Properties of Thymine Dimers

Abstract

Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross‐sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross‐sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.

Photochemical cross sections in cytidylyl-(3′-5′)-cytidine

“…In spite of those reservations we believe that the dimer reverse cross sections and extinction coefficients are of the right order of magnitude since the quantum yield which is based on the ratio of these quantities is very nearly 1.0 (see Fig. 6a) and sensibly independent of wavelength, a property which we have observed for all dimers so far studied (21).…”

“…Figure 7 shows the extinction coefficient of CpC and two of the five dimers of thymine (type C* and B) which have been isolated (23) and studied (21) in our laboratory. Of the five dimers of thymine, dimers B and C+ have the largest extinction at 230 mp, but this extinction is smaller by a factor of nearly 4 than that of the mixture of CpC dimers, as shown in Fig.…”

Splitting of Cis-syn Cyclobutane Thymine-thymine Dimers by Radiolysis and Its Relevance to Enzymatic Photoreactivation

“…This is unlikely to be due to any change in the state of protonation of the dimer (pKa =10 . 7) for deprotonation (Herbert et al 1969) and by comparison with other thymines pK a for protonation is expected to be <2) . Even at the P. F. Heelis et al . lowest pH used (pH 3 .5, 2 .3 x 10 -3 moldm -3 dimer) around 80% of the solvated electrons react with the dimer, rather than H + (k(e aq -+H + =2x10'0, k(eaq -+ dimer) =1.5 x 1010 mol -1 dm s _ 1) (Santus et al 1972) .…”

Pyrimidine Dimers: UV‐Induced DNA Damage

“…However, Taylor and coworkers [64] re-evaluated its structure by 1 H À 13 C heteronuclear NMR experiments, and proposed a new structure containing an eight-membered ring (15), which was produced by a ring-expansion reaction of the cyclobutane intermediate, as shown in Figure 5.3. Recently, the reaction mechanism for the formation of this photoproduct was analyzed by quantum-chemical calculations [65].…”

“…In this case, the trans-syn-I thymine dimer (3), which is formed between the 5 0 thymidine in the syn conformation and the 3 0 thymidine in the anti conformation, is the major product [12], and the yield of the trans-syn-II product (4) is extremely low [13]. Formation of the thymine dimer reaches a plateau at a high UV dose, because it is a reversible process [14][15][16] that depends on the wavelength. At longer wavelengths (UV-B), the dimer is formed in a relatively efficient manner, but when DNA is irradiated with short-wavelength UV (UV-C), the photoequilibrium is inclined to the reversion of the thymine dimer to the original thymine bases, as shown in Figure 5.1.…”