The Use of X-Ray Absorption Spectroscopy for Detection of Metal-Metal Interactions. Application to Copper-Containing Enzymes

Abstract

The use of the extended x-ray absorption fine structure (EXAFS) technique for the identification of metal-metal distances in biomacromolecules is discussed. The Cu EXAFS data for a number of structurally characterized copper-containing bi-nuclear complexes are analyzed to determine the viability of detecting a Cu[sbnd]M (M=Cu, Fe) scattering interaction at ∼3 Å in the presence of Cu[sbnd]C interactions at approximately the same distance (deriving from the outer-shell atoms of heterocyclic ligands). The techniques developed are then applied to the oxidized and reduced forms of the copper-containing enzyme dopamine β-hydroxylase. Although in principle the EXAFS technique has the ability to distinguish C from M scatterers, based on differences in the backscattering amplitude and phase, in the absence of a priori knowledge of the distribution of C atoms about the Cu site, often no unambiguous determination of the presence or absence of a Cu[sbnd]M interaction can be made. It is suggested that caution be used in attempting to assign Cu[sbnd]M distances based on the analysis of outer-shell Fourier-transform peaks.

Key Words:

• EXAFS
• metalloenzymes
• binuclear sites
• metal-metal distance
• active site structure