THE PHOTOCHEMISTRY OF 14 C-5-BROMO-2'-DEOXYURIDYLYL-(3'→5')-THYMIDINE DETERMINATION OF QUANTUM YIELDS AS A FUNCTION OF pH

Abstract— Quantum yields for the formation of the major photoproduct of 2-¹⁴C-5-bromo-2'-deoxyuridylyl-(3'→5')-thymidine (BrdUpT) have been determined by irradiation of the 2-¹⁴C-BrU-labeled dinucleotide at pH 2.3, 5.9, 7.05, 8.0 and 10.25 with U.V. light at 280nm. At acidic and neutral pH the quantum yield was 0.0063; the value decreased markedly above pH 8.0 to 0.0025 at pH 10.25. Some evidence of the formation of additional photoproducts at high and low pH was found. Some aspects of the mechanism of the reaction are discussed.
Consideration of p K_a values calculated for singlet and triplet excited states indicates that the decrease in the quantum yield of main photoproduct at high pH is due to dissociation of the excited bromodeoxyuridine moiety. It is suggested that the formation of BrdUpT photoproduct and the debromination of bromouracil-labeled DN A occur via different excited states.