Ms. Vinchurkar et al., Kinetics, spectral and redox behaviour of OH adducts of methylxanthines: aradiation chemical study, J CHEM S P2, (3), 1999, pp. 609-617
Citations number
48
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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.