Characteristics of the Excited Triplet States of Thiolated Guanosine Derivatives and Singlet Oxygen Generation

Miyata, Shoma; Tanabe, Shunsuke; Isozaki, Tasuku; Xu, Yao-Zhong and Suzuki, Tadashi (2018). Characteristics of the Excited Triplet States of Thiolated Guanosine Derivatives and Singlet Oxygen Generation. Photochemical & Photobiological Sciences, 17(10) pp. 1469–1476.

DOI: https://doi.org/10.1039/C8PP00240A

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. The 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 (∅_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 precise analysis of the quantum yield of ¹O₂* generation (∅_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 ∅_Δ values at low oxygen concentrations reveal these thioguanosines, particularly taDTGuo, can still effectively generate ¹O₂* under low molecular oxygen concentration like carcinomatous microenvironments. These findings indicate that taDTGuo would perform well as a potential agent for photo–induced cancer therapies.