Monolayer properties of membrane lipids of the extreme halophile Halobacterium cutirubrum, at the air/water interface

Abstract

Compression isotherms relating surface pressure with mean molecular area of the two major membrane lipids of Halobacterium cutirubrum have been recorded at the air/water interface. The lipid constituents studied were diphytanyl ether analogues of ammonium salts of phosphatidylglycerol phosphate and glycolipid sulphate. The influence of subphase pH and NaCl concentration on surface characteristics such as limiting molecular area, collapse pressure, collapse area, surface compressional modulus were investigated. The effect of pH (0 to 5.6) and electrolyte concentration (0.01 to 1 M NaCl) on these parameters is not significant in the case of phosphatidylglycerol phosphate in contrast with glycolipid sulphate where pH and particularly electrolytes cause major changes in film characteristics. There is a marked increase of molecular area and decrease in both collapse pressure and surface compressional modulus of glycolipid sulphate monolayers with increasing subphase NaCl concentration. Theoretical cross-sectional areas of the phytanyl chains and of the polar groups have been calculated for different conformations and these have been used to interpret the experimental data. It is concluded that the polar group of phosphatidylglycerol phosphate is readily accommodated beneath the phytanyl chains and both limiting molecular area and area occupied per molecule at collapse pressure are determined by the chains rather than the polar head group. The collapse area of glycolipid sulphate, however, is determined by its polar head group, which penetrates deeply into the aqueous subphase at low electrolyte concentrations, but at high NaCl concentrations it is oriented up into the interface, presumably, due to a salting out effect. These characteristics are discussed in terms of the properties of the lipids in aqueous dispersions and in purple membrane of Halobacterium cutirubrum.