Saturation photodimerization of thymines in DNA

Acetophenone sensitizes the uv photodimerization of adjacent thymines in nucleic acids and under steady state irradiation induces irreversible saturation dimerization. Monte Carlo and analytical techniques are used to calculate the saturation dimerization limits for various nucleic acids, under the assumption that acetophenone-sensitized dimerization occurs by a random irreversible process. A comparison of these calculated results with experimental values indicates that sensitized dimerization occurs by an effectively random process only in single-stranded DNA or in native DNA of very high A.T content. In general, nucleation phenomena operate in native DNA that serve to stereochemically restrict saturation dimerization limits below those expected for random dimerization.