Uric acid photo‐oxidation assay: in vitro comparison of sunscreening agents

Abstract

We present a new method to evaluate the photo-oxidative activity of sunscreening agents based on the photodynamic oxidation of uric acid. Uric acid was selected as the oxidant probe for its high reactivity to singlet oxygen and oxygen radicals, high sensitivity of detection using electrochemical (EC) techniques, low light absorptivity and high photochemical stability in the UVA/B region of interest, and stability to autoxidation. The method is demonstrated by the photodynamic oxidation of uric acid on co-irradiation with Rose Bengal, a highly efficient photosensitizing dye for the production of singlet oxygen (1O2). Using this assay we found that the relative photodynamic oxidation rates of UVB-absorbing sunscreens in 80% methanol on irradiation with >290 nm light decreased in the order 2-ethylhexyl 4-dimethylaminobenzoate (DMABA-2EH) >> 2-ethylhexyl 4-methoxycinnamate (MCA-2EH) and the experimental sunscreens, 1-(1,1-dimethylethyl)-3-octanoyl-4,4-dimethyl- 1,4,5,6,-tetrahydropyridine (ICI-319) and 1-(2-methylpropyl)-3-propionyl-4,4-dimethyl-1,4,5,6-tetrahydropyridine (ICI-855). The relative photodynamic oxidation rates of UVA-absorbing sunscreens decreased in the order 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM) and 4-(2-propyl)benzophenone (PB) > 2-hydroxy-4'-methoxy-benzophenone (HMB) and 2,2'-dihydroxy-4-methoxybenzophenone (DHMB). We have confirmed the photodynamic activity of DMABA-2EH for the production of singlet oxygen (1O2) using electron paramagnetic resonance (EPR) spectroscopy and the reagent 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP). We failed to detect the photodynamic production of the oxyradicals, superoxide (O2.-) and hydroxyl radical (HO.) using N-tert-butyl-a-phenylnitrone (PBN) and 5,5-dimethyl-1-pyrrolidine-1-oxide (DMPO) as a result of photochemical interference caused by these spin-trapping reagents. The uric acid photo-oxidation assay was also used to compare the photodynamic reactivity of light-reflective, microfine oxides TiO2, ZnO and ZrO2 suspended in aqueous 80% methanol. All of the microfine oxides (uncoated) showed greater photodynamic reactivity in equimolar dispersion than did any of the organic UVA- and UVB-absorbing sunscreens in homogeneous solution. In this assay the photodynamic oxidation rates for the microfine oxides decreased in the order ZnO >> TiO2 (anatase) > ZrO2 > TiO2 (rutile).

Study of photo-oxidative reactivity of sunscreening agents based on photo-oxidation of uric acid by kinetic Monte Carlo simulation

“…As seen in Table 1 The concentrations of UA-OX versus time curves for different mineral oxides have been obtained by simulation and results were shown in Figure 1. As seen in this Figure, there is a very good agreement between simulated and experimental data [13]. This agreement demonstrates that the proposed mechanism can be suitable to study kinetics of ROS production upon photo excitation of some sun care agents.…”

“…Dunlap and his coworkers [13] have studied uric acid photo-oxidation assay for determination of ROS production by some of sun care agents such as TiO 2 , ZnO and ZrO 2 .…”

“…To study the effect of initial concentration of sun care agents on the rate of uric acid photo- [13], the obtained steps of mechanism and rate constants of them (Table 1). ZnO on the photo-oxidation of uric acid.…”

“…To study the effect of initial concentration of uric acid on the rate of photo-oxidation assay, different initial concentrations of uric acid have been chosen. The input data for the simulations are temperature (310 K), initial concentrations of uric acid, initial concentration of mineral oxides ([MO] ○ = 5×10 -5 M) [13], the mechanism and the obtained rate constants for each step (Table 1). This treatment were also observed for TiO 2 (anatase), ZnO and ZrO 2 (Figures 3(b)-3(d)).…”

“…The input data for the simulations are temperature (310 K), initial concentrations of mineral oxide (MO) ([MO] ○ = 5×10 -5 M), initial concentrations of uric acid (UA) ([UA] ○ = 2×10 -6 )[13], the steps of mechanism and rate constant of each step.…”