title: Kinetics of photoinduced electron transfer reactions of ruthenium(II) complexes and phenols, tyrosine, N-acetyl-tyrosine and tryptophan in aqueous solutions measured with modulated fluorescence spectroscopy.
abstract: Photooxidation kinetics of phenol, 1-naphthol, 2-naphthol, tyrosine (TyrOH) and N-acetyl-tyrosine (AcTyrOH), tryptophan (TrpH) by ruthenium(II) polypyridyl complexes: [Ru(bpy)3]Cl2(1), [Ru(phen)3]Cl2(2), [Ru(bpy)(phen)(bpg)]Cl2(3), and [Ru(dpq)2(bxbg)]Cl2(4) where bpy is 2,2'-bipyridine, phen - 1,10-phenanthroline, bpg - bipyridine-glycoluril, dpq - dipyrido[3,2-d:2',3'-f]quinoxaline, and bxbg - bis(o-xylene)bipyridine-glycoluril are investigated. Rate constants have been measured by steady-state luminescence and phase-modulation fluorometry in aqueous solutions at different pH's. The rates for the oxidation of the phenols and phenolic aromatic amino acids spreads over a wide range from 4.2×106to 6.8×109M-1s-1, depending on pH and the nature of solutes. At pH>pKaof the quenchers, the presence of reactive species (PhO-) in the alkaline solutions is accounted for the rapid ET rates. In the pH range between 4 and 10 (pH<pKa), the ETPT mechanism becomes dominate and the rate constants are relatively low. It reveals that the important parameters that influence the quenching reaction rates, others than the driving forces ∆G0are the steric and hydrophobic interactions arising from the structure of the compounds. This is clearly seen in the case of photoreaction between the Ru(phen)32+complex and AcTyrOH. Phen ligands and acetyl group cause a steric effect, but strengthen the hydrophobic interactions and thus promote the quenching process. The pH-dependent equation of the observed rate constant for PhOH/AcTyrOH oxidation is expressed as a sum of rates for its protonated, neutral and deprotonated forms.
authors.affiliation: Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria; School of Chemical Engineering, Hanoi University of Technology, Dai Co Viet 1, Hanoi, Vietnam. Electronic address: truong.nguyenxuan@tugraz.at.
authors.forename: Truong X
authors.initials: TX
authors.lastname: Nguyen
authors.affiliation: Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria.
authors.forename: Stephan
authors.initials: S
authors.lastname: Landgraf
authors.affiliation: Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria. Electronic address: grampp@tugraz.at.
authors.forename: Günter
authors.initials: G
authors.lastname: Grampp
mesh_terms: D004579:Electron Transport; D007700:Kinetics; D010636:Phenols / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000737:chemistry*; D012996:Solutions; D013050:Spectrometry, Fluorescence / Q000379:methods*; D014364:Tryptophan / Q000737:chemistry*; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D010636:Phenols; D017975:Ruthenium Compounds; D012996:Solutions; D014867:Water; D014443:Tyrosine; D014364:Tryptophan; C025787:N-acetyltyrosine
keywords: Hydrophobic interaction; N-acetyl-tyrosine; Phase-modulated fluorescence; Phenols; Photoinduced electron transfer; Ruthenium(II) polypyridine complexes; Steric; Tryptophan; Tyrosine