title: Structural character and energetics of tyrosyl radical formation by electron/proton transfers of a covalently linked histidine-tyrosine: a model for cytochrome C oxidase.
abstract: The structural, energetic, and electronic and IR spectroscopic properties for a model of the cross-linked histidine-tyrosine (His-Tyr) residues as found in cytochrome c oxidase (CcO) are investigated by ab initio methods. The formation of a His-Tyr radical is studied by two paths: proton release followed by electron release and vice versa. The energetics for the proton/electron releases of the Tyr depend modestly on the cross-linked His substituent and, more sensitively, on the charge of the cation attached to the imino N site of the His residue. Protonation of the imino N site significantly increases the electron ionization potential and decreases the proton dissociation energy, making them competitive processes. A positive charge placed at the imino N site, whose value is scanned from zero to one, shows a continuous increase in ionization potential and a decrease in proton dissociation energy, with the +1 limit agreeing well with the protonated imino N site result, indicating a dominant electrostatic effect. The charge populations and the spin density distributions of the His-Tyr model, the radical cation formed by electron ionization, the anion formed by proton dissociation, and the final His-Tyr radical depend sensitively on the substituents, implying a modulation role on the charge transfer between the phenol and imidazole rings, especially for the charged species. His-Tyr and protonated His-Tyr exhibit differences among their respective structural isomers with consequences on their IR absorptions. Small barriers between their pseudo-cis and pseudo-trans rotamers demonstrate the relative flexibility between the two rings, and these may facilitate proton release and charge transfer. The cation effect demonstrates that the cationized cross-linked His-Tyr should be the best candidate to mimic the covalently ring-linked histidine-tyrosine structure in CcO.
authors.affiliation: Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, USA.
authors.forename: Yuxiang
authors.initials: Y
authors.lastname: Bu
authors.forename: R I
authors.initials: RI
authors.lastname: Cukier
mesh_terms: D003576:Electron Transport Complex IV / Q000737:chemistry*; D004583:Electrons; D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D006639:Histidine / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D011522:Protons; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D005609:Free Radicals; D011522:Protons; D014443:Tyrosine; D006639:Histidine; D003576:Electron Transport Complex IV