(322j) Computational Analysis and Atomic Force Microscopy of Physical Properties of Lung Surfactant and Lipid Monolayers

Lung surfactant is a complex lipid-protein mixture made up of 50-75% enantiospecific (r)-dipalmitoylphosphatidylcholine ((r)-DPPC), with smaller fractions of unsaturated phospholipids and fatty acids along with ~ 10 wt% of lung surfactant specific proteins. The role of cholesterol and palmitic acid in native and replacement surfactants remains a contested issue; certain clinical replacement surfactants carefully remove all cholesterol, while others retain 4-5 wt%. Other clinical replacement surfactants add up to 15% palmitic acid. Understanding how these chemical species influence monolayer domain morphology and dynamics would be helpful to developing improved synthetic replacement surfactants and provide fundamental understanding of monolayer phase behavior and the interactions of cholesterol with lipids important to the raft hypothesis of cell membrane organization.

On the micron scale, lung surfactant monolayers exhibit coexisting liquid condensed (LC) and liquid expanded (LE) phases; the LC phase has long range orientational order of the molecular tilt direction and short-range crystalline packing of the alkane chains and the LE phase is disordered and liquid-like. In mixtures with cholesterol and hexadecanol or palmitic acid, unstable growth of the LC domains results in a fingering instability in which curved linear “fingers” grow out of circular domains (1). Here we report the curvature and growth angles of the fingers to show that the finger orientation changes by 120°, consistent with the underlying hexagonal packing of the alkane lattice and the twist induced by the chiral center of DPPC inducing twist grain boundaries at which the lattice reorients abruptly. We also report initial work on transferring monolayers of DPPC and mixtures from the air-water interface to a mica substrate for fluorescence confocal and atomic force microscope imaging using a direct vertical deposition method (2). Transferring monolayers at LC-LE coexistence causes the brittle LC domains to fracture forming smaller domains in the surrounding LE phase. This complicates analysis of the LC domain shapes and sizes but does not seem to affect the local chemical composition of the LC phase as revealed by infra-red AFM.

1. C. Valtierrez-Gaytan, J. M. Barakat, M. Kohler, K. Kieu, B. L. Stottrup, J. A. Zasadzinski, Spontaneous evolution of equilibrium morphology in phospholipid-cholesterol monolayers. Science Advances 8, (2022).
2. K. Kim, S. Q. Choi, T. M. Squires, J. A. Zasadzinski, Cholesterol nanodomains: their effect on monolayer morphology and dynamics. Proc. Nat. Acad. Sci. USA 110, E3054-E3060 (2013).