A molecular orbital treatment of Cu(II) proteins

A qualitative molecular orbital treatment for the binding of Cu(II) to deprotonated peptide backbone nitrogens is offered. A square planarD_4h local symmetry is assumed. Thed⁹ electron of Cu(II) is promoted to the 4p₂ (a_2u) level. The energy for this promotion is regained by interaction of the promoted copper electron with the ligand {A figure is presented} orbital; thea_2u molecular orbital is stabilized sufficiently that it is doubly occupied and the unpaired electron remains a3d. (a) Copper is stop the Irving-Williams series because thea_2u molecular orbital creates additiona back-bonding capacity. (b) The enhanced visible absorbance shown by nonblue copper proteins is attributed to an alloweda_2u to a_1a transition. (c) The intense circular dichroism bands seen in copper proteins at 251, 305, and 345 nm are charge-transfer bads between copper and the peptide ligands. (d) Cu(I) proteins may also exhibit the 251 and 305 nm charge-transfer bands. (e) The distinction between blue and nonblue copper proteins may depend on whether the a_2u molecular orbital is sufficiently stabilized to be doubly occupied. It will be singly occupied in the blue proteins. (f) A double ground state is postulated for the nonblue proteins. Thed_2y and d₂z orbitals will be singly occupied simultaneously. (g) These various aspects are taken as presumptive evidence for the dominant role of deprotonated peptide nitrogens in protein copper binding.