Structural elucidation of a unique macrocyclic membrane lipid from a new, extremely thermophilic, deep-sea...

The membrane lipid of a new deep-sea hydrothermal vent methanogen, Methanococcus jannaschii, was isolated, purified, and structurally characterized. The total lipid extract, amounting to 32.2 micrograms/mg, dry cell weight, was fractionated on silica gel into a neutral lipid and polar lipid fraction. The neutral lipid fraction consisted of a series of isoprenoid hydrocarbons and free (nonphospholipid) alkylglycerol ethers. The polar phospholipid and glycolipid fraction (8.44 micrograms/mg, dry cell weight) was hydrolyzed with methanolic HCl, and the resulting alkylglycerol ethers were analyzed by a combination of chemical and spectroscopic techniques. The hydrolyzed polar lipid was primarily (95%) a unique, macrocyclic glycerol diether, heretofore unknown. High-field (250 MHz) proton nuclear magnetic resonance and infrared spectra of this novel macrocyclic compound are nearly identical and overlapping those of the known bis-(phytanyl)glycerol diether and bis-(diphytanyl)diglycerol tetraether. A field desorption mass spectrum revealed a molecular weight of 650 for the macrocyclic glycerol diether, 2 mass units less than that of bis-(phytanyl)glycerol diether. Degradation of the macrocyclic ether with boron tribromide resulted in diphytanyl dibromide, and further reaction of this dibromide with lithium aluminium hydride resulted in diphytane as determined by gas chromatography-mass spectrometry. The significance of the predominance of this structure in M. jannaschii is discussed. A survey of selected methanogenic Archaebacteria, including three thermophiles, failed to indicate the presence of the macrocyclic glycerol diether in any other microorganism, including two species of order Methanococcales, one species of Methanobacteriales, and three strains belonging to the order Methanomicrobiales.