Thymine dimers, monomerization

Despite the frequent use of photoreversal as a measure of dimer formation in many biological materials, few detailed studies of this phenomenon have been made. The action spectrum for photoreversal of uracil dimer (and thymine dimer) has been reported by Swenson and Setlow,48 and is shown in Figure 1. Purified, crystalline dimer was used in these studies. The quantum yield for dimer breaking was found to be 0.45, one-half of that for monomerization of thymine dimers.49... 

Thymine dimers may be monomerized from DNA by enzymatic photoreactivation. In this case no nucleotides are removed in the repair reaction. 

The simplest example of an enzymic repair process is that of photoreactivation. This phenomenon was rediscovered in 1949 by Kelner, who found that UV-irradiated spores of Streptomyces showed higher survival if exposed to white light after the UV treatment (Kelner, 1949). At the time, this was not interpreted as an enzymic repair process but it was so. Enzymic action became demonstrable when it was shown that it could occur in vitro if, for example, transforming DNA of Haemophilus influenzae was incubated with cell-free extracts of yeast or E. coli after UV-treatment and then exposed to 400 nm light. (Rupert, 1958 1960). This assay made it possible to purify the enzyme from a variety of sources (e.g., Sutherland, 1974 Rupert, 1975). Once an assay for thymine dimers became available, it was shown that the action of the enzyme was to restore the dimers to monomeric thymidine in situ. 

The photosensitized monomerization of thymine dimers has been achieved in the presence of reduced flavin,and pyrimidine dimers undergo cleavage by light-utilizing enzymes which are thought to act by a photoinduced electron transfer mechanism. ... 

Scheme 32. General mechanism of flavin- or S-deazaflavin-photosensitized monomerization of thymine dimers (16/).

To elucidate further the possible role of flavin as a sensitizer in photolyase catalysis, Joms conducted a study on the light-catalyzed monomerization of thymine dimers by reduced flavins and flavin analogs as models for the DNA-PL reaction of E. coli and yeast (162). The reduced forms of 1-deazariboflavin, A(3)-methyllumiflavin, 7,8-dimethyl-1,10-ethyleneisoalloxazinium perchlorate, and 5-deazariboflavin were generated anaerobically with excess dithionite. Of these four flavins, cleavage of cij-syn-[me//iy/- H]thymine dimer to [methyl- H]-thymine was observed with reduced 1-deazariboflavin, and to a lesser extent with... 

DNA Photolyase. - Ultraviolet light (> 300 nm) damages cellular DNA by the formation of thymine dimers from adjacent pyrimidine bases on the same DNA strand. The two main photoproducts are (cis,syn)-cyclobutane pyrimidine dimers (CPDs) and the (6-4) photoproduct. Such dimers may be restored to their monomeric form through the action of two photoactive (300 < X < 500 nm) damage-specific DNA repair enzymes, named CPD photolyase (also called DNA photolyase) and (6-4) photolyase. These enzymes are found in various... 

The effect of phase upon the monomer-dimer equilibrium is pronounced. The quantum yields for dimer formation in liquid-aerated water solution are low (from zero for thymine to 10"2 for other pyrimidines) but the quantum yields for dimer formation in frozen aqueous solutions or in single crystals are much higher (reaching unity in frozen water solution for thymine). The quantum yields for monomerization are uniformly high and are about the same in solution or in solid phase. The net result of this phase effect is that even at optimum wavelengths for dimer formation, the yields of dimers are low in solution and high in solid phases, for all the single bases, nucleosides, or nucleotides. 

The increase in absorbance at 240 nm upon saturation of the 5,6 double bond is considerably more pronounced for cytosine than for thymine and uracil. Thus the cross section of the cytosine dimer for monomerization at 240 nm is high, and the equilibrium concentration of dimer at various wavelengths is smaller for cytosine than for the other two pyrimidines.57... 

When Nm is the concentration of monomeric dinucleotide (pTpT), Nd is the concentration of dinucleotide with dimerized thymine residues, and Nm0 is the total concentration of dinucleotide, it can be shown that the concentration of Nm at a particular dose, D, of ultraviolet light is given by eq. (8). 

Photodimerization of thymin to butane type dimers is one of the important causes of UV radiation damage to genetic material DNA in its native double helix form. The reaction occur from the triplet state as confirmed by acetone sensitized photodimerization of thymine. These dimers can be monomerized when irradiated by short wavelength radiations. which are absorbed by the dimer only. 

In a series of our interaction studies, the photodimerization of thymine bases along the polymer chain was carried out to estimate the degree of intramolecular selfassociation of the bases (Fig. 14). The photodimerization reaction was also studied for monomeric and dimeric model compounds (Fig. 15)27). The formation of thymine photodimers by UV irradiation onto nucleic add is well-kown and the photodimerization of nucleic add, its model compounds and pyrimidine bases has been reported by many groups, though the work on the synthetic polymers containing thymine bases has hitherto been unknown. 

Thymine derivatives " are very resistant to UV irradiation in dilute solution because the triplet state is the precursor of the photodimer and intersystem crossing of thymine is very low. The yield of the photodimer in solution is low, but a high yield of photodimer can be obtained with acetone as the sensitizer. In DNA polymers and solid state, however, the photodimerization reaction is fast and proceeds through the singlet state. The photodimerizations of thymine oHgomers and polymers were studied in the presence of isoprene as a triplet quencher. The photodimerization of the thymine, monomeric and dimeric model, were quenched by isoprene, but the photodimerization of a polymer with thymine units was not quenched. This result appears to indicate that the photodimerization of aggregated thymine compounds occurs almost completely from the singlet state. ... 

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