PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING .1. REACTIONS INVOLVING PYRIMIDINE RADICAL-CATION INTERMEDIATES

The light-induced splitting of pyrimidine dimers was studied using the electron acceptor anthraquinone-2-sulfonate (AQS) as a photosensitize r. To this end, photochemically induced dynamic nuclear polarization ( photo-CIDNP) experiments were performed on a series of pyrimidine mono mers and dimers. The CIDNP spectra demonstrate the existence of both t he dimer radical cation, which is formed by electron transfer from the dimer to the photoexcited sensitizer AQS, and its dissociation produ ct, the monomer radical cation. In spectra of 1,1'-trimethylene bridge d cis,syn pyrimidine dimers, polarization is observed that originates from a spin-sorting process in the dimer radical pair. This points to a relatively long lifetime of the dimer radical cation involved, which is presumably due to stabilization by the trimethylene bridge. Polari zation originating from a dimer radical pair is detected in the spectr um of trans,anti (1,3-dimethyluracil) dimer as well. The spectra of th e bridged pyrimidines also demonstrate the reversibility of the dissoc iation of dimer radical cation into monomer radical cation, which is c oncluded from the observation of polarization in the dimer as a result of spin sorting in the monomer radical pair.