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HR-ESI-MS/MS spectra of 1 and 2, and interpretation of the fragment ions. HR-ESI-MS/MS spectra of 1 and 2 were measured by setting the [M−H]− ions (1: m/z 973.1077, 2: m/z 973.1080) as precursor ions in negative mode. The methanolic solution (1 μL) of purified 1 (2 ng) and 2 (2 ng) were applied to an ESI-Q-TOF MS spectrometer (see Section 4.4 for details).
