Four isomeric photodimers of dimethylthymine (I syn, head to head, II anti, head to head, III syn, head to tail, IV anti, head to tail), have been irradiated in the crystalline state with60 Co γ-rays. The exclusive radiolysis product of all four dimers is dimethylthymine. The initial G value for this reaction is 6300, 1700, 14, and 19 for dimers I, II, III, and IV, respectively. For all four compounds, the G values decrease with increasing conversion of the dimer to dimethylthymine. For I and II the initial G values are lower for smaller and less perfect crystals. In aqueous solution the radiolysis proceeds by a different mechanism and dimethylthymine is only a minor reaction product. In this case, the G values for destruction of I, II, III, and IV are 1.2, 1.5, 1.2, and 1.2, respectively. For the radiolysis in the crystalline state, the possibility of a chain reaction mechanism is discussed.

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