Time-Resolved Study of Thymine Dimer Formation

The formation of thymine dimers in the single-stranded oligonucleotide, (dT)₂₀, is studied at room temperature by laser flash photolysis using 266 nm excitation. It is shown that the (6-4) adduct is formed within 4 ms via a reactive intermediate. The formation of cyclobutane dimers is faster than 200 ns. The overall quantum yield for the (6-4) formation is (3.7 ± 0.3) × 10^-3, and that of the cyclobutane dimers is (2.8 ± 0.2) × 10^-2. No triplet absorption is detected, showing that either the intersystem crossing yield decreases by 1 order of magnitude upon oligomerization (<1.4 × 10^-3) or the triplet state reacts with unit efficiency in less than 200 ns to yield cyclobutane dimers.