Generation of superoxide and singlet oxygen from alpha-tocopherolquinone and analogues

Abstract

Three potential routes to generation of reactive oxygen species (ROS) from alpha-tocopherolquinone (alpha-TQ) have been identified. The quinone of the water-soluble vitamin E analogue Trolox C (Trol-Q) is reduced by hydrated electron and isopropanol alpha-hydroxyalkyl radical, and the resulting semiquinone reacts with molecular oxygen to form superoxide with a second order rate constant of 1.3 x 10(8) dm(3)/mol/s, illustrating the potential for redox cycling. Illumination (UV-A, 355 nm) of the quinone of 2,2,5,7,8-pentamethyl-6-hydroxychromanol (PMHC-Q) leads to a reactive short-lived (ca. 10(- 6) s) triplet state, able to oxidise tryptophan with a second order rate constant greater than 10(9) dm(3)/mol/s. The triplet states of these quinones sensitize singlet oxygen formation with quantum yields of about 0.8. Such potentially damaging reactions of alpha-TQ may in part account for the recent findings that high levels of dietary vitamin E supplementation lack any beneficial effect and may lead to slightly enhanced levels of overall mortality.

Scheme 1

Figure 1

Transient absorption spectra, as product of radiation chemical yield (G-value) and extinction coeeficient, from pulse radiolysis of N₂-saturated aqueous solutions containing Trol-Q (200 μmol dm⁻³), 2-methylpropanol (0.1 mol dm⁻³) and phosphate buffer (10 mmol dm⁻³) at pH 9.2 (□) and pH 1.8 (○) (left hand scale). Also shown is the transient spectrum from N₂O-saturated solutions of Trol-Q (200 μmol dm⁻³) and propan-2-ol (20% v/v) in phosphate buffer (10 mmol dm⁻³) at pH 7.3 (■) (right hand scale). Inset:- First order rate constant for decay of the hydrated electron at 700 nm versus Trol-Q concentration in N₂-saturated solutions containing 2-methylpropanol (0.1 mol dm⁻³) and phosphate buffer (10 mmol dm⁻³, pH 7.3).

Figure 2

Effect of pH on the transient absorbance at 445 nm from pulse radiolysis of N₂-saturated aqueous solutions containing Trol-Q (200 μmol dm⁻³), 2-methylpropanol (0.1 mol dm⁻³) and phosphate buffer (10 mmol dm⁻³). Inset:- First order rate constant for decay of the transient absorbance of the semiquinone radical at 440 nm versus oxygen concentration in solutions containing 2-methylpropanol (0.1 mol dm⁻³) and phosphate buffer (10 mmol dm⁻³, pH 7.3).

Figure 3

Laser flash photolysis (355 nm, 10 ns) of PMHC-Q (2 mmol dm⁻³) in deaerated methanol. Transient spectra were measured 150 ns (■), 1.5 μs (○) and 9 μs (▲) after the laser flash. Inset: Absorption transients recorded at (a) 450 nm, and (b) 520 nm.

Figure 4

Laser flash photolysis of PMHC-Q (2 mmol dm⁻³) in deaerated n-hexane. Spectra are shown 100 ns (◆), 300 ns (□), 500 ns (▲) and 1000 ns (○) after the laser flash. Inset: decay of the transient at 460 nm with the solid line indicating the fitted exponential decay with τ = 0.41 μs.

Figure 5

Intensities of 1270 nm singlet oxygen luminescence versus fraction of light absorbed (1-10^−A) in solutions of perinaphthenone (■) and PMHC-Q (○) in oxygen-saturated acetonitrile. Inset: First order rates of decay of triplet PMHC-Q versus oxygen concentration in methanol (■) and ethanol/water (1:1 v/v buffered to pH 7 with 10 mmol dm⁻³ phosphate) (□).

Figure 6

Effect of oxygen concentration on the singlet oxygen yield from PMHC-Q (■) and perinaphthenone (□) in methanol, plotted according to equation (7).

Figure 7

Transient spectrum measured 10 μs after laser flash photolysis of a deaerated solution of PMHC-Q (200 μmol dm⁻³) and tryptophan (2 mmol dm⁻³) in ethanol-water (1:1 v/v buffered to pH 7 with 10 mmol dm⁻³ phosphate). Inset: Effect of tryptophan concentration on the first order rate constant for decay of triplet states of DQ (□) and PMHC-Q (◆) in the same solvent.