title: Excited-State Dynamics of the Thiopurine Prodrug 6-Thioguanine: Can N9-Glycosylation Affect Its Phototoxic Activity?
abstract: 6-Thioguanine, an immunosuppressant and anticancer prodrug, has been shown to induce DNA damage and cell death following exposure to UVA radiation. Its metabolite, 6-thioguanosine, plays a major role in the prodrug's overall photoreactivity. However, 6-thioguanine itself has proven to be cytotoxic following UVA irradiation, warranting further investigation into its excited-state dynamics. In this contribution, the excited-state dynamics and photochemical properties of 6-thioguanine are studied in aqueous solution following UVA excitation at 345 nm in order to provide mechanistic insight regarding its photochemical reactivity and to scrutinize whether N9-glycosylation modulates its phototoxicity in solution. The experimental results are complemented with time-dependent density functional calculations that include solvent dielectric effects by means of a reaction-field solvation model. UVA excitation results in the initial population of the S₂(ππ*) state, which is followed by ultrafast internal conversion to the S₁(nπ*) state and then intersystem crossing to the triplet manifold within 560 ± 60 fs. A small fraction (ca. 25%) of the population that reaches the S₁(nπ*) state repopulates the ground state. The T₁(ππ*) state decays to the ground state in 1.4 ± 0.2 μs under N₂-purged conditions, using a 0.2 mM concentration of 6-thioguanine, or it can sensitize singlet oxygen in 0.21 ± 0.02 and 0.23 ± 0.02 yields in air- and O₂-saturated solution, respectively. This demonstrates the efficacy of 6-thioguanine to act as a Type II photosensitizer. N9-glycosylation increases the rate of intersystem crossing from the singlet to triplet manifold, as well as from the T₁(ππ*) state to the ground state, which lead to a ca. 40% decrease in the singlet oxygen yield under air-saturated conditions. Enhanced vibronic coupling between the singlet and triplet manifolds due to a higher density of vibrational states is proposed to be responsible for the observed increase in the rates of intersystem crossing in 6-thioguanine upon N9-glycosylation.
authors.affiliation: Department of Chemistry and Center of Chemical Dynamics, Case Western Reserve University, Cleveland, OH 44106, USA. bga11@case.edu.
authors.forename: Brennan
authors.initials: B
authors.lastname: Ashwood
authors.affiliation: Department of Chemistry, Columbia University, New York, NY 10027, USA. sj67@columbia.edu.
authors.forename: Steffen
authors.initials: S
authors.lastname: Jockusch
authors.affiliation: Department of Chemistry and Center of Chemical Dynamics, Case Western Reserve University, Cleveland, OH 44106, USA. carlos.crespo@case.edu.
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Crespo-Hernández
mesh_terms: D006031:Glycosylation; D006151:Guanosine / Q000031:analogs & derivatives* / Q000737:chemistry; D015394:Molecular Structure; D017319:Photosensitizing Agents / Q000737:chemistry*; D011355:Prodrugs / Q000737:chemistry*; D011789:Quantum Theory; D013872:Thionucleosides / Q000737:chemistry*
chemical_list: D017319:Photosensitizing Agents; D011355:Prodrugs; D013872:Thionucleosides; D006151:Guanosine; C005783:6-thioguanosine
keywords: TD-DFT; excited-state dynamics; glycosylation; low-temperature emission; phototoxicity; prodrugs; singlet oxygen generation; sulfur-substituted DNA bases; transient absorption