Hybrid density functional study of ligand coordination effects on the magnetic couplings and the dioxygen binding of the models of hemocyanin

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Abstract

Ligand coordination effects on the magnetic interaction and the reversible dioxygen binding process for the models of hemocyanin have been investigated with hybrid density functional theory. Hemocyanin is an oxygen transport protein found in mollusks and arthropods. The active site of hemocyanin contains a pair of copper atoms, each of which is coordinated by three histidine residues. We utilized NH3 (a), methylimidazole (b), and histidine (c) as the ligands of the models of hemocyanin, which coordinate to the binuclear copper core in the active site of oxygenated (1z) (z = ac) and deoxygenated hemocyanin (2z). The difference of the ligand coordination in the magnetic couplings for the model of hemocyanin 1z complexed with peroxide (1z-Omath image) shows that the substitution from histidine to methylimidazole group is not serious, but that the substitution from imidazole to NH3 is influential. In the dioxygen binding process, the ligands influence the binding energy and the shapes of potential energy surfaces of the models of hemocyanin 1z and 2z with dioxygen due to the strength of the mixing between the ground state and the charge transfer one. The active dxy ± dxy orbitals of the copper core of 1b and 1c strongly interact with πmath image and πmath image orbitals of the dioxygen than those of 1a, leading to the larger binding energy of the model 1b (1c) with dioxygen. The orbital energies of the active orbitals for the synthetic model, [Cu(HB(3,5-iPr2-Pz)3)]2 (HB(3,5-iPr2-Pz)3 = hydrotris{3,5-diisopropyl-pyrazolyl}borate), imply that it irreversibly binds dioxygen because HB(3,5-iPr2-Pz)3 groups show stronger electron-release character and raise the orbital energies nearer that to π* orbitals of O2. The effective bond-order, localized electron density, and information entropy for 1z with dioxygen were calculated in terms of occupation numbers of the natural orbitals of the broken-symmetry UB2LYP solution in order to elucidate the nature of the Cu2O2 bonding. These chemical indices indicate that the copper–oxygen bonds in Hc exhibit an intermediate covalent-bonding character (half bonding), which is the origin of an active control of oxygen transport: addition and dissociation of O2 in hemocyanin by the conformational changes. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

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