Alteration of Guanine Residues During Proflavine Mediated Photosensitization of Dna

Mild photodynamic treatments of proflavine‐calf thymus DNA complexes induce a unique and quantitatively important alteration of the guanine residues which can be related to the lethal lesions due to the combined action of proflavine and light on phages. The ‘altered guanine’ is destroyed by HClO 4 but is recovered after partial DNA depurination under the form of two photoproducts. The first product, G ox , elutes as guanine on a Sephadex column but has a modified UV absorbance spectrum. It gives rise by further irradiation to another product, X, which elutes at pH 9.7 as a pyrimidine compound and presented a maximal UV absorbance at 246 nm. Product X is also selectively released by piperidine fixation onto the photo‐damaged DNA. The guanine degradation process is markedly decreased in the presence of the singlet oxygen quencher, NaN 3 . The photodynamic lesion inhibits the enzymatic degradation of the DNA but generates locally denatured regions that are sensitive to S1 endonuclease.