Monolayer properties of archaeol and caldarchaeol polar lipids of a methanogenic archaebacterium, Methanospirillum hungatei, at the air/water interface

Abstract

Monolayer studies at the air/water interface were carried out on the major tetraether (caldarchaeol-) derived phosphoglycolipid, Glc p- α(1-2)-Gal f- β(1-1)-caldarchaeol-phosphoglycerol (PGC-I), the major diether (archaeol-) derived glycolipid, Glc p- α(1-2)-Gal f- β(1-1)-archaeol (DGA-I), the major archaeol-derived phospholipids, phosphatidyl- N, N dimethylaminopentanetetrol (PPDAA) and phosphatidyl- N, N, N-trimethylaminopentanetetrol (PPTAA) and the minor caldarchaeol-derived glycolipid, Glc p- α(1-2)-Gal f- β(1-1)-caldarchaeol (DGC-I) isolated from the methanogenic archaebacterium, Methanospirillum hungatei. The compression isotherms obtained showed that the two tetraether lipids had molecular surface areas about twice those of the diether lipids at all surface pressures, suggesting that both polar headgroups of the tetraether lipids are anchored into the aqueous subphase, even at the collapse pressure π c. A U-shaped hydrocarbon chain conformation thus appears to be preferred for the tetraether lipids at the air/water interface, rather than an extended chain arrangement. The compression isotherms of the two tetraether lipids PGC-I and DGC-I were very similar at pH 0, both molecules being uncharged, but at pH 5.6 or 8, PGC-I films were much more expanded than the neutral DGC-I, due to ionization of the phosphate group in PGC-I and the resulting charge-charge repulsion. Monolayers of the zwitterionic diether phospholipids PPDAA and PPTAA were much less compressible than the glycosylated lipids, PGC-I, DGC-I and DGA-I, because the latter lipids contain the more compressible diglycosyl headgroup, oriented in horizontal conformation at low surface pressures, compared to the lower compressibility of the zwitterionic headgroup in the vertical conformation, particularly at pH 0 and 5.6. Values of the collapse molecular surface area ( A c), collapse pressure (π c), limiting molecular area ( A o) and surface compressional modulus [(C SO) −1], obtained from the compression isotherms, are reported and the effects of polar headgroup conformation (vertical or horizontal) have been calculated from molecular models for various types of surface lattices.