Functionalized magnetic particles coupled with LC-MS strategy facilitated discovery of trace thioalkaloids with potent immunosuppressive activity

[Display omitted] •Magnetic particles integrated LC-MS strategy guides isolation of trace compounds.•Fifteen trace alkaloids including seven new ones were obtained from Nuphar pumila.•The ΔδC of methylenes at sulfur-ring concludes the stereochemistry of spiroatoms.•The immunosuppressive effects of Nuphar dimeric alkaloids were revealed. Trace natural products (TNPs) are still the vital source of drug development. However, the mining of novel TNPs is becoming increasingly challenging due to their low abundance and complex interference. A comprehensive strategy was proposed in which the functionalized magnetic particles integrated with LC-MS for TNPs discovery. Under the guidance of the approach, fifteen trace Nuphar alkaloids including seven new ones, cyanopumiline A sulfoxide (1), cyanopumiline C sulfoxide (8) and cyanopumilines A–E (4–5, 10, 12–13) featuring an undescribed nitrile-containing 6/6/5/6/6 pentacyclic ring system were isolated from the rhizomes of Nuphar pumila. Their structures and absolute configurations were determined on the basis of detailed spectroscopic data analysis and single-crystal X-ray diffraction analysis. Notably, a concise method based on 13C NMR spectroscopy was established to determine the relative configurations of spiroatoms. Biologically, compounds 1–12 exhibited potent immunosuppressive activities with IC50 values ranging from 0.1–12.1 μM against anti-CD3/CD28 induced human peripheral T cell proliferation. Mechanistic studies revealed that 4 could dose-dependently decrease pro-inflammatory cytokines and the expression levels of CD25 and CD71.