Ribose Alters the Photochemical Properties of the Nucleobase in Thionated Nucleosides

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.