Galactolipids from the temperate red marine alga Gracilariopsis lemaneiformis

Abstract

Chemical examination of the temperate red marine alga Gracilariopsis lemaneiformis from the coast of Oregon has led to the isolation of several new lipids as stabilized peracetate derivatives. The simplest was identical to the recentlv described and novel icosanoid, 12( R), 13( S)-diHEPE, isolated from another red alga, Farlowia mollis. Two more polar metabolites contained either one or two of these diHEPE structural units esterified to a glycerol unit which also contained either a mono- or digalactosyl substituent. The gross structures and relative stereochemical aspects of these major subunits, as well as their placement relative to one another, followed from analysis of various spectroscopic features including 2D NMR data. The structure of the monogalactosyldiglyceride metabolite was deduced as 1,2-bis- O-[12( R*),13( S*)-dihydroxyicosa-5( Z),8( Z),10( E),14( Z),17( Z)-pentaenoyl]-3- O-β- d-galactopyranosyl glycerol. The absolute stereochemistry of the galactose residue in this metabolite was determined from its optical rotation following isolation from the crude acid hydrolysis product. The structure of the digalactosyldiglyceride metabolite was deduced as 1- O-[12( R*),13( S*)-dihydroxyicosa-5( Z),8( Z),10( E),14( Z),17( Z)-pentaenoyl]-2- O-palmitoyl-3- O-[-β- d-galactopyranosyl-6,1α-galactopyranosyl]-glycerol. Isolation of these two metabolites documents the production of both ‘prokaryotic’ and ‘eukaryotic’ forms of galactolipids by this marine plant as well as illustrating a novel lipoxygenase-type metabolism of polyunsaturated fatty acids presumably while esterified to galactosylglycerol, a relatively unusual structural feature in glycerol-based lipids.