Two Acyclic Kahalalides from the Sacoglossan Mollusk Elysia rufescens

The sacoglossan mollusk Elysia rufescens elaborates six cyclic depsipeptides, kahalalides A−F. Its green algal diet ( Bryopsis sp.) contains kahalalide G, which is an acyclic analogue of kahalalide F. Further analysis of the molluskan extract revealed two new acyclic peptides, kahalalide H ( 1 ) and kahalalide J ( 2 ), which share only four amino acids (leucine, phenylalanine, serine, and valine) with kahalalide F. Both contain aspartic acid and 4-hydroxyproline residues, and kahalalide J ( 2 ) also contains lysine. They have in common a β-hydroxy fatty acid, 3-hydroxy-9-methyldecanoic, previously encountered in kahalalide E. For kahalalide H ( 1 ) we succeeded in determining the sequential positions of the antipodal d - and l -phenylalanine. In common with the acyclic constituent of the alga, kahalalide G, the new compounds lack significant cytotoxicity.