Experimental and Computational Studies of the Macrocyclic Effect of an Auxiliary Ligand on Electron and Proton Transfers Within Ternary Copper(II)-Histidine Complexes

摘要

The dissociation of [Cu-II(L)His](center dot 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] (center dot 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(center dot+) from prototypical [Cu-II(L) His](center dot 2+) systems. DFT revealed that the relative energy barriers of the same electron-transfer (ET) dissociation pathways of [Cu(II()9-aneN(3))His](center dot 2+) and [Cu-II(dien)His](center dot 2+) are very similar, with the ET reactions of [Cu-II(9-aneN(3))His](center dot 2+) leading to the generation of two distinct His(center dot+) species; in contrast, the proton transfer (PT) dissociation pathways of [Cu(II()9-aneN(3))His](center dot 2+) and [CuII(dien)His](center dot 2+) differ considerably. The PT reactions of [Cu-II(9-aneN(3))His](center dot 2+) are associated with substantially higher barriers (>13 kcal/mol) than those of [CuII(dien)His](center dot 2+) . Thus, the sterically encumbered auxiliary 9-aneN, ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto nonobservable histidine radical cations. (J Am Soc Mass Spectrom 2009, 20, 972-984) .