A molecular orbital treatment of Cu(II) proteins

Kenneth W. Nickerson, Nelson F. Phelan

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


A qualitative molecular orbital treatment for the binding of Cu(II) to deprotonated peptide backbone nitrogens is offered. A square planarD4h local symmetry is assumed. Thed9 electron of Cu(II) is promoted to the 4p2 (a2u) level. The energy for this promotion is regained by interaction of the promoted copper electron with the ligand {A figure is presented} orbital; thea2u 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 thea2u molecular orbital creates additiona back-bonding capacity. (b) The enhanced visible absorbance shown by nonblue copper proteins is attributed to an alloweda2u to a1a 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 a2u 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. Thed2y 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.

Original languageEnglish (US)
Pages (from-to)79-91
Number of pages13
JournalBioinorganic Chemistry
Issue number1
StatePublished - 1974


  • Cu(II)-ribonuclease complex
  • Molecular orbital theory
  • allowed transitions
  • biuret reaction
  • blue copper proteins
  • charge-transfer bands
  • circular dichroism
  • hemocyanin

ASJC Scopus subject areas

  • Biochemistry


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