The temperature–pressure phase diagram of a DPPC–ergosterol fungal model membrane—a SAXS and FT-IR spectroscopy study

We investigated the phase behaviour of the binary lipid mixture dipalmitoylphosphatidylcholine (DPPC)–ergosterol (78:22 mol%) as a function of temperature and pressure using Fourier transform-infrared (FT-IR) spectroscopy and small-angle X-ray scattering (SAXS). A temperature range from 10 to 70 °C and a pressure range from 1 to ∼8500 bar was covered to be able to establish a p,T-phase diagram of this model mixture of a fungal plasma membrane. Our results show a liquid-ordered/solid-ordered (lo + so) two-phase coexistence region up to 41 °C, followed by a liquid-disordered/liquid-ordered (ld + lo) coexistence region up to ∼57.5 °C, giving way to an all-fluid-like ld phase at higher temperatures. With increasing pressure, all phase boundaries shift upwards to higher temperatures. The width of the raft-like ld + lo two-phase coexistence region is found to be about 2 kbar over the temperature range covered. At higher pressures, transitions to a lo + so two-phase region and, at still higher pressures and low temperatures, an all-ordered so phase is detected. Finally, the biological (high pressure-induced upregulation of ergosterol synthesis) and biotechnological (inactivation of fungal microorganisms) relevance of these studies is discussed. The results may be particularly interesting for understanding the function of membrane proteins under external stress conditions, such as high pressure. Interestingly, in this pressure range of about 2 kbar, ceasing of membrane protein function has been observed in natural membrane environments. They may be inactivated under pressure/temperature conditions when the plasma membrane becomes too rigid, i.e., reaches an overall ordered conformational state.