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

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.