7 MORE MICROCYSTINS FROM HOMER-LAKE CELLS - APPLICATION OF THE GENERAL-METHOD FOR STRUCTURE ASSIGNMENT OF PEPTIDES CONTAINING ALPHA,BETA-DEHYDROAMINO ACID UNIT(S)

Larger scale isolation of microcystins, cyclic heptapeptide hepatotoxi ns, from a water bloom of Microcystis spp. collected from Homer Lake ( Illinois) gave the previously reported 1-5, additional quantities of [ L-MeSer(7)]microcystin-LR (6), and microcystin-(H-4)YR [8, (H-4)Y = 1' ,2',3',4'-tetrahydrotyrosine], which were previously isolated in insuf ficient amounts to complete the structure assignment, and seven more m icrocystins, 9-15. A general method for assigning the structures of cy clic peptides containing alpha,beta-unsaturated amino acid unit(s) dev eloped with nodularin, a cyclic pentapeptide hepatotoxin, was applied to confirm the previously assigned structures of 1-5 and to assign the structures of [D-Asp(3)]microcystin-LR (9) and the. new microcystin-H ilR (10, Hil = homoisoleucine). The method consists of linearization o f a cyclic molecule by a one-pot reaction sequence (ozonolysis followe d by NaBH4 reduction) and tandem FARMS (FABMS/CID/MS) analysis of the product (linear peptide). A new microcystin, 11, was assigned the stru cture [L-MeLan(7)]microcystin-LR (MeLan = N-methyllanthionine) and syn thesized from 1 and L-Cys. Four linear peptides 12-15, which are reaso nable biogenetic precursors of the cyclic compounds, were also assigne d structures based on their FABMS/CID/MS data.