Laboratório de Estudos Computacionais em Sistemas Moleculares, eCsMolab, Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Pampulha, 31270-901 Belo Horizonte, Minas Gerais, Brazil.
J Phys Chem B. 2021 Aug 12;125(31):8932-8943. doi: 10.1021/acs.jpcb.1c05463. Epub 2021 Jul 29.
Photosensitizer (PS) molecules play a critical role in photodynamic therapy of cancer and the understanding of the molecular mechanism involved in the photophysics of these compounds, and their reactions in the excited state are, therefore, of great interest for the development of this technique. In this article, the photophysics of the cationic PS 4,5-dibromorodamine methyl ester (TH9402), its electron- and energy-transfer reactions in the excited triplet state, with molecular oxygen, nitric oxide, guanosine-5'-monophosphate (GMP), and guanine, and the interaction with DNA were evaluated. Time-dependent density functional theory calculations at the TPSSh/Def2-TZVP//B3LYP/Def2-TZVP level of theory in water solution reveals that the PS has a bright S state 2.33 eV above the ground state that produces a fluorescent rate constant of 5.40 × 10 s, calculated using Fermi's golden rule within a path integral formalism. Once excited to the bright state, the main intersystem crossing (ISC) channel involves the coupling with the T state just below S (S → T → T) with an overall ISC rate constant of 10.1 × 10 s, in good agreement with the experimental data. Excited-state reaction thermodynamics, computed at the M06-2X/Def2-TZVP//B3LYP/Def2-TZVP level of theory in water, showed that from all the excited-state electron-transfer reactions studied, only the transfer from GMP to the PS is thermodynamically favorable, independent of the protonation state of guanosine, which indicates a possible DNA photo-oxidation mechanism for the PS. Triplet-triplet energy-transfer reactions from TH9402 to molecular oxygen, producing reactive singlet oxygen, and to the deprotonated guanosine, producing GMP, are also thermodynamically favorable, with Δ = -2.0 and -24.0 kcal//mol, respectively. However, the energy transfer to the monoprotonated guanosine is not favorable, (Δ = 36.1), suggesting that in the DNA double-strand environment, this energy-transfer process may not be observed. The results show that the PS can act through electron transfer and triplet-triplet energy-transfer reactions involved in mechanism types I and II in photodynamic therapy. Interactions of TH9402 with the d(AGACGTCT) octanucleotide revealed that the PS can intercalate between the d(GpC)-d(CpG) base pairs in three different orientations and, upon intercalation, the π → π* transition of the PS shows a bathochromic shift up to 90 nm and up to 60% decrease in intensity. Interactions through groove binding showed a smaller bathochromic shift of 52.2 nm and a 56% decrease in intensity of the main transition band.
光敏剂 (PS) 分子在癌症的光动力疗法中起着关键作用，并且了解这些化合物光物理过程中涉及的分子机制及其在激发态下的反应对于该技术的发展非常重要。在本文中，评估了阳离子 PS 4,5-二溴罗丹明甲酯 (TH9402) 的光物理性质，及其在激发三重态下与分子氧、一氧化氮、鸟苷-5'-单磷酸 (GMP) 和鸟嘌呤的电子和能量转移反应，以及与 DNA 的相互作用。在水溶液中，使用时间相关密度泛函理论 (TD-DFT) 在 TPSSh/Def2-TZVP//B3LYP/Def2-TZVP 理论水平上进行计算，结果表明 PS 具有明亮的 S 态，比基态高出 2.33 eV，产生荧光速率常数为 5.40×10 s，这是通过路径积分形式中的费米黄金规则计算得出的。一旦被激发到明亮状态，主要的系间窜越 (ISC) 通道涉及与 S 态以下的 T 态的耦合（S→T→T），整体 ISC 速率常数为 10.1×10 s，与实验数据吻合良好。在水相中使用 M06-2X/Def2-TZVP//B3LYP/Def2-TZVP 理论水平计算的激发态反应热力学表明，在所研究的所有激发态电子转移反应中，只有从 GMP 到 PS 的转移是热力学有利的，与鸟苷的质子化状态无关，这表明 PS 可能存在一种 DNA 光氧化机制。TH9402 到分子氧的三重态-三重态能量转移反应，产生活性单线态氧，以及到去质子化的鸟苷，产生 GMP，也是热力学有利的，分别为 Δ=-2.0 和-24.0 kcal/mol。然而，向单质子化鸟苷的能量转移是不利的（Δ=36.1），这表明在 DNA 双链环境中，这个能量转移过程可能不会发生。结果表明，PS 可以通过电子转移和三重态-三重态能量转移反应参与光动力疗法的 I 型和 II 型机制。TH9402 与 d(AGACGTCT) 八核苷酸的相互作用表明，PS 可以在三个不同的方向插入 d(GpC)-d(CpG)碱基对之间，并且在插入后，PS 的π→π*跃迁会发生红移，最大可达 90nm，强度降低 60%。通过沟结合的相互作用，主跃迁带的红移为 52.2nm，强度降低 56%。
