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.