1. pH-Dependent Singlet O 2 Oxidation Kinetics of Guanine and 9-Methylguanine: An Online Mass Spectrometry and Spectroscopy Study Combined with Theoretical Exploration.
- Author
-
Lu W, Sun Y, Zhou W, and Liu J
- Subjects
- Chlorine chemistry, Guanidines chemical synthesis, Guanosine analogs & derivatives, Guanosine chemical synthesis, Hydantoins chemical synthesis, Hydrogen Peroxide chemistry, Hydrogen-Ion Concentration, Kinetics, Models, Chemical, Molecular Conformation, Oxidation-Reduction, Spectrometry, Mass, Electrospray Ionization, Spiro Compounds chemical synthesis, Tandem Mass Spectrometry, Guanine analogs & derivatives, Guanine chemistry, Singlet Oxygen chemistry
- Abstract
We report a kinetic and mechanistic study on the title reactions, in which
1 O2 was generated by the reaction of H2 O2 with Cl2 and bubbled into an aqueous solution of guanine and 9-methylguanine (9MG) at different pH values. Oxidation kinetics and product branching ratios were measured using online electrospray ionization mass spectrometry coupled with absorption and emission spectrophotometry, and product structures were determined by collision-induced dissociation (CID) tandem mass spectrometry. Experiments revealed strong pH dependence of the reactions. The oxidation of guanine is noticeable only in basic solution, while the oxidation of 9MG is weak in acidic solution, increases in neutral solution, and becomes intensive in basic solution. 5-Guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) were detected as the major oxidation products of guanine and 9MG, and Sp became dominant in basic solution. A reaction intermediate was captured in mass spectra, and assigned to gem-diol on the basis of CID measurements. This intermediate served as the precursor for the formation of Gh. After taking into account solution compositions at each pH, first-order oxidation rate constants were extracted for individual species: that is, 3.2-3.6 × 107 M-1 s-1 for deprotonated guanine, and 1.2 × 106 and 4.6-4.9 × 107 M-1 s-1 for neutral and deprotonated 9MG, respectively. Guided by approximately spin-projected density-functional-theory-calculated reaction potential energy surfaces, the kinetics for the initial1 O2 addition to guanine and 9MG was evaluated using transition state theory (TST). The comparison between TST modeling and experiment confirms that1 O2 addition is rate-limiting for oxidation, which forms endoperoxide and peroxide intermediates as determined in previous measurements of the same systems in the gas phase.- Published
- 2018
- Full Text
- View/download PDF