X-ray Absorption Edge and EXAFS Studies of the Blue Copper Site in Stellacyanin:  Effects of Axial Amide Coordination

The blue copper sites in Cucumis sativus stellacyanin and Rhus vernicifera stellacyanin have been investigated by X-ray absorption spectroscopy (XAS) in order to probe the effects of axial amide coordination. EXAFS results show a slight lengthening of the Cu−S(Cys) bond in both stellacyanins compared to the classic blue copper site in azurin. The 1s → 3d transition in the Cu K-edge of stellacyanin is ∼0.5 eV lower in energy than those of the classic blue copper sites, consistent with a stronger ligand donor set. A configuration interaction (CI) model applied to the 1s → 4p and 1s → 4p + shakedown transitions in the edge predicts less covalent bonding for stellacyanin than for plastocyanin, which is attributed to a weaker thiolate bond. These studies demonstrate that the strength of the axial ligand interaction inversely affects the strength of the thiolate−copper bond.