When measuring isobars (Fig. 2) THBH shows no phase transition in the applied temperature range, while DPPC shows the known phase transition, Also while measuring isobars beyond the phase transition temperature of DPPC, a condensation of DPPC molecules was observed in the presence of THBH. In the presence of various amounts of THBH a quenching of the phase transition of DPPC and a shift to lower temperatures is observed (Fig. 2). Condensation of lipids and phase transition quenching by a hopanoid, isolated from a prokaryote, were so far only demonstrated for sterols occurring in prokaryotes. Very recently similar properties in combination with DPPC could be shown for the glycolipid of THBH [71. These properties of hopanoids are probably the basis for the increased stability and diminished permeability of the cytoplasmic membrane of the thermo-acidophilic B. acidocaldarius. In further experiments we shall investigate these possible consequences on the permeability of liposomes containing THBH and on physiological properties of
B. acidocaldarius cells containing different amounts of hopanoids.
It is also of interest to note that our results corroborate the idea of Ourisson, Rohmer, and Albrecht [1,2] that the hopanoids are one group of compounds which can be candidates for phylogenetic precursors of ster01s in the primordial atmosphere devoid of molecular oxygen. This work was supported by the Deutsche Forschungsgemeinschaft (Po117/6). We thank H. Zfihner (SFB 76) for facilities for culturing large amounts of bacteria and J. Schultz for improving our English.