Preparation of chlorophyll compounds and simulation of DFT optical properties assisted screening of photosensitizers from Ginkgo biloba leaves

Chlorophyll compounds (Chls) from Ginkgo biloba leaves (GBL) are non-toxic natural pigments characterized by highly conjugated molecules and special spectra, making them ideally potential photosensitizers (PSs) in photodynamics therapy (PDT). In this study, 4 Chls were isolated from Ginkgo biloba polyprenols (GBPPs) wastes by using silica gel column separation and high performance liquid chromatography (HPLC). In addition, two new chlorophyll derivatives were successfully synthesized with chlorin e6 as the primary raw material. To evaluate and identify superior chlorophyll PSs, Density Functional Theory (DFT) and Density Functional Theory (TD-DFT) were employed to decode the microscopic and spectral properties of Ginkgo biloba Chls (GBChls). The spectral absorption, optical stability and reactive oxygen species (ROS) production of GBChls were further analyzed through comparisons of their frontier molecular orbital layout, charge distribution and excitation energy properties. Our results show that about 63 % absorption in the Q band for 14 GBChls comes from the pi pi* transition from the highest occupied orbital (HOMO) to the lowest unoccupied orbital (LUMO), indicating the favorable delocalization effect of these GBChls. In particular, those GBChls with the long chain of the phytyl group exhibited higher total electron energy and thus could significantly increase the light stability. However, the large spatial volume and hydrophobicity of these compounds with the phytyl group chain was unfavorable for cellular uptake, leading to low ROS in cells. The electronegativity of porphin macrocycles in GBChls positively correlates with the intracellular ROS production rate, meaning that the former likely promotes the energy transfer between PSs and molecular oxygen during type II photoreaction. Among the GBChls, chlorin e6 demonstrated the highest ROS yields in DMF, while purpurin 18 compounds displayed long-wavelength absorption (698 nm), strong Q-band absorption, good optical stability, and high intracellular ROS production. Our findings highlight that chlorin e6 and purpurin 18 are most potential PSs anticancer drugs and offer new research perspectives for the processing and utilization of GBL resources.