Acid-alkaline properties of triplet state and radical of kynurenic acid

Kynurenic acid (KNA), a degradation product of endogenous UV filter of the human lens kynurenine, is a convenient model compound for studying reactions of photoexcited chromophores with amino acids, peptides and proteins. In this work, transient absorption spectra of triplet KNA (^TKNA) and KNA radical (KNA[rad]⁻) were obtained for aqueous solutions with different pH's, and the dissociation constants for ^TKNA (pKa = 3.7) and KNA[rad]⁻ (pKa = 5.5) have been determined. The quenching of ^TKNA by oxygen, tryptophan and ascorbate proceeds with nearly diffusion rate constants, weakly dependent on the charge on ^TKNA and on quencher. The quenching by tyrosine and histidine are much slower, and the quenching rate constants depend on solution pH's. The most pronounced pH-dependence was found for ^TKNA quenching by thiols cysteine and glutathione, which can be attributed to the switch of the quenching mechanism from the electron transfer in alkaline solutions to the hydrogen atom transfer under neutral and acidic conditions.