Branched-Chain Fatty Acid Content Modulates Structure, Fluidity, and Phase in Model Microbial Cell Membranes.

Recent progress in understanding the importance and origins of lipid rafts in microbial cell membranes has focused attention on membranes containing branched-chain fatty acids. The working hypothesis is that branched fatty acids increase the fluidity of the bilayer, analogous to unsaturated fatty acids in membranes of higher organisms. Here, we perform a series of 7 μs long atomistic simulations on biomimetic, branched-chain lipid containing bilayer patches, systematically varying the amount of the straight-chain fatty acid component, 16:0, from 7.0 to 47.3 mol %. The simulations reveal thickening and ordering of the bilayer as well as higher bilayer viscosity and bending modulus with increasing 16:0 content, thus providing quantitative support that branched fatty acids increase the bilayer fluidity. A sharp transition in these properties is observed at ∼20% 16:0 content, resembling a phase change. The simulations provide the first access to ordered disordered phases in a bacterial cell membrane mimic containing branched-chain lipids. Granted several assumptions, a comparison of these phases provides estimates of physical properties such as hydrophobic mismatch (∼1.2 Å), difference in bending moduli (∼15.7 ), and the line tension (∼0.6 pN) for a putative lipid raft in the cell membrane of an organism such as or .