Kinetics and Mechanism of the Sensitized Photodegradation of Uracil-Modeling the Fate of Related Herbicides in Aqueous Environments†

Abstract

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Delta(g))] and, to a lesser extent, superoxide radical anion (O2*-)-both species photogenerated from triplet excited Rf, 3Rf*-participate in the photodegradation. At pH 7, UR is slowly degraded through an O2*- -mediated mechanism, whereas Rf disappears through its reaction with O2(1Delta(g)) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species-a back electron transfer that also produces O2*- -and the elimination from the medium of O2(1Delta(g)) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Delta(g)) were in the range 5 x 10(5) M(-1) s(-1) (at pH 7) to 1.3 x 10(8) M(-1) s(-1) (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Delta(g)0-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Delta(g)0-reactive quenching with UR was observed.

Photo‐Fenton and Riboflavin‐photosensitized Processes of the Isoxaflutole Herbicide

“…Besides that, it has been reported that Rf is present in natural waters and could participate in the degradation of different organic pollutants , and in this case, it is expected that the reaction of 3 Rf*, O 2 ( 1 Δ g ) or is not a significant process in the degradation of IXF since, although it is a continuous process, it would not compete with the hydrolysis rate at pH close to 7.…”

Photodegradation of antibiotic 5-sulfaminouracil in the presence of vitamin B2: A kinetic study

“…It is interesting to compare the kinetic constants for SFU at pHs 7 and 12 with those of uracil previously published. 20 The values of these constants (Table 2) suggest that the presence of the sulfamino-group as a substituent in the uracil moiety exerts an activating power on its nucleus. This is based on the increase of the constants for the global and for the reactive processes in the interaction with SFU compared to the uracil in alkaline medium.…”

“…This degradation happens in the presence of either dissolved organic matter or other reactive species that can be formed in natural surface waters. [17][18][19] Regarding the uracil molecule, which is part of the SFU structure, it has been reported by our group 20 that its sensitized photodegradation takes place with the involvement of reactive oxygen species (ROS) such as singlet molecular oxygen [O 2 ( 1 Δ g )] and superoxide radical anion (O 2…”

Oxygen Uptake in the Vitamin B2‐sensitized Photo‐oxidation of Tyrosine and Tryptophan in the Presence of Uracil: Kinetics and Mechanism

“…A daylight‐absorbing pigment of particular interest is the naturally occurring vitamin B 2 , riboflavin (Rf). The vitamin is endogenously present in living organisms, and has been postulated as a possible sensitizer for the in vivo photo‐oxidative degradation of proteins, puric bases and fatty acids, among other biologically relevant substrates (8–13).…”

“…Studies with puric bases showed that Rf‐photosensitized processes could be responsible for photodamaging of DNA (14). Recently we reported kinetic studies on the Rf‐sensitized photo‐oxidation of ur, uric acid, xanthine and hypoxanthine in aqueous solution, mediated by ROS (9,15). The aerobic light‐induced interaction of Rf with proteins is well known to occur through ROS‐mediated oxidation of one or several of the five photo‐oxidizable AAs: tyr, trp, methionine, histidine and cysteine (16–19).…”