Characteristics of the excited triplet states of thiolated guanosine derivatives and singlet oxygen generation

Abstract

Thioguanine is sensitive to UVA light and generates singlet molecular oxygen (¹O₂*) when exposed to UVA. Three thioguanosine derivatives, 2′,3′,5′-tri-O-acetyl-6-thioguanosine (ta6TGuo), 2′,3′,5′-tri-O-acetyl-8-thioguanosine (ta8TGuo), and 2′,3′,5′-tri-O-acetyl-6,8-dithioguanosine (taDTGuo) were explored photophysically and photochemically. Nanosecond transient absorption and time-resolved near-infrared emission measurements were carried out to investigate the characteristics of their excited triplet states in acetonitrile solution. The quantum yield of intersystem crossing (Φ_ISC), the intrinsic decay rate constant (k₀), the quenching rate constant by ³O₂ (k_q) and the self-quenching rate constant (k_SQ) of their triplet states were all determined. From the precise analysis of the quantum yield of ¹O₂* generation (Φ_Δ) against the concentration of dissolved molecular oxygen, the fraction of the triplet states quenched by dissolved oxygen which gives rise to ¹O₂* formation (S_Δ) was successfully obtained with high accuracy. The Φ_Δ values at low oxygen concentrations reveal that these thioguanosines, particularly taDTGuo, can still effectively generate ¹O₂* at low molecular oxygen concentrations like carcinomatous microenvironments. These findings indicate that taDTGuo would perform well as a potential agent for photo-induced cancer therapies.