Cyanobacteria of Microcystis species have been promptly studied from the environmental, toxicological, biological, and chemical point of views because they are responsible for water blooms frequently producing potent hepatotoxins. During our works on the toxins, we found that an HPLC fraction contained a peptide, which exhibited tyrosinase inhibitory activity. This report describes elucidation of its marvelous structure. The methanol extract of an axenical clonal strain of M. viridis was separated by HPLC to afford a colorless solid, microviridin. Microviridin (1) produced 14 amino acids on hydrolysis. The GCMS analysis using a chiral column indicated that all the amino acids are in an L-form. The structure of 1 was elucidated by spectroscopic analyses including 2D NMR and chemical reactions. Microviridin possesses the novel tricyclic structure containing two ester linkages, which is the first example of tricyclic depsipeptide from natural resources.
