Bueno Hernán Rueda, Pinzón Julio R, Daza Martha C, Doerr Markus
Grupo de Bioquímica Teórica, Universidad Industrial de Santander, Carrera 27, Calle 9, Bucaramanga, Colombia.
Laboratorio de Síntesis Orgánica, Universidad Industrial de Santander, Carrera 27, Calle 9, Bucaramanga, Colombia.
Phys Chem Chem Phys. 2025 May 14;27(19):10376-10386. doi: 10.1039/d4cp04829f.
Pheophorbide a is a dephytylation and demetallation product of chlorophyll a isolated from plants and algae. Pheophorbide a has been used as a photosensitizer to treat microbes, cancer and multidrug resistance. Methylpheophorbide a (MPh) or its methyl ester is another photosensitizer with interesting photophysical properties such as stronger absorption at longer wavelengths compared to the absorption of porphyrins and a high singlet oxygen production quantum yield ( = 0.62). To gain deeper insight into the photophysics of MPh, a computational protocol was employed that allows the elucidation of the photophysical properties of methylpheophorbide a (MPh). This protocol uses Fermi's golden rule within a path integral formalism. Time-dependent density functional theory (TD-DFT) calculations at the CAM-B3LYP/def2-SVP(C-PCM) level of theory were performed. Our calculations reproduce acceptably well the vibronic structure of the Q-band of the absorption spectrum of MPh. After photoexcitation, MPh can decay to the ground state fluorescence or it can undergo intersystem crossing. Three triplet excited states (T, T and T) are found below the S state with an overall spin-vibronic ISC rate constant of 6.14 × 10 s, in good agreement with the experimental value of 7.90 × 10 s. The calculated fluorescence rate is approximately five times higher than the experimental value, which can be attributed to an overestimation of the adiabatic energy of the S state and to the inherent limitations of the approach employed. Consistent with the experimentally observed behavior, our calculations predict that MPh is not phosphorescent.
脱镁叶绿酸a是从植物和藻类中分离出的叶绿素a的脱植基和脱金属产物。脱镁叶绿酸a已被用作光敏剂来治疗微生物、癌症和多药耐药性。甲基脱镁叶绿酸a（MPh）或其甲酯是另一种具有有趣光物理性质的光敏剂，例如与卟啉的吸收相比，在更长波长处具有更强的吸收，并且单线态氧产生量子产率高（= 0.62）。为了更深入地了解MPh的光物理性质，采用了一种计算方法，该方法可以阐明甲基脱镁叶绿酸a（MPh）的光物理性质。该方法在路径积分形式中使用费米黄金规则。在CAM - B3LYP/def2 - SVP(C - PCM)理论水平上进行了含时密度泛函理论（TD - DFT）计算。我们的计算能够较好地重现MPh吸收光谱Q带的振动结构。光激发后，MPh可以通过荧光衰减到基态，或者它可以发生系间窜越。在S态以下发现了三个三重激发态（T₁、T₂和T₃），总的自旋 - 振动系间窜越速率常数为6.14×10⁷ s⁻¹，与7.90×10⁷ s⁻¹的实验值吻合良好。计算得到的荧光速率大约比实验值高五倍，这可以归因于对S态绝热能量的高估以及所用方法的固有局限性。与实验观察到的行为一致，我们的计算预测MPh不产生磷光。
