Physicochemical characterization of tetraether lipids from Thermoplasma acidophilum. Calorimetric studies on miscibility with diether model lipids carrying branched or unbranched alkyl chains

Abstract

The thermal properties of the main tetraether lipid of Thermoplasma acidophilum membranes, as monitored by differential thermoanalysis of hydrated samples, are compared with those of two synthetic diether glucolipids one of which is comprised of two phytanyl chains as apolar moieties, the other of two palmityl chains. The tetraether lipid undergoes a broad phase transition between −30°C and −5°C; the enthalpy change of the transition is low, amounting to −14 kJ · mol −1. Among the two model lipids, diphytanylglucosylglycerol does not show any phase transition between −60°C and 80°C. In contrast, dipalmitylglucosylglycerol passes a sharp transition at 62°C, which is correlated with a large enthalpy change of ΔH = −98 kJ · mol −1 . Thus, it is concluded that the presence of repetitive methyl branches along the hydrocarbon chains hinders the formation of condensed structures. At very low temperatures the two branched-chain lipids undergo a thermal transition which is not observed with the straight-chain lipid. This transition is connected with a considerable change in heat capacity. The upper and lower temperature limits of the transitions are −50°C and −90°C for the tetraether lipid and −61°C and −84°C for diphytanylglucosylglycerol. The changes in heat capacity amount to 1.26 kJ · mol −1 · K −1 (tetraether lipid) and 74 kJ · mol −1 · K −1 (diether lipid), respectively. Phenomenologically, these transitions appear to be glass transitions. Studies on lipid mixtures reveal that the bipolar tetraether lipid is able to form mixed phases with the monopolar diphytanylglucosylglycerol. In contrast, the miscibility of tetraether lipid with dipalmitylglucosylglycerol is limited depending on the experimental conditions. The miscibility of fluid tetraether lipid with dipalmitylglucosylglycerol in the gel state is very low. It is significantly increased at temperatures above 62°C which is the transition temperature of the diether lipid. However, if mixtures prepared above 62°C are cooled to lower temperatures, a metastable state is formed which slowly transforms into a stable state.