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Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross‐sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross‐sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.

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