PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING .2. REACTIONS INVOLVING PYRIMIDINE RADICAL-ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarizati on) experiments were performed on pyrimidine monomers and dimers, usin g the electron-donor N alpha-acetyltryptophan (AcTrp) as a photosensit izer. The CIDNP spectra give evidence for the existence of both the di mer radical anion, which is formed by electron transfer from the excit ed AcTrp to the dimer, and its dissociation product, the monomer radi cal anion. The AcTrp spectra are completely different from those obtai ned with an oxidizing sensitizer like anthraquinone-2-sulfonate, becau se of different unpaired electron spin density distributions in pyrimi dine radical anion and cation. In the spectra of the anti (1,3-dimethy luracil) dimers, polarization is detected that originates from a spin- sorting process in the dimer radical pair, pointing to a relatively lo ng lifetime of the dimer radical anions involved. Although the dimer r adical anions of the 1,1'-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hy perfine interaction, which explains why no polarization originating fr om the dimer radical pair is detected. In the spectra of the bridged p yrimidines, polarized dimer protons are observed as a result of spin s orting in the monomer radical pair, from which it follows that the dis sociation of dimer radical anion into monomer radical anion is reversi ble. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that ori ginates from spin-sorting in the monomer radical pair. At higher pH th e geminate recombination polarization is partly cancelled by escape po larization arising in the same product.