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

The mechanistic sequence(s) in the TiO2-photocatalytic oxidation of constit uent 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 ch romatography to assess how the chemical structure of the bases affects thei r photocatalyzed mineralization. Rates of formation of NH4+ and NO3- ions i n the pyrimidine bases are closely dependent on the existence of the carbon yl and amino groups; for example, formation of NO3- ions is faster than for mation of NH4+ ions for uracil (Ura) and thymine (Thy) having the carbonyl function. By contrast, NH4+ ions are produced faster than NO3- ions in the case of cytosine (Cyt) which possesses a primary amine function. In compari son with uric acid, which has no amino group, the photocatalyzed mineraliza tion of the purine bases adenine (Ade) and guanine (Gua) generates a greate r quantity of NH4+ ions than NO3- ions, in the initial stages. In nearly al l cases examined, formation of NO3- ions takes place only after an inductio n period and originates mostly from the ring nitrogen atoms of the bases. ( C) 1999 Elsevier Science S.A. All rights reserved.