Abstract
Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.
Highlights
In 1950, George Hitchings and Gertrude Elion [1] obtained, by exchanging oxygen with sulphur in nucleobases, a potent drug for the treatment of numerous diseases [2,3,4,5]
Unlike the canonical bases, which quickly repopulate the ground state after UV photoexcitation via an internal conversion (IC) process mediated by conical intersections (CIs) [20,21], the thiobases population mostly decays to a long-lived triplet state with a high quantum yield via an ultrafast intersystem crossing (ISC) process [12,22,23,24]
The intense band from 430 nm to 550 nm was attributed to the phosphorescence emission, which peaks at 480 nm, in reasonable agreement with the vertical excitation energy calculated for the T1(3ππ*) state at 454 nm [45] in water
Summary
In 1950, George Hitchings and Gertrude Elion [1] obtained, by exchanging oxygen with sulphur in nucleobases, a potent drug for the treatment of numerous diseases [2,3,4,5]. Due to the high transfer efficiency to the triplet states and, the high rate of singlet oxygen formation, several applications of the thiobases have been demonstrated They are used as chemotherapeutic agents for the treatment of some types of skin cancer, as they enhance the polymerase chain reaction for DNA/RNA replication, they are used as chromophores for investigating specific interactions between nucleic acids and proteins [25,26,27,28,29,30,31,32,33]. One of the open questions in the ultrafast dynamics of thiobases concerns the mechanism behind the efficient triplet manifold formation by ISC, which occurs shortly after the bright, excited singlet state is populated. Our results demonstrate that the decay channel S2(ππ*)→S1(nπ*)→T1(ππ*), experimentally observed so far only in thiouracils [36,39], is more general and can be found in purine thiobases
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