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

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 × 10^6 to 6.8 × 10^9 M^-1 s^−1, depending on pH and the nature of solutes. At pH > pKa 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 < 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 ∆ G0 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.