Substitution of exocyclic oxygen with sulfur was shown to substantially influence the properties of RNA/DNA bases, which are crucial for prebiotic chemistry and photodynamic therapies. Upon UV irradiation, thionucleobases were shown to efficiently populate triplet excited states and can be involved in characteristic photochemistry or generation of singlet oxygen. Here, we show that the photochemistry of a thionucleobase can be considerably modified in a nucleoside, that is, by the presence of ribose. Our transient absorption spectroscopy experiments demonstrate that thiocytosine exhibits 5 times longer excited-state lifetime and different excited-state absorption features than thiocytidine. On the basis of accurate quantum chemical simulations, we assign these differences to the dominant population of a shorter-lived triplet nπ* state in the nucleoside and longer-lived triplet ππ* states in the nucleobase. This explains the distinctive photoanomerziation of thiocytidine and indicates that the nucleoside will be a less efficient phototherapeutic agent with regard to singlet oxygen generation.

Department of Astronomy Harvard Smithsonian Center for Astrophysics 60 Garden Street Cambridge Massachusetts 02138 United States

Department of Physical and Quantum Chemistry Faculty of Chemistry Wrocław University of Science and Technology Wybrzeże Wyspiańskiego 27 50 370 Wrocław Poland

EaStCHEM School of Chemistry University of Edinburgh Joseph Black Building David Brewster Road Edinburgh EH9 3FJ U K

Howard Hughes Medical Institute Department of Molecular Biology and Center for Computational and Integrative Biology Massachusetts General Hospital 185 Cambridge Street Boston Massachusetts 02114 United States

Institute of Biophysics Czech Academy of Sciences Královopolská 135 61265 Brno Czech Republic

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacky University Olomouc Slechtitelu 241 27 783 71 Olomouc Holice Czech Republic

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