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 similar to0.5 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.