title: Experimental and computational studies of the macrocyclic effect of an auxiliary ligand on electron and proton transfers within ternary copper(II)-histidine complexes.
abstract: The dissociation of [Cu(II)(L)His](*2+) complexes [L = diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN(3))] bears a strong resemblance to the previously reported behavior of [Cu(II)(L)GGH](*2+) complexes. We have used low-energy collision-induced dissociation experiments and density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level to study the macrocyclic effect of the auxiliary ligands on the formation of His(*+) from prototypical [Cu(II)(L)His](*2+) systems. DFT revealed that the relative energy barriers of the same electron-transfer (ET) dissociation pathways of [Cu(II)(9-aneN(3))His](*2+) and [Cu(II)(dien)His](*2+) are very similar, with the ET reactions of [Cu(II)(9-aneN(3))His](*2+) leading to the generation of two distinct His(*+) species; in contrast, the proton transfer (PT) dissociation pathways of [Cu(II)(9-aneN(3))His](*2+) and [Cu(II)(dien)His](*2+) differ considerably. The PT reactions of [Cu(II)(9-aneN(3))His](*2+) are associated with substantially higher barriers (>13 kcal/mol) than those of [Cu(II)(dien)His](*2+). Thus, the sterically encumbered auxiliary 9-aneN(3) ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto nonobservable histidine radical cations.
authors.affiliation: Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Song
authors.forename: Corey N W
authors.initials: CN
authors.lastname: Lam
authors.forename: Dominic C M
authors.initials: DC
authors.lastname: Ng
authors.forename: Galina
authors.initials: G
authors.lastname: Orlova
authors.forename: Julia
authors.initials: J
authors.lastname: Laskin
authors.forename: De-Cai
authors.initials: DC
authors.lastname: Fang
authors.forename: Ivan K
authors.initials: IK
authors.lastname: Chu
mesh_terms: D001372:Aza Compounds / Q000737:chemistry; D003198:Computer Simulation; D003300:Copper / Q000737:chemistry*; D004583:Electrons; D006639:Histidine / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry*; D010880:Piperidines / Q000737:chemistry; D011073:Polyamines / Q000737:chemistry; D011522:Protons; D013816:Thermodynamics
chemical_list: D001372:Aza Compounds; D007477:Ions; D009942:Organometallic Compounds; D010880:Piperidines; D011073:Polyamines; D011522:Protons; C482777:triazacyclononane; C005391:diethylenetriamine; D006639:Histidine; D003300:Copper