PHOTOCHEMISTRY OF MODEL OLIGO‐ AND POLYNUCLEOTIDES—IV. HETERO‐OLIGONUCLEOTIDES AND HIGH MOLECULAR WEIGHT SINGLE AND TWIN‐STRANDED POLYMER CHATNS.*†

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Abstract
Summary: A study has been made of the photochemical transformations undergone by pyrimidine residues in a number of di‐, tri‐ and tetra‐nucleotides, and in high molecular weight single and twin‐stranded model polynucleotides. The results confirm and extend previous conclusions as to resistance of purines and indicate that one of the predominant reactions exhibited by uridylic and cytidylic acid residues is the reversible nucleophilic addition of water to the 5, 6 double bond of the pyrimidine rings. Results for poly‐A‐poly‐U provide an explanation for the apparent resistance of DNA to U.V. irradiation (as measured by changes in U.V. absorption spectra), due to the fact that photolysis of a uracil residue is accompanied by dissociation of the hydrogen bonds to the adenine of the A‐U base pair and the resultant reduction in hypochromicity partially compensates for loss of absorption of the uracil residue. Quantum yield calculations have been made for the various polymers. A tabulation is presented of data hitherto accumulated with regard to the photochemical behaviour of pyrimidines in nucleotide chains, as well as the kinetic constants for the reverse thermal reaction. Attention is drawn to the significance of the results with reference to models for biological thermal and photoreactivation. From a consideration of the quantum yields for pyrimidine residues in various types of nucleotide chains, it is concluded that energy transfer in nucleic acids, insofar as it exists, is not of primary importance in the production of photochemical lesions.