Thymine dimer photoreversal in purine-containing trinucleotides

Area of Science:

• Photochemistry
• DNA Damage and Repair
• Molecular Biophysics

Background:

• Cyclobutane-pyrimidine dimers (CPDs) are major UV-induced DNA lesions.
• CPD formation is influenced by neighboring bases, with purines often suppressing yields.
• The mechanism behind this suppression, particularly potential repair via electron transfer, remains unclear.

Purpose of the Study:

• To investigate if electron transfer from adjacent purine bases repairs CPDs.
• To quantify the yields and dynamics of the reverse photoreaction in CPDs.
• To elucidate the role of neighboring bases in UV-induced DNA damage and repair.

Main Methods:

• Studied trinucleotides with a cis-syn dimer (T<>T) flanked by adenine or guanine.
• Varied irradiation wavelength (240-280 nm) to selectively excite the dimer or purine base.
• Utilized pump-probe spectroscopy to measure excited-state lifetimes and reaction dynamics.

Main Results:

• Cleavage quantum yields decreased significantly with increasing wavelength, becoming <1% at 280 nm.
• Excitation of the purine base (>95% probability at 280 nm) did not enhance dimer cleavage.
• Conditional quantum yields of cleavage were similar to unmodified T<>T, indicating direct photoreversal.
• Excited-state lifetimes of adenine or guanine were unperturbed, ruling out significant electron transfer to the dimer.
• Dimer cleavage occurred on a picosecond or subpicosecond timescale.

Conclusions:

• Photoreversal of CPDs in these substrates is primarily due to direct electronic excitation of the dimer itself.
• Photolyase-like repair via electron transfer from adjacent purines is negligible.
• Purine bases may exert a modest quenching effect on the dimer's excited state, influencing photoreversal yields.