron 4p atomic orbitals mixed into the predominantly iron 3d molecular orbitals. This correlation is expected for a dipolar transition. The ovotransferrin complexes exhibited preedge peak areas consistent with the coordination numbers 6 and 7, while the dioxygenase complexes exhibited values that range from 8 to 13.4 units, indicating an active site that can be five- or six-coordinate.