Kinetics, spectral and redox behaviour of OH adducts of methylxanthines: a radiation chemical study1

Abstract

The reactions of ˙OH, O^˙– and SO₄^˙– with mono- (MX), di- (DMX) and tri- (TMX) methylxanthines were investigated by pulse radiolysis. The second order rate constants for the ˙OH reaction range from 8.5 × 10⁹ dm³ mol^–1 s^–1 for 1,3,7-TMX to 2.4 × 10⁹ dm³ mol^–1 s^–1 for 1-MX, the order being 1,3,7-TMX > 1,3-DMX ≈ 3,7-DMX ≈ 7-MX > 3-MX > 1-MX. The second order rate constants (k ≈ 3.2–4.5 × 10⁹ dm³ mol^–1 s^–1) for the SO₄^˙– reaction are comparable to those obtained in the ˙OH reaction which are considerably reduced in the case of O^˙– reaction (k = 0.33–1 × 10⁹ dm³ mol^–1 s^–1). The transient absorption spectra of the OH adducts of methylxanthines (X–4OH˙ and X–8OH˙) have exhibited maxima at 320–330 nm and a broad peak around 490 nm whereas the latter peak was not observed in the spectra obtained in the O^˙– and SO₄^˙– reactions. The rates of decay of absorption around 500 nm due to the dehydration of the X–4OH˙ adducts are in the range 1.5–4 × 10⁴ s^–1. In contrast to the behaviour observed with 1,3,7-TMX, the X–8OH˙ adducts of mono- and dimethylxanthines undergo ring opening with k = (4–11) × 10⁴ s^–1. The addition of O₂ (N₂O∶O₂ (4∶1 v/v)) to the X–4OH˙ adduct of methylxanthines (k = 1 × 10⁹ dm³ mol^–1 s^–1) is much more effective than its dehydration. While O₂ addition predominates over the ring opening in 1-MX, 3-MX and 1,3-DMX, the X–8OH˙ adducts of 7-MX and 3,7-DMX were found to be relatively unreactive.