posted on 2023-08-16, 14:37authored byDaniel
J. Speer, Marta Salvador-Castell, Yuqi Huang, Gang-Yu Liu, Sunil K. Sinha, Atul N. Parikh
The hydrophobic effect, a ubiquitous process in biology,
is a primary
thermodynamic driver of amphiphilic self-assembly. It leads to the
formation of unique morphologies including two highly important classes
of lamellar and micellar mesophases. The interactions between these
two types of structures and their involved components have garnered
significant interest because of their importance in key biochemical
technologies related to the isolation, purification, and reconstitution
of membrane proteins. This work investigates the structural organization
of mixtures of the lamellar-forming phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and two zwitterionic
micelle-forming surfactants, being n-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate
(Zwittergent 3-12 or DDAPS) and 1-oleoyl-2-hydroxy-sn-glycero-3-phosphocholine (O-Lyso-PC), when assembled by water vapor
hydration with X-ray diffraction measurements, brightfield optical
microscopy, wide-field fluorescence microscopy, and atomic force microscopy.
The results reveal that multilamellar mesophases of these mixtures
can be assembled across a wide range of POPC to surfactant (POPC:surfactant)
concentration ratios, including ratios far surpassing the classical
detergent-saturation limit of POPC bilayers without significant morphological
disruptions to the lamellar motif. The mixed mesophases generally
decreased in lamellar spacing (D) and headgroup-to-headgroup
distance (Dhh) with a higher concentration
of the doped surfactant, but trends in water layer thickness (Dw) between each bilayer in the stack are highly
variable. Further structural characteristics including mesophase topography,
bilayer thickness, and lamellar rupture force were revealed by atomic
force microscopy (AFM), exhibiting homogeneous multilamellar stacks
with no significant physical differences with changes in the surfactant
concentration within the mesophases. Taken together, the outcomes
present the assembly of unanticipated and highly unique mixed mesophases
with varied structural trends from the involved surfactant and lipidic
components. Modulations in their structural properties can be attributed
to the surfactant’s chemical specificity in relation to POPC,
such as the headgroup hydration and the hydrophobic chain tail mismatch.
Taken together, our results illustrate how specific chemical complexities
of surfactant–lipid interactions can alter the morphologies
of mixed mesophases and thereby alter the kinetic pathways by which
surfactants dissolve lipid mesophases in bulk aqueous solutions.