Absorption spectra of isomeric OH adducts of 1,3,7-trimethylxanthine

Abstract

The reactions of OH^•, O^•-, and SO₄^•-with 1,3,7-trimethylxanthine (caffeine) were studied by pulse radiolysis with optical and conductance detection techniques. The absorption spectra of transients formed in OH^• reaction in neutral solutions exhibited peaks at 310 and 335 nm, as well as a broad absorption maximum at 500 nm, which decayed by first-order kinetics. The rate (k = (4.0 ± 0.5) × 10⁴ s^-1) of this decay is independent of pH in the range 4-9 and is in agreement with that determined from the conductance detection (k = 4 × 10⁴ s^-1). The spectrum in acidic solutions has only a broad peak around 330 nm with no absorption in the higher wavelength region. The intermediates formed in reaction of O^•- absorb around 310 and at 500 nm, and the first-order decay at the latter wavelength was not seen. The OH radical adds to C-4 (X-4OH^•) and C-8 (X-8OH^•) positions of caffeine in the ratio 1:2 as determined from the redox titration and conductivity measurements. H abstraction from the methyl group is an additional reaction channel in O^•- reaction. Dehydroxylation of the X-4OH^• adduct occurs, whereas the X-8OH^• adduct does not undergo ring opening. The rate constant for addition of O₂ to X-4OH^• is estimated to be 1 × 10⁹ M^-1 s^-1, whereas it is unreactive toward X-8OH^•. The spectrum obtained for OH^• reaction in oxygenated solutions is similar to that observed in SO₄^•- reaction in basic solutions. The radical cation of caffeine formed from its reaction with SO₄^•- ( λ _max = 320-340 nm) is hydrolyzed in basic solutions to yield the X-8OH^• adduct. The molar absorptivities of the X-8OH^• and the X-4OH^• adducts at 300 and 335 nm are 6500 and 5300 M^-1 cm^-1, respectively. The yield of 1,3,7-trimethyluric acid in OH^• reaction in the absence of O₂ (28%) is reduced by more than 50% in its presence. The differences in the mechanism of OH^• reaction with caffeine and its isomer 1,3,9-trimethylxanthine (isocaffeine) are discussed.