Flavin-sensitized photochemically induced dynamic nuclear polarization detection of pyrimidine dimer radicals

Repair of a Dimeric Azetidine Related to the Thymine-Cytosine (6-4) Photoproduct by Electron Transfer Photoreduction

Photolyases are intriguing enzymes that take advantage of sunlight to restore lesions like cyclobutane pyrimidine dimers or (6-4) photoproducts. This work focused on the photoreductive process responsible for splitting of the azetidine ring proposed to occur during (6-4) photoproduct repair at a thymine-cytosine sequence. A model compound formed by photocycloaddition between thymine and 6-azauracil has been designed to mimic the elusive azetidine intermediate. The photoinduced electron transfer process has been investigated by means of steady-state and time-resolved fluorescence using photosensitizers with oxidation potentials in the singlet excited state ranging from -3.3 to -2.1 V vs. SCE. Azetidine ring splitting and recovery of "repaired" bases were proven by HPLC analysis.

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.

Flavin-photosensitized monomerization of dimethylthymine cyclobutane dimer in the presence of magnesium perchlorate

We have investigated the photosensitized monomerization of the cis,syn-cyclobutane dimer of 1,3-dimethylthymine using riboflavin tetraacetate and a 5-deazaflavin derivative as photosensitizer. Although little monomerization of the dimer is induced by photoexcitation of the flavins in the absence of any additives, the flavins can function as an efficient photosensitizer in the presence of magnesium perchlorate. Mechanistic studies involving spectroscopic, quantum-yield and flash-photolysis measurements demonstrated that the photosensitized monomerization exclusively proceeds through electron transfer from the dimer to the triplet flavins complexed with Mg2+. The effects of magnesium perchlorate are compared with those on the chloranil-photosensitized monomerization and also with the effects of HClO4 on the flavin-photosensitized reaction.

The photo repair of pyrimidine dimers by DNA photolyase and model systems

Pyrimidine dimers are eliminated from DNA by a number of different mechanisms known as DNA repair. Photoreactivation, the reversal of the harmful effects of short wavelength radiation by subsequent exposure to longer wavelengths, is one such mechanism. In photoreactivation, the enzyme DNA photolyase utilises light in order to catalyse the cleavage of the cyclobutane ring of the pyrimidine dimer. The results of recent studies of E. coli DNA photolyase and model systems using techniques such as steady state and flash photolysis, time resolved fluorescence and photo CIDNP are surveyed. A mechanism is proposed for the in vitro reaction of E. coli DNA photolyase which involves photoreduction of the FAD radical cofactor followed by electron donation to the dimer from the excited singlet state of reduced FAD.