Fig. 1: Schematic structure of a phospholipid and different possible lateral packing structures.
a A schematic of a lipid bilayer membrane containing two lipids (red and purple headgroups) showing their well-defined out-of-plane directionality (black arrow) and ordered domains enriched in one lipid (red) surrounded by a disordered phase enriched in the other (purple). b A schematic of such ordered domains formed in a model lipid monolayer. c 3D representation of the (R)-DPPC lipid viewed from the side showing the two lipid tails [highlighted green (tail 1) and blue (tail 2)] on either side of a plane which cuts through the headgroup (highlighted in grey) and contains the defined in-plane direction (black arrow). This direction (as well as the highlighted plane) is defined as exactly opposite the in-plane component of the symmetric methyl stretch (CH3SS) transition dipole (red arrows) for the two terminal methyl groups on the acyl tails, as demonstrated by projecting this component into the plane. The molecular stick structure of the tails highlights the terminal CH3 groups in red and the CH2 groups in green for emphasis. d 3D representation of the lipid viewed from the top, showing the separating plane (grey) containing the defined in-plane direction (black arrow) opposite to the CH3SS in-plane component (red arrows). This top-view structure is also presented with a trapezoidal outline (blue, dashed line) as a representation of its asymmetric in-plane morphology. e Examples of possible in-plane packing arrangements, representing the lipid as its trapezoidal in-plane morphology (d) and in-plane direction (black arrow). CW clockwise, ACW anti-clockwise. Co- and counter-directional packing is defined by the specific adjacent lipid alignment directions.
