The fluid lamellar phase of lipid bilayers is associated with the normal function of the plasma membrane. The extension of the range of the fluid phase can support an increase of the operational range of environmental conditions for the normal physiological functioning of the membrane. A range of enzymes may be utilized to provide to the lipid bilayer with a means of adapting to changes in environmental conditions. Methyl substitution is particularly common in Archaea and may have useful biotechnological applications.[1, 2] The effect of such substitutions and their localization have been explored with molecular dynamics simulations. In this study we examine and compare the changes in dynamics and structure with humidity and temperature of stacked bilayers of 1,2-diphytanoyl-sn-glycero-3-phosphocholine (diphytanoyl-PC), a lipid containing 4 methyl substitutions on a C16 carbon chain, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) for experimental comparison with simulations. We have utilized the cold triple-axis spectrometer MIRA (Heinz Maier-Leibnitz FRM II, Garching, Germany) as a two-circle neutron diffractometer with low background to study the packing of lipid molecules into bilayer stacks as a function of humidity. The backscattering spectrometer EMU (ANSTO, Lucas Heights, Australia) provides information about the short range motions of lipid chains that occur within the short time scales of < 1ns. DPPC exhibits clear gel-to-fluid transition while diphytanoyl-PC exhibits dynamics consistent with a fluid phase over a wide range of temperatures.
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