ACTION SPECTRA FOR THE PHOTOLYSIS OF 5-IODODEOXYURIDINE IN DNA AND RELATED MODEL SYSTEMS: EVIDENCE FOR SHORT-RANGE ENERGY TRANSFER*

Abstract— A variety of polynucleotides containing 5-iodouracil residues were irradiated in aqueous solution with wavelengths between 240 and 313 nm. From the rate of deiodination the photochemical cross sections (a_B) were determined as a function of the irradiation wavelength (A). The expression was used to relate the observed values of _B to the intrinsic quantum yield, φ_INT, and to the absorption cross sections, and for the iodinated and noniodinated residues, respectively. φ_INT is the probability an excited iodouracil residue will deiodinate, while the parameter b is a measure of the number of noniodinated bases which contribute their excitation energy to the deiodination process. For IdUrd and poly(5-iodouridylic acid), the average values of φ_INT calculated from the experimental _B values were 0.0202 and 0.0188, respectively, for irradiation in air. In native, denatured, and depurinated DNA in which IdUrd was substituted for 10% of the Thd, the average φ_INT values were 0.0069, 0.0088, and 0.0153, respectively, indicating an enhancement in φ_INT upon decreasing the order of the polynucleotide. In contrast, the average values of b bor the same set of compounds were found to be 4 , 2 and 0.4, respectively, indicating a decrease in b with decreasing polynucleotide order, i.e. a loss of base stacking decreases the extent of energy transfer. A value of b= 4 for native DNA is assumed to mean that the extent of energy transfer in native DNA is limited to four base donors per iodouracil residue serving as an energy trap.