Photo-CIDNP study of pyrimidine dimer splitting. I: 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 photosensitizer. To this end, photochemically induced dynamic nuclear polarization (photo-CIDNP) experiments were performed on a series of pyrimidine monomers and dimers. The CIDNP spectra demonstrate the existence of both the dimer radical cation, which is formed by electron transfer from the dimer to the photoexcited sensitizer AQS*, and its dissociation product, the monomer radical cation. In spectra of 1,1'-trimethylene bridged 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. Polarization originating from a dimer radical pair is detected in the spectrum of trans,anti (1,3-dimethyluracil) dimer as well. The spectra of the bridged pyrimidines also demonstrate the reversibility of the dissociation of dimer radical cation into monomer radical cation, which is concluded from the observation of polarization in the dimer as a result of spin sorting in the monomer radical pair.