Photo‐cidnp Study of Pyrimidine Dimer Splitting Ii: Reactions Involving Pyrimidine Radical Anion Intermediates

Abstract

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor N alpha-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1'-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. 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 originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.

Density functional investigation on electron‐transfer catalysis of cycloreversion of cyclobutane: Radical anion mechanism

“…24,25 It is also shown that the cycloreversion of c-C 4 H 8 ϩ may lead to low-lying C 4 H 8 ϩ isomers rather than C 2 H 4 ϩ C 2 H 4 ϩ as final products mainly due to very low H-shift barriers for radical cation intermediates. 21 We notice that recent experimental 22 and theoretical 23 data have suggested a radical anion mechanism for the photoenzymic repair of cyclobutane pyrimidine dimers. Thus, from the organic synthetic point of view, it is interesting to make clear if the cycloreversion/cycloaddition reactions of small substituted cyclobutanes can be catalyzed by electron-attachment.…”

Urocanic Acid Photochemistry and Photobiology

“…As for dsDNA. the UA-photosensitizcd formation of cyclobutane pyrimidine dimers could also be occurring via an electron transfer mechanism because several papers have recently been issued that confirm that pyrimidine radical cations can react with the neutral base to form dimers (112)(113)(114). In this regard one should note that the environment surrounding the radical ions plays an important role in the successful dimerization reaction.…”

Photo‐cidnp Study of Pyrimidine Dimer Splitting I: Reactions Involving Pyrimidine Radical Cation Intermediates

“…1, and the reducing sensitizer N α‐acetyltryptophan (AcTrp). Reactions involving AcTrp are treated in part II, the accompanying paper 26 . The pyrimidines investigated are shown in Fig.…”