CIDNP DETECTED FLASH PHOTOLYSIS OF cis–syn 1,3 DIMETHYLTHYMINE DIMER

Abstract

Abstract— Theanthraquinone–2‐sulfonate photo‐sensitized splitting of cis‐syn 1,3‐dimethylthymine dimer gives rise to large CIDNP effects in the reaction product 1,3‐dimethylthymine. The polarization originates from a radical ion pair formed by electron transfer from the dimer to the triplet state sensitizer. In a deoxygenated solution the sign of the polarization of theC–6 proton is reversed compared to the predicted one on account of the CIDNP sign rules. In an aerated solution the correct sign is observed. This can be accounted for by assuming reduction of the lifetime of the radical pair in the presence of oxygen. The time‐resolved photo‐CIDNP technique was used to study the time dependence of the 1,3‐dimethylthymine signal. To account for this time‐dependence a cation radical disproportionation reaction is proposed.

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

“…The monomer and the cs ‐dimer of DMT yield identical spectra (Fig. 8), which contain polarized monomer signals, as has been reported previously by Roth and Lamola 19 and Kemmink et al 20 A spin‐sorting process in the AQS − M + radical pair, in which all polarized protons have positive hyperfine couplings, is responsible for recombination polarization observed in emission. The absence of polarized dimer protons indicates that the dimer radical pair produces the geminate monomer radical pair before spin sorting has taken place (Scheme II).…”

“…3 for both syn and anti dimer configurations. The monomer radical cation M + has been detected long ago, 19–20 and evidence for the existence of the dimer radical cation D + has recently been obtained from photo‐CIDNP experiments as well 21 . The exact mechanism of the dissociation has not been demonstrated experimentally until now, and it is not known whether the splitting is a concerted or a stepwise process, in which case the radical cation I + as drawn in Fig.…”

“…Radicals that have escaped the geminate pair contain the opposite polarization, which either disappears by relaxation processes or arises in escape products (ε < 0). One of the escape products is the monomer itself, which may result from a reaction with a neutral monomer M (exchange), with another monomer radical cation M + (disproportionation) or with an AQS* radical anion (random recombination) 20–29 . Here, these mechanisms are not treated in detail, but it suffices to know that there is a polarization transfer pathway from M + to M. Escape polarization may also arise in other escape products, such as an addition product denoted M‐R.…”

Photochemistry, Photophysics, and Mechanism of Pyrimidine Dimer Repair by Dna Photolyase

“…Direct detection of the dimer radical cation@) and/or pyrimidine monomer cation in model reactions has been reported from photo-CIDNP studies of anthraquinone-sensitized splitting of Pyr< >Pyr. [57][58][59] Even though these model studies provided experimental support for dimer splitting by both electron abstraction and donation mechanisms, studies with natural DNA photolyase overwhelmingly suggest that an electron is being transferred to the Pyr< >Pyr in the initial stage of catalysis. Recently, we found that the active form of the flavin in photolyase is present as an anion (Enz-FADH; S. T. Kim, C. Essenmacher, G. T. Babcock and A. Sancar, unpublished observation) in agreement with the prediction made by Okamura and coworkers."…”

Photo-CIDNP study of the splitting of the dinucleotidecis, syn-thymine dimer with reduced flavin as a sensitizer: Evidence for a thymine radical anion intermediate

“…Photochemically induced dynamic nuclear polarization (photo-ClDNP) has proven a powerful method for studying the splitting of pyrimidine dimers [7][8][9][10][11][12]. With anthraquinone-2-sulfonate [7][8][9][10] or oxidized flavin [11] asa sensitizer the existence of pyrimidine radical cations has been demonstmted, and pyrimidine radical anions have been observed during indole-sensitized pyrimidine dimer splitting [10,12]. The two splitting mechanisms give rise to completely different CIDNP spectra, because the electron spin density distribution in pyrimidine radical anions is different from that in radical cations.…”