In continuing the previous research on the photoreactions between DNA and skin-photosensitizing furocoumarins, the Authors have irradiated at 3,655 Å an aqueous solution of DNA added with psoralen. The DNA, precipitated and washed with ethyl alcohol, has been hydrolyzed by heating with 70 % perchloric acid. Among the products of hydrolysis the following substances have been isolated through preparative paper chromatography and identified. 1) A photoadduct psoralen-thymine, with violet fluorescence when observed at 3,655 Å, already obtained by the Authors through irradiation of an aqueous frozen solution of thymine and psoralen. 2) A photoadduct psoralen-cytosine, also with violet fluorescence and already obtained through irradiation of cytosine and psoralen. 3) A photoadduct psoralen-uracil, not present in the macromolecule but which takes place by a deamination of the photoadduct psoralen-cytosine. All these compounds derive from a C4-cyclo-addition, in which the positions 4', 5' of psoralen and the positions 5, 6 of pyrimidine bases are involved. Hydrolysis of DNA, irradiated in the presence of psoralen, has been performed also in more mild conditions, that is by heating with 0.4 N hydrochloric acid. In this condition, in addition to the preceding substances, a fourth compound was identified, that is a photo adduct psoralen-thymine, without fluorescence at 3,655 Å, deriving also from a C4-cyclo-addition of the substances, but in which the positions 3, 4 of psoralen and the positions 5, 6 of thymine are involved. Also this compound has been previously obtained by irradiation of psoralen and thymine. The results obtained clearly show that the stable combination of psoralen to DNA which occurs through the irradiation at 3,655 Å, takes place at the pyrimidine bases level with the formation of C4-cyclo-additions, in which the 5, 6-double bond of thymine and cytosine and the double bonds 4', 5' or 3, 4 of the furocoumarin are involved. The fluorescent photoadduct psoralen-thymine 1) forms in a grater extent.
Referenze Bibliografiche
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Con il contributo del Ministero per i Beni e le Attività Culturali