title: Mechanism of intramolecular electron transfer in the photoexcited Zn-substituted cytochrome c: theoretical and experimental perspective.
abstract: Photoinduced electron transfer (ET) in zinc-substituted cytochrome c (Zn-cyt c) has been utilized in many studies on the long-range ET in protein. Attempting to understand its ET mechanism in terms of electronic structure of the molecule, we have calculated an all-electron wave function for the ground-state of Zn-cyt c on the basis of density functional theory (DFT). The four molecular orbitals (MOs) responsible for excitation by UV-vis light (Gouterman's 4-orbitals) are assigned on the basis of the excited states of chromophore model for Zn-porphine complex calculated with the time-dependent DFT method. ET rates between each Gouterman's 4-orbitals and other MOs were estimated using Fermi's golden rule. It appeared that the two occupied MOs of the 4-orbitals show exclusively higher ET rate from/to particular MOs that localize on outermost amino acid residues (Lys 7 or Asn 54), respectively, whereas ET rates involving the two unoccupied MOs of the 4-orbitals are much slower. These results imply that the intramolecular ET in photoexcited Zn-cyt c is governed by the hole transfer through occupied MOs. The couplings of MOs between zinc porphyrin core and specific amino acid residues on the protein surface have been demonstrated in Zn-cyt c immobilized on an Au electrode via carboxylic acid group-terminated self-assembled monolayer. The Zn-cyt c-modified electrode showed photocurrents responsible for photoillumination. The action spectrum of the photocurrent was identical with the absorption spectrum of Zn-cyt c, indicating photoinduced electron conduction via occupied MOs. The voltage dependence of the photocurrent appeared to be linear and bidirectional like a photoconductor, which strongly supports the intramolecular ET mechanism in Zn-cyt c proposed on the basis of the theoretical calculations.
authors.affiliation: Materials Laboratories, Sony Corporation, 4-16-1 Okata, Atsugi, Kanagawa 243-0021, Japan. yuichi.tokita@jp.sony.com
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Tokita
authors.forename: Jusuke
authors.initials: J
authors.lastname: Shimura
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nakajima
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Goto
authors.forename: Yoshihito
authors.initials: Y
authors.lastname: Watanabe
mesh_terms: D000818:Animals; D045304:Cytochromes c / Q000737:chemistry* / Q000378:metabolism*; D004566:Electrodes; D004579:Electron Transport; D006046:Gold / Q000737:chemistry; D006736:Horses; D008958:Models, Molecular; D008968:Molecular Conformation; D010777:Photochemistry; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism
chemical_list: D011166:Porphyrins; C050095:zinc cytochrome c; C107018:porphine; D006046:Gold; D045304:Cytochromes c