  Since the lipid raft model was developed at the end of the last century, it
became clear that the specific molecular arrangements of phospholipid
assemblies within a membrane have profound implications in a vast range of
physiological functions. Studies of such condensed lipid islands in model
systems using fluorescence and Brewster angle microscopies have shown a wide
range of sizes and morphologies, with suggestions of substantial in-plane
molecular anisotropy and mesoscopic structural chirality. Whilst these
variations can significantly alter many membrane properties including its
fluidity, permeability, and molecular recognition, the details of the in-plane
molecular orientations underlying these traits remain largely unknown. Here, we
use phase-resolved sum-frequency generation microscopy on model membranes of
phospholipid monolayers with mixed molecular chirality, which form micron-scale
circular domains of condensed lipids, to fully determine their
three-dimensional molecular structure. We find that the domains possess curved
molecular directionality with spiralling mesoscopic packing. By comparing
different enantiomeric mixtures, both the molecular and spiral turning
directions are shown to depend on the lipid chirality, but with a clear
deviation from mirror symmetry in the formed structures. This demonstrates
strong enantioselectivity in the domain growth process, which has potential
connections to the evolution of homochirality in all living organisms as well
as implications for enantioselective drug design.
