272519 Molecular Packing and Phase Behavior of Heterogeneous Lipid Monolayers in the Presence of n-Butanol
Interest in butanol as a biofuel is increasing due to its physicochemical properties (high energy density, low vapor pressure, hydrophobicity) and the ability to use it directly in gasoline engines. However, there are barriers to producing butanol cost-effectively at the industrial scale. Butanol production via fermentation is limited to low bacterial solvent tolerance due to cell membrane fluidization or destabilization. Bacteria respond to membrane destabilization by adjusting their membrane composition; typically modifying the ratio of saturated to unsaturated lipids (S/U). Nevertheless, mechanisms of lipid restructuring due to butanol partitioning are unclear.
In this research we are examining the effects of n-butanol on the molecular packing, phase behavior, and mechanical properties of monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, a saturated lipid) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, an unsaturated lipid). Surface pressure-area per molecule isotherms were conducted at the air/water interface at 25 oC as a function of the DPPC:DOPC ratio (1:0, 3:1, 1:1, 1:3, and 0:1). DPPC undergoes a liquid expanded (LE) to liquid condensed (LC) phase transition upon compression, whereas DOPC remains in a LE phase.
In the absence of n-butanol, the presence of DOPC within the monolayer increased the surface pressure, relative to pure DPPC, and the position of the LE-LC transition of DPPC. DOPC also decreased the area compressibility modulus of DPPC/DOPC monolayers, denoting a more rigid structure. These results can be attributed lipid immiscibility that arises due to tail mismatch between saturated DPPC and unsaturated DOPC. When n-butanol was added to the sub-phase it partitioned into the monolayers and increased the surface pressure, the effective area per lipid molecule, and compressibility modulus. This effect was most pronounced when DPPC was the continuous phase. However, when DOPC was the continuous phase, n-butanol partitioning had little effect on the isotherm. Differential effects of n-butanol on DPPC and DOPC are attributed to lipid structure. DOPC can accommodate n-butanol within pockets created by tail kinking, while DPPC cannot.