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 ]Cl 2 ( 1 ), [Ru(phen) 3 ]Cl 2 ( 2 ), [Ru(bpy)(phen)(bpg)]Cl 2 ( 3 ), and [Ru(dpq) 2 (bxbg)]Cl 2 ( 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 × 10 6 to 6.8 × 10 9  M − 1  s − 1 , depending on pH and the nature of solutes. At pH > pK a of 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  a ), 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 ∆ G 0 are 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) 3 2 + 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.