PARA-AMINOBENZOIC ACID-PHOTOSENSITIZED DIMERIZATION OF THYMINE .2. INPUC19 PLASMID DNA

Para-aminobenzoic acid (PABA) photosensitizes the dimerization of cont iguous thymine bases in DNA upon irradiation at wavelengths greater th an 300 nm which are not absorbed by the DNA bases themselves. The kine tics and mechanism of this reaction has been studied for pUC19 plasmid DNA. The dependence of dimer yield on a variety of reaction condition s has been determined. A mechanism has been proposed to account for th e major processes taking place and rate constants for the constituent reactions have been acquired from the literature, from previous studie s on the simpler systems of PABA-photosensitized dimerization of free thymine base and thymidylyl-3',5'-thymidine (TpT) and from calculation based upon a Stern-Volmer steady state analysis of the proposed mecha nism and experimental data. Computer simulation of the mechanism with its associated rate constants gives dimer yields that are generally in reasonable agreement with the experimental data. Where discrepancies occur they have been explained in terms of production of other photopr oducts.