Photocatalyzed degradation of polymers in aqueous semiconductor suspensions.: IV Theoretical and experimental examination of the photooxidative mineralization of constituent bases in nucleic acids at titania/water interfaces

Abstract

The mechanistic sequence(s) in the TiO 2-photocatalytic oxidation of constituent pyrimidine and purine bases in nucleic acids is examined theoretically by molecular orbital calculations of frontier electron densities and point charges on all atoms, and experimentally by UV–Vis spectroscopy and gas chromatography to assess how the chemical structure of the bases affects their photocatalyzed mineralization. Rates of formation of NH 4 + and NO 3 − ions in the pyrimidine bases are closely dependent on the existence of the carbonyl and amino groups; for example, formation of NO 3 − ions is faster than formation of NH 4 + ions for uracil (Ura) and thymine (Thy) having the carbonyl function. By contrast, NH 4 + ions are produced faster than NO 3 − ions in the case of cytosine (Cyt) which possesses a primary amine function. In comparison with uric acid, which has no amino group, the photocatalyzed mineralization of the purine bases adenine (Ade) and guanine (Gua) generates a greater quantity of NH 4 + ions than NO 3 − ions, in the initial stages. In nearly all cases examined, formation of NO 3 − ions takes place only after an induction period and originates mostly from the ring nitrogen atoms of the bases.