Isolation and Characterization of [DLeu]microcystin-LY from CPCC-464.

[D-Leu]MC-LY () ([M + H]/ 1044.5673, Δ 2.0 ppm), a new microcystin, was isolated from strain CPCC464. The compound was characterized by H and C NMR spectroscopy, liquid chromatography-high resolution tandem mass spectrometry (LC-HRMS/MS) and UV spectroscopy. A calibration reference material was produced after quantitation by H NMR spectroscopy and LC with chemiluminescence nitrogen detection. The potency of in a protein phosphatase 2A inhibition assay was essentially the same as for MCLR (). Related microcystins, [D-Leu]MC-LR () ([M + H]/ 1037.6041, Δ 1.0 ppm), [D-Leu]MC-M(O)R () ([M + H]/ 1071.5565, Δ 2.0 ppm) and [D-Leu]MC-MR () ([M + H]/ 1055.5617, Δ 2.2 ppm), were also identified in culture extracts, along with traces of [D-Leu]MC-M(O)R () ([M + H]/ 1087.5510, Δ 1.6 ppm), by a combination of chemical derivatization and LC-HRMS/MS experiments. The relative abundances of , , , and 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-Leu]-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.