[D-Leu1]MC-LY (1) ([M + H]+m/z 1044.5673, Δ 2.0 ppm), a new microcystin, was isolated from Microcystis aeruginosa strain CPCC464. The compound was characterized by 1H and 13C NMR spectroscopy, liquid chromatography-high resolution tandem mass spectrometry (LC-HRMS/MS) and UV spectroscopy. A calibration reference material was produced after quantitation by 1H NMR spectroscopy and LC with chemiluminescence nitrogen detection. The potency of 1 in a protein phosphatase 2A inhibition assay was essentially the same as for MCLR (2). Related microcystins, [D-Leu1]MC-LR (3) ([M + H]+m/z 1037.6041, Δ 1.0 ppm), [D-Leu1]MC-M(O)R (6) ([M + H]+m/z 1071.5565, Δ 2.0 ppm) and [D-Leu1]MC-MR (7) ([M + H]+m/z 1055.5617, Δ 2.2 ppm), were also identified in culture extracts, along with traces of [D-Leu1]MC-M(O2)R (8) ([M + H]+m/z 1087.5510, Δ 1.6 ppm), by a combination of chemical derivatization and LC-HRMS/MS experiments. The relative abundances of 1, 3, 6, 7 and 8 in a freshly extracted culture in the positive ionization mode LC-HRMS were ca. 84, 100, 3.0, 11 and 0.05, respectively. These and other results indicate that [D-Leu1]-containing MCs may be more common in cyanobacterial blooms than is generally appreciated but are easily overlooked with standard targeted LC-MS/MS screening methods.
Keywords: PP2A inhibition; cyanobacteria; cyanotoxin; liquid chromatography; mass spectrometry; microcystin; structure.