Calorimetry of archaeal tetraether lipid—indication of a novel metastable thermotropic phase in the main phospholipid from Thermoplasma acidophilum cultured at 59°C

Abstract

The main glycophospholipid (MPL) from the archaeon Thermoplasma acidophilum is composed of a di-isopranol-2,3-glycerotetraether. The fraction of pentacyclizations of its hydrocarbon chains increases with the growth temperature of the source organism (39 and 59°C), the respective lipids being named MPL39 and MPL59. MPL has a main phase transition between −15 and −30°C. Non-hydrated and hydrated samples of MPL59 have been studied by differential thermal analysis (DTA). Non-hydrated MPL59 does not exert any phase transition. Computer simulation of an unhydrated MPL molecule with four pentacycles and another without pentacyclations demonstrates similar behavior, i.e. the MPL molecules form coils with both polar ends getting closely together. The molecule without pentacyclation coils faster than that with pentacycles. With hydrated samples, DTA scanning conditions were varied. Under certain conditions, the shape of the calorimetric scans, i.e. occurrence of an additional (endotherm) phase transition peak at +17°C and enthalpy changes of the phase transitions indicate a (metastable) solid-analogue phase in MPL59 in addition to the well-known liquid–crystalline phase. Only lipid samples from T. acidophilum with a high degree of acyclic hydrocarbon chains (MPL39) had thus far been reported to form a metastable solid-analogue phase (Blöcher, D., Gutermann, R., Henkel, B., Ring, K., 1990. Biochim. Biophys. Acta 1024, 54–60). A phase transition model is presented for MPL59 which includes the existence of a metastable solid-analogue phase.