Some effects of hydrostatic pressure, inert gases and anaesthetics on lipid bilayer
The phase transition in Dipalmitoyl phosphatidylcholine (DPPC) liposomes was studied using a light absorbance technique. Pure hydrostatic pressure increased the transition temperature by 0.024°K/ATM. Helium, nitrogen, nitrous oxide, cyclopropane and n-propane all reduced transition temperature by 1°K, from an equivalent hydrostatic compression, at membrane concentrations, expressed in MOL%, of 10.2, 13.2, 9.04, 12.8 and 6.3 respectively. Nitrous oxide had identical effects on 16.5 MOL% cholesterol/DPPC liposomes. The helium and nitrous oxide effects on pure DPPC liposomes were confirmed using differential thermal analysis. The ATP-ase activity of membrane fragments of ACHOLEPLASMA LAIDLAWII was monitored by measuring inorganic phosphate release over a thirty minute period at various temperatures. Arrhenius plots of activity were constructed and the observed discontinuities contained therein, could be accommodated either by proposals of a membrane phase transition, or direct inactivation of the enzyme. Arrhenius profiles were described in the enzyme inactivation proposal, using enthalpy entropy free volume changes. The membrane phase transition suggestion was investigated further by using principles derived from the liposome experiments.