Excited States in Photodimerization of Aqueous Tyrosine at Room Temperature

Abstract

Abstract— The photoionization of tyrosine in aqueous solution in the liquid state was studied at room temperature by analyzing the kinetics of formation of bityrosine upon irradiation of tyrosine, and the external heavy‐atom effect on the formation of bityrosine. The relative number of bityrosine molecules was determined by measuring the fluorescence intensity at 400 nm. A kinetic model for the formation of bityrosine was formulated on the assumption that the rate constants were first‐order for production of radicals and for recombination with ejected electrons. The applicability of this model to the present case was confirmed by the experimental data. On the basis of the model, we found that the electronic process of photoionization of tyrosine at room temperature is different in acidic and alkaline media. In acidic media a tyrosine molecule absorbs one light quantum and photoionizes through a singlet excited state, while in alkaline media a tyrosinate ion photoionizes after absorption of two light quanta. The intermediate product that absorbs the second photon is in a triplet excited state.

Thermal and Photochemical Effects on the Fluorescence Properties of Type I Calf Skin Collagen Solutions at Physiological pH

“…The value of r 2 /r 1 = 2.31 ± 0.40. Covalent dityrosine (325/400 nm fluorescence) is formed from a metastable steady-state tyrosyl radical, where the tyrosyl steady state concentration is proportional to the amount of tyrosine [9]. In such cases, plots of I f (t) vs. t are quasi-linear, [9]; (Figure 3(b)), with r 2 /r 1 = 0.246 ± 0.097 (see Table 1 for results).…”

“…Both r 1 and r 2 are proportional to fluorophore concentration [9] [11]. Changes in the other fluorescence pairs were kinetically complicated alterations and were not amenable to simple analysis (see [12] for results with Skh-1 hairless mouse collagen).…”

“…The build-up of 325/400 nm fluorescence attending photolysis (dityrosine formation) was plotted as I(t) vs. t to give a quasi-linear plot [9] whose slope affords a first order rate parameter, proportional to a molecular rate constant for build-up of the dityrosine fluorophore at 325/400 nm, is:…”

“…These compounds are photolabile, the predominant photo-product being photo-dimerization to dityrosine [9] [10]. Photo-induced dityrosine formation and fluorescence disappearance ("fading") of DOPA oxidation products ("oxidation products") attending exposure to UV radiation have been used as convenient markers of the extent of photolysis [11] [12].…”

The effect of neighboring amino acid residues and solution environment on the oxidative stability of tyrosine in small peptides

“…Shimizu 31 reported that at pH 3.5 with 283 nm excitation, the rate of reaction was directly proportional to light intensity. The experiments in the present study were conducted with steady-state photolytic exposure with a broad spectrum of sunlight.…”