title: Hydroxyl radical induced oxidation of theophylline in water: a kinetic and mechanistic study.
abstract: Oxidative destruction and mineralization of emerging organic pollutants by hydroxyl radicals (˙OH) is a well established area of research. The possibility of generating hazardous by-products in the case of ˙OH reaction demands extensive investigations on the degradation mechanism. A combination of pulse radiolysis and steady state photolysis (H2O2/UV photolysis) followed by high resolution mass spectrometric (HRMS) analysis have been employed to explicate the kinetic and mechanistic features of the destruction of theophylline, a model pharmaceutical compound and an identified pollutant, by ˙OH in the present study. The oxidative destruction of this molecule, for intermediate product studies, was initially achieved by H2O2/UV photolysis. The transient absorption spectrum corresponding to the reaction of ˙OH with theophylline at pH 6, primarily caused by the generation of (T8-OH)˙, was characterised by an absorption band at 330 nm (k2 = (8.22 ± 0.03) × 10(9) dm(3) mol(-1) s(-1)). A significantly different spectrum (λmax: 340 nm) was observed at highly alkaline pH (10.2) due to the deprotonation of this radical (pKa∼ 10.0). Specific one electron oxidants such as sulphate radical anions (SO4˙(-)) and azide radicals (N3˙) produce the deprotonated form (T(-H)˙) of the radical cation (T˙(+)) of theophylline (pKa 3.1) with k2 values of (7.51 ± 0.04) × 10(9) dm(3) mol(-1) s(-1) and (7.61 ± 0.02) × 10(9) dm(3) mol(-1) s(-1) respectively. Conversely, oxide radicals (O˙(-)) react with theophylline via a hydrogen abstraction protocol with a rather slow k2 value of (1.95 ± 0.02) × 10(9) dm(3) mol(-1) s(-1). The transient spectral studies were complemented by the end product profile acquired by HRMS analysis. Various transformation products of theophylline induced by ˙OH were identified by this technique which include derivatives of uric acids (i, iv & v) and xanthines (ii, iii & vi). Further breakdown of the early formed product due to ˙OH attack leads to ring opened compounds (ix-xiv). The kinetic and mechanistic data furnished in the present study serve as a basic frame work for the construction of ˙OH induced water treatment systems as well as to understand the biological implications of compounds of this kind.
authors: School of Environmental Sciences, Mahatma Gandhi University, Kottayam, India. cta@mgu.ac.in.
authors: M M
authors: MM
authors: Sunil Paul
authors: U K
authors: UK
authors: Aravind
authors: G
authors: G
authors: Pramod
authors: A
authors: A
authors: Saha
authors: C T
authors: CT
authors: Aravindakumar
mesh_terms: D006861:Hydrogen Peroxide / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D017665:Hydroxyl Radical / Q000737:chemistry*; D006900:Hydroxylation; D007700:Kinetics; D008027:Light; D013058:Mass Spectrometry; D008745:Methylation; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis; D013057:Spectrum Analysis; D013806:Theophylline / Q000737:chemistry*; D014527:Uric Acid / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D014867:Water / Q000737:chemistry*; D019820:Xanthine / Q000138:chemical synthesis / Q000737:chemistry
chemical_list: D014867:Water; D019820:Xanthine; D014527:Uric Acid; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide; D013806:Theophylline; C031274:1,3-dimethyluric acid