Ether lipids from Botryococcus braunii and their biosynthesis

Abstract

Six new series of ether lipids, named botryococcoid ethers, have been isolated from the green microalga Botryococcus braunii (A race). These high molecular weight lipids are structurally related to compounds previously identified in this alga, such as n-alkadienes, very long chain alkenylphenols and botryals (C 52-C 64 α-branched, α-unsaturated aldehydes). The structures were determined by a combination of methods including NMR, mass spectrometry and size exclusion HPLC, as di- O-alkenyl ethers, alkenyl- O-alkylphenol ethers, alkenyl- O-botryalyl ethers and alkenyl- O-epoxybotryalyl ethers. All these compounds possess two very long chains ether-linked at C-9′ and C-10, one hydroxyl at C-10′ and a second hydroxyl, or an ester group, at C-9. The presence of oxygen functions at C-9, C-10, C-9′ and C-10′ in the two hydrocarbon branches suggested that they could be biosynthesized by coupling of two epoxides, normally synthesized by the alga, and derived from the parent compounds. On the basis of data from feeding experiments with 14C-labelled cis-9,10-epoxynonacos-28-ene which show the incorporation of radioactivity into the ether lipids without prior degradation into acetate units, a biosynthetic mechanism is proposed, with epoxides as close precursors.