CALORIMETRY OF ARCHAEAL TETRAETHER LIPID - INDICATION OF A NOVEL METASTABLE THERMOTROPIC PHASE IN THE MAIN PHOSPHOLIPID FROM THERMOPLASMA-ACIDOPHILUM CULTURED AT 59-DEGREES-C

The main glycophospholipid (MPL) from the archaeon Thermoplasma acidop hilum is composed of a di-isopranol-2,3-glycerotetraether. The fractio n of pentacyclizations of its hydrocarbon chains increases with the gr owth temperature of the source organism (39 and 59 degrees C), the res pective lipids being named MPL39 and MPL59. MPL has a main phase trans ition between -15 and -30 degrees C. Non-hydrated and hydrated samples of MPL59 have been studied by differential thermal analysis (DTA). No n-hydrated MPL59 does not exert any phase transition. Computer simulat ion of an unhydrated MPL molecule with four pentacycles and another wi thout pentacyclations demonstrates similar behavior, i.e. the MPL mole cules form coils with both polar ends getting closely together. The mo lecule without pentacyclation coils faster than that with pentacycles. With hydrated samples, DTA scanning conditions were varied. Under cer tain conditions, the shape of the calorimetric scans, i.e, occurrence of an additional (endotherm) phase transition peak at +17 degrees C an d enthalpy changes of the phase transitions indicate a (metastable) so lid-analogue phase in MPL59 in addition to the well-known liquid-cryst alline phase. Only lipid samples from T. acidophilum with a high degre e of acyclic hydrocarbon chains (MPL39) had thus far been reported to form a metastable solid-analogue phase (Blocher, D., Gutermann, R., He nkel, 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, (C) 1998 Published by Elsevier S cience Ireland Ltd, All rights reserved.