On the surface of the Earth, DNA oligonucleotides have been exposed to UV-irradiation since very early stages of evolution . The role of UV-light is ambivalent: As it can cause damage, it imposes a selection pressure on oligonucleotides but it can also repair photolesions in a photolyase-like mechanism via charge transfer from adjacent bases . Both processes strongly depend on the oligonucleotide sequence and are likely to have influenced the early selection. Among the canonical nucleotides, Guanine (dG) is the strongest electron donor and therefore plays a major role in the photoinduced self-repair via charge transfer . In this work, we studied the ultrafast photodynamics of short dG-containing oligonucleotides of different sequences and strand lengths with transient UV (266 nm) pump, IR (~5-7 µm) probe spectroscopy . We determined the quantum efficiencies and lifetimes of the short-lived intermediate states and found marker bands which unambiguously identify the intermediates as charge transfer states. Oligonucleotides, which consisted of purines only, showed lifetimes of the charge transfer states on the order of several hundred picoseconds, regardless of the strand length. On the contrary, dinucleotides with mixed purine-pyrimidine sequences, such as d(GT) and d(GC), showed much shorter intermediate lifetimes in the range of 10 ps to 30 ps. However, for all studied oligonucleotides the efficiency of the charge transfer state formation was on the order of ~50%. These findings explain the highly reduced self-repair activity of cyclobutane-pyrimidine dimers in the vicinity of d(GT) and d(GC) in contrast to d(GA) and have implications for the photostability and sequence selectivity of short oligonucleotides under UV exposure in very early stages of evolution.
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