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The formation of Thy*Thy in DNA in the presence of tyramine, tyrosine and-tyrosine-containing peptides such as Lys-Tyr and Lys-Tyr-Lys was studied with monochromatic UV irradiation. The formation of Thy* Thy by UV irradiation was enhanced in the presence of these compounds. The action spectrum of the photosensitization has a peak near 280 nm corresponding to the absorption spectrum of tyrosine. The triplet quencher reduced the sensitization substantially. The sensitization in native DNA was more than six times larger than that in denatured DNA. Increasing the concentration of salts suppressed the sensitization. The nature of the interaction between DNA and the sensitizer is discussed.

Abstract. Photodimerization of thymine in aqueous solution in the presence of tyrosine was studied with monochromatic UV irradiation. The total dimer formation was sensitized in the presence of tyrosine. The action spectrum of sensitized total dimer formation has a peak near 280 nm corresponding to the absorption maximum of tyrosine. Triplet quenchers reduced the sensitization substantially. It seems probable that tyrosine‐sensitized photodimerization of thymine occurred via triplet‐triplet energy transfer from tyrosine to thymine.

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