Cross-sections for some TpT photoproducts

The kinetics and cross-sections (probability σ of a photon to give rise to the photochem. event) of some photoproducts of TpT (thymidylylthymidine) irradiated by UV light were studied, based on the reaction scheme of G. E. Johns, et al. (1964).  The optical absorbance variations of 2 wavelengths (2650 anad 3250 Å) were measured as a function of the incident dose at 2450 Å.  At small doses, a rapid dimer formation and a slower formation of product TpT4 were obsd.  The equil. percentage of dimers was calculated to be 12%.  The equil. between products TpT4 aad TpT3 was studied by the optical absorbance changes at 3250 Å as a function of the dose at different wavelengths.  The cross-section values for TpT4 → TpT3 and TpT3 → TpT4 were calcd. as σ4,3 = 0.24 and σ3,4 = 1.7 cm2μEinstein, resp.  The σ4,3 was detd. as a function of the irradn. wavelength and was similar to the TpT4 absorption spectrum.  Thus, the quantum yield .vphi.4,3 is nearly const. in the obsd. range.  The effect of UV radiation on DNA is discussed.