The gas phase acid/base properties of 1,3,-dimethyluracil, 1-methyl-2-pyridone, and 1-methyl-4-pyridone:: relevance to the mechanism of orotidine-5′-monophosphate decarboxylase

A combination of experimental and theoretical approaches have been used to probe the gas phase acidity and basicity of 1,3-dimethyluracil, 1-methyl-2-pyridone, and 1-methyl-4-pyridone. For the acidity measurements, bracketing experiments were completed in an electrospray/quadrupole ion trap mass spectrometer. The conjugate bases of the title species were formed by collision activated decarboxylation of appropriate carboxylate precursors. The data indicates only a small variation in the acidities (Delta H-acid = 369.9-377.0 kcal/mol) with the uracil derivative being similar to 7 kcal/mol more acidic than both the pyridones. To determine the basicities of the title compounds, Cooks' kinetic method was used and a much larger variation (19 kcal/mol) was observed in the proton affinities of the neutral species: 1,3-dimethyluracil, 213.7 +/- 3.0 kcal/mol; 1-methyl-2-pyridone, 222.3 +/- 2.9 kcal/mol; and 1-methyl-4-pyridone, 233.1 +/- 3.0 kcal/mol. Delta H-acid and proton affinity values were also computed at the MP2/6-31 + G(d,p)//HF/6-31 + G(d), and B3LYP/6-31 + G(d,p)//HF/6-31+ G(d) levels. There is very good agreement between the experimental values and those from both levels of theory. (C) 2000 Elsevier Science B.V.