A Series of β‑Carboline Alkaloids from the Seeds of <i>Peganum harmala</i> Show G‑Quadruplex Interactions

In this study, we screened 17 medicinal plants for binding activity to G-quadruplex d­(TTGGGTT)4 by 1H NMR spectroscopy and found that the crude extract of Peganum harmala L. seeds showed the most potential binding activity. Subsequently, 1H NMR- and bioassay-guided isolation of the extract of P. harmala L. was performed to obtain four pairs of partially racemized β-carboline alkaloids, pegaharmines A–D (1–4). Their structures and absolute configurations were determined by extensive NMR analyses, X-ray crystallography, ECD calculations, and CD exciton chirality approaches. Interestingly, pegaharmine D (4), which showed the strongest G-quadruplex interaction, exhibited significant cytotoxic activity against three cancer cell lines. This work contributed a practical strategy for the discovery of novel G-quadruplex ligands from natural products and provided potential insights for using β-carboline alkaloids as anticancer lead compounds specifically targeting G-quadruplexes.