Properties of an Unusual Photoproduct of U.V. Irradiated Thymidylyl‐thymidine

One of the four major photoproducts formed by UV irradiation of TpT in aqueous solution has a number of unusual properties. This compound, which we arbitrarily call TpT 4 , is produced irreversibly from TpT with single‐hit kinetics and seems to represent a novel pathway for TpT photolysis different from the conventional cyclobutane ring dimer formation. This communication describes the preparative isolation of TpT 4 by ion‐exchange chromatography on DEAE‐cellulose and its UV, fluorescence and IR spectra. TpT 4 has an absorption maximum at 325 mμ, fluoresces at 405 mμ when excited at 325 mμ, and has an IR spectrum consistent with the assumption that one of the thymine rings has been converted to a pyrimidin‐2‐one structure. In addition, TpT 4 appears to have both pyrimidine rings linked together and does not contain a peroxide group as previously supposed. Spectrophotometric titration shows that TpT 4 has a pKâ of 10.75 and is unstable to prolonged treatment at extreme pH. A model structure consistent with the present data is given. This model contains the glycosidic and phosphodiester bonds intact with a 5‐methylpyrimidine‐2‐one ring and a thymine ring joined in ether linkage at the C 4 positions, with an OH at the C 4 position in the thymine ring.