Electronic Spectroscopic Studies of Ni(II)‐Substituted Blue Copper Proteins

Abstract

AbstractThe nickel(II) derivative of Rhus vernicifera stellacyanin has been prepared. The near‐infrared, visible, and near‐ultraviolet absorption spectra of the nickel(II) derivatives of stellacyanin and Pseudomonas aeruginosa azurin are reported. Charge transfer bands in the visible and near‐ultraviolet regions are assigned as follows: nickel(II)‐stellacyanin, 335 nm (29,900 cnr−1) (πN(his)→Ni(II)), 410 nm (24,400 cm−1) (σS(cys) → Ni(II)), and 470 nm (21,300 cm−1)(πS(cys)→ Ni(II)); nickel(II)‐azurin, 355 nm (28,200 cm−1) (πN(his)→Ni(II)), 440 nm (22,700 cm−1) (σS(cys)→ Ni(II)), and 500 nm (20,000 cm−1) (πS(cys)→ Ni(II)). Relatively weak bands at 550 (18,200) and 590 nm (16,900 cm−1) (Ni(II)St) and 540 (18,500) and 565 nm (17,700 cm−1) (Ni(II)Az) are attributed to d–d excitations. No evidence for lower energy d‐d bands has been found for either Ni(II)–protein. The data now in hand raise the possibility that the Ni(II) site structure differs from the flattened tetrahedral geometry exhibited by Cu(II) in these proteins.