The ultraviolet photochemistry of thymidylyl-(3′→5′)-thymidine

Irradiation of ³²P-labelled TpT‡ by monochromatic light in the wavelength range 225 to 289 mμ. results in the production of four chromatographically separable photoproducts. Two isomeric dimers, TpT1¯ and TpT2¯, approach an equilibrium with TpT with 2·5% in the dimer form at 225 mμ and 95% at 289 mμ. The relative yields of the dimers are 5 : 1 and independent of wavelength. One of the unknown photoproducts, TpT⁴, which has an absorption maximum at 325 mμ, is produced irreversibly from TpT at a yield comparable to the least plentiful dimer, TpT2¯. This photoproduct can in turn be converted to a second unknown, TpT³, by irradiation at 313 mμ. TpT³ can be reconverted to TpT⁴ by re-irradiation at 240 mμ. Since TpT³ and TpT⁴ have an extinction at 267 mμ which is about one-third of one thymine, it is likely that in both these compounds both thymine moieties are altered. None of the photoproducts is hydrolysed by venom phosphodiesterase; this also suggests that the thymine moieties in all four photoproducts are joined together. None of the photoproducts is completely stable at 85°C at pH 1 or 13. No conclusive evidence for the production of a heat-labile hydrate photoproduct of TpT has been found. The cross-sections and quantum yields for the production of photoproducts from TpT, the reversal of dimers to TpT, and the interconversion of TpT⁴ and TpT³ have been determined as a function of wavelength, using chromatographically purified photoproducts. Mathematical analysis of the dose-effect curves permits the calculation of the cross-sections.