Syn Dimer Research Articles

Effect of sequence, adduct type, and opposing lesions on the binding and repair of ultraviolet photodamage by DNA photolyase and (A)BC excinuclease

The cis,syn and (6-4) products of dipyrimidine sites are the major mutagenic and lethal UV photoproducts in DNA. To investigate their relative susceptibilities to repair and other factors such as sequence context and lesions in the complementary strand that might influence repair efficiencies, we constructed 49-mer duplexes containing site-specific photoproducts of thymidylyl(3',5')thymidine sites at central locations. Using these substrates, we measured the binding of Escherichia coli DNA photolyase to cis,syn dimers in four sequence contexts and to two cis,syn dimers in close proximity and on opposing strands. We found that the sequence within a 10-base pair region had little effect on binding and that two enzyme molecules bound to substrate containing two dimers in the 5'-staggered orientation, but not in the 3'-staggered one. Similarly, the excision of a cis,syn dimer by (A)BC excinuclease was not influenced by the sequence in the immediate vicinity of the dimer, and the enzyme was active on 5'-staggered cis,syn dimers, but not on 3'-staggered ones. Of special significance, we found that (A)BC excinuclease removed the cis,syn, trans,syn-I, (6-4), and Dewar photoproducts at vastly differing relative rates of 1:6:9:9, respectively.

CHLORANIL‐PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMERS and EFFECT OF MAGNESIUM PERCHLORATE

The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.

STRUCTURE‐REACTIVITY RELATIONSHIPS IN REDOX‐PHOTOSENSITIZED SPLITTING OF PYRIMIDINE DIMERS AND UNUSUAL ENHANCING EFFECT OF MOLECULAR OXYGEN *

Abstract Redox photosensitization using the phenanthrene‐p‐dicyanobenzene pair in acetonitrile has been applied to the respective four isomeric dimers of N.N′‐dimethylthymine (DMT) and N,N′‐dimethyluracil (DMU) as well as to several related cyclobutane compounds. The head‐to‐head (syn) dimers of both DMT and DMU can undergo photosensitized splitting in the following order of efficiency: cis, syn dimer of DMT > cis, syn dimer of DMU > trans, syn dimer of DMT. On the other hand, the head‐to‐tail (anti) dimers are totally unreactive and have higher oxidation potentials than the corresponding syn dimers. It is suggested that the key mechanistic pathway is the formation of π complexes between the dimers and the photo‐generated cation radical of phenanthrene by way of which splitting of the cyclobutane ring catalytically occurs without the formation of the discrete cation radical of the dimers. Structure‐reactivity relationships are interpreted in terms of through‐bond interactions between the n orbitals of N(l) and N(l′) involving the C(6)‐C(6′) bond, as well as in terms of steric repulsion. It was found that aeration of solution greatly enhances the quantum yields of photosensitized splitting; the limiting quantum yield for splitting of the cis, syn dimer of DMT is 100.

Photoelectron spectra and SCF MO calculations for the dimers of cyclobutadiene

Abstract The He I photoelectron spectra of the syn and anti dimers of cyclobutadiene have been measured. Accurate MINDO/2 SCF MO calculations, in which the geometries of both dimers were optimized, have been used in interpreting the spectra. The experimental ionization energies derived from the spectra are in reasonable accord with those reported recently by Heilbronner and co-workers. However, the current interpretation of the photoelectron spectrum of the syn dimer differs significantly from that of the earlier one.