Structural Character and Energetics of Tyrosyl Radical Formation by Electron/Proton Transfers of a Covalently Linked Histidine-Tyrosine:A Model for Cytochrome c Oxidase

摘要

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.