Photocycloaddition of the T1 excited state of thioinosine to uridine and adenosine

Abstract

Novel photoadducts were obtained by irradiation of thioinosine (6-thiopurine riboside, TI) in deaerated aqueous solution without and in the presence of uridine and adenosine. Excitation (λ > 300 nm) of TI to its excited S₂ state yields a single bimolecular photoproduct. It is a purine–pyrimidine diriboside in which the purine ring is attached to the amide nitrogen of 6-amino-4-thioxo-5-formamidopyrimidine. When TI was irradiated in the presence of an excess of adenosine, two photoproducts were isolated: diribosides of N-(4,6-diaminopirymidin-5-yl)-N-formyl-6-aminopurine and N-(4-amino-6-formylamino-pyrimidin-5-yl)-6-aminopurine, both containing a purine and a formylaminopyrimidine (Fapy) fragment. The photoreaction of TI with uridine gave two regioisomeric photoproducts identified as diribosides containing either 5- or 6-(purin-6-yl)uracil as aglycones. A multistep mechanism leading to the stable photoproducts is proposed. In the first step of the mechanism, the CS group of the excited TI undergoes a [2 + 2] cycloaddition regioselectively to the N(7)C(8) bond of the purine ring or adds in a non-regioselective manner to the C(5)C(6) bond of uracil. The unstable photoproducts thus formed undergo a series of dark reactions at room temperature. The photocycloaddition reactions originate from the excited T₁ state of TI. This conclusion is supported by a combination of evidence from reaction quenching studies using both steady-state quantum yield determinations and kinetics results from nanosecond laser flash photolysis. The T₁ state of TI is quenched by other TI molecules in their S₀ state (self-quenching) and also by uridine and adenosine, all with large rate constants (0.8–5) × 10⁹ M⁻¹ s⁻¹. The quantum yields of the reactions are in general very low (ϕ_R≤ 8 × 10⁻³). The sources of the inefficiency in the photocycloaddition of TI to uridine and adenosine are discussed. The photoproducts containing the Fapy residue undergo deformylation and isomerization of the ribosyl moiety (anomerization, furanose/pyranose transformation) upon heating in aqueous solution. Products of the transformations were identified.