Algicidal hydroxylated C18 unsaturated fatty acids from the red alga Tricleocarpa jejuensis: Identification, synthesis and biological activity

Abstract

Bioassay-guided separation of a methanol extract of Tricleocarpa jejuensis by monitoring algicidal activity against the red tide phytoplankton Chattonella antiqua led to the isolation of an active fraction consisting of a mixture of four isomeric compounds. The active compounds were identified as (E)-9-hydroxyoctadec-10-enoic acid (1), (E)-10-hydroxyoctadec-8-enoic acid (2), (E)-11-hydroxyoctadec-12-enoic acid (3) and (E)-12-hydroxyoctadec-10-enoic acid (4) by NMR, IR and mass spectral data. The structures were confirmed by comparison of the NMR and MS data with those of authentic samples of 1–4 obtained by unambiguous syntheses. Synthesized hydroxy acids 1–4 and related compounds were assessed for algicidal activity against C. antiqua and it was found that all of 1–4 had high activity (>80% mortality at 24 h) at a concentration of 20 μg/mL. A structure–activity relationship study using 11 related compounds revealed that the presence of the hydroxyl group is important for the activity and the double bond may be replaced with a triple bond.