Physicochemical characterization of tetraether lipids from Thermoplasma acidophilum. V. Evidence for the existence of a metastable state in lipids with acyclic hydrocarbon chains.

The main glycophospholipid of Thermoplasma acidophilum, grown at 39° C, is composed of a di-isopranol-2,3-glycerotetraether. It has been characterized in hydrated systems by calorimetry. Unlike its equivalent grown at 59° C, it shows complex phase properties, which include at least three different phases, (1) a liquid-analogue state (C), which is stable above 20° C, (2) a metastable solid-analogue state (A) formed by supercooling of the liquid-analogue state (C) and (3) a stable solid-analogue state (B), which is slowly formed and may include a close chain packing of lipids and a network of hydrogen bonds between the headgroups. A high fraction of acyclic isopranol chains seems to be a prerequisite for the formation of state (B). A phase diagram, displaying the observed states and the transitions between them is proposed.