Photolysis of 5 -Bromouracil and Some Related Compounds in Solution

The steady sta te photolysis of 5-bromouracil (BU) in aqueous solution has been studied as a function of wavelength, pH , tem perature, and hydrogen-donor concentration. U nder all conditions studied, the prim ary reaction is shown to be C-Br bond cleavage followed by abstraction from the hydrogen-donor to give uracil and H B r. A t pH > 12 further products are formed. In deoxygenated aqueous solution a t pH 6, 20 °C, and 254 nm, the quantum yield of BU consumption, 0 (-BLT), is 1.8 X 10~3 independent of hydrogen-donor type or concentration (e.g. 3 X 10“ 2 to 2 m MeOH). W ith increasing pH , & (-BU) increases stepwise to 0.012 a t pH 10 and to 0.28 a t pH 14. pK -values calculated from these da ta are the same as ground sta te pK values. 0 (-BU) increases w ith tem perature w ith an activation energy of approx. 3.4 kcal/mol. 0 (-BU) increases w ith photon energy. Above 2 M MeOH 0 (-BU) increases reaching 0 (-BU) — 0.025 in neat MeOH. Similar high, solvent dependent, values are obtained for other hydrogen-donor solvents. In neat organic solvents an additional reaction w ith BU induced by solvent radicals was observed. These results have been explained in term s of a homolytic dissociation of the C -B r bond of the excited BU followed by recom bination or H atom abstraction by the radicals. A t high hydrogen-donor concentration H atom abstraction can compete w ith cage re ­ combination. A comparison has been m ade between BU photolysis in organic, hydrogendonor solvents and BU photolysis w ithin the DNA of bacteria or phages. I t has been concluded th a t the m uch higher quantum yields observed for chain breaks in the pho to ­ lysis of DNA containing BU com pared to photolysis of BU in aqueous solution is due to the high local concentration of hydrogen-donors (sugar molecules) w ithin the DNA molecule, even in dilute aqueous solution.