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

The reactions of (OH)-O-., O.- and SO4.- with mono- (MX), di- (DMX) and tri - (TMX) methylxanthines were investigated by pulse radiolysis. The second o rder rate constants for the (OH)-O-. reaction range from 8.5 x 10(9) dm(3) mol(-1) s(-1) for 1,3,7-TMX to 2.4 x 10(9) dm(3) mol(-1) s(-1) for 1-MX, th e order being 1,3,7-TMX > 1,3-DMX approximate to 3,7-DMX approximate to 7-M X > 3-MX > 1-MX. The second order rate constants (k similar to 3.2-4.5 x 10 (9) dm(3) mol(-1) s(-1)) for the SO4.- reaction are comparable to those obt ained in the (OH)-O-. reaction which are considerably reduced in the case o f O.- reaction (k = 0.33-1 x 10(9) dm(3) mol(-1) s(-1)). The transient abso rption spectra of the OH adducts of methylxanthines (X-4OH(.) and X-80H(.)) 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 SO 4.- reactions. The rates of decay of absorption around 500 nm due to the de hydration of the X-40H(.) adducts are in the range 1.5-4 x 10(4) s(-1). In contrast to the behaviour observed with 1,3,7-TMX, the X-80H . adducts of m ono- and dimethylxanthines undergo ring opening with k = (4-11) x 10(4) s(- 1). The addition of O-2 (N2O:O-2 (4: 1 v/v)) to the X=4OH(.) adduct of meth ylxanthines (k = 1 x 10(9) dm(3) mol(-1) s(-1)) is much more effective than its dehydration. While O-2 addition predominates over the ring opening in 1-MX, 3-MX and 1,3-DMX, the X-80H(.) adducts of 7-MX and 3,7-DMX were found to he relatively unreactive.