Nanoparticle self-assembly in mixtures of phospholipids with styrene/maleic acid copolymers or fluorinated surfactants

Abstract

Self-assembling nanostructures in aqueous mixtures of bilayer-forming lipids and micelle-forming surfactants are relevant to in vitro studies on biological and synthetic membranes and membrane proteins. Considerable efforts are currently underway to replace conventional detergents by milder alternatives such as styrene/maleic acid (SMA) copolymers and fluorinated surfactants. However, these compounds and their nanosized assemblies remain poorly understood as regards their interactions with lipid membranes, particularly, the thermodynamics of membrane partitioning and solubilisation. Using (19)F and (31)P nuclear magnetic resonance spectroscopy, static and dynamic light scattering, and isothermal titration calorimetry, we have systematically investigated the aggregational state of a zwitterionic bilayer-forming phospholipid upon exposure to an SMA polymer with a styrene/maleic acid ratio of 3 : 1 or to a fluorinated octyl phosphocholine derivative called F(6)OPC. The lipid interactions of SMA(3 : 1) and F(6)OPC can be thermodynamically conceptualised within the framework of a three-stage model that treats bilayer vesicles, discoidal or micellar nanostructures, and the aqueous solution as distinct pseudophases. The exceptional solubilising power of SMA(3 : 1) is reflected in very low membrane-saturating and solubilising polymer/lipid molar ratios of 0.10 and 0.15, respectively. Although F(6)OPC saturates bilayers at an even lower molar ratio of 0.031, this nondetergent does not solubilise lipids even at >1000-fold molar excess, thus highlighting fundamental differences between these two types of mild membrane-mimetic systems. We rationalise these findings in terms of a new classification of surfactants based on bilayer-to-micelle transfer free energies and discuss practical implications for membrane-protein research.

Effect of Polymer Composition and pH on Membrane Solubilization by Styrene-Maleic Acid Copolymers

“…This will speed up the thermodynamically favorable formation of nanodisks (1,29). The distribution of the monomer units is important, because a more homogenous distribution along the polymer chain can promote uncoiling of the styrene-rich hydrophobic domains (30) into the specific conformation that is required to stabilize a nanodisk.…”

Solubilization of Membrane Proteins into Functional Lipid‐Bilayer Nanodiscs Using a Diisobutylene/Maleic Acid Copolymer

“…In 31 P NMR experiments, the signal of slow‐tumbling DMPC LUVs was broadened beyond detection, but an isotropic peak emerged upon addition of DIBMA (Figure 2 d), the intensity of which reflected the amount of solubilized lipid 14, 15. The peak areas at each DMPC concentration revealed two breakpoints marking the onset (SAT) and completion (SOL) of solubilization (Figure 2 e).…”

Membrane proteins: is the future disc shaped?

“…Different ratios of styrene and maleic anhydride produce different polymers. In their simplest form, SMALPs are stable, self-assembling, water-soluble particles consisting of SMA and approximately 140 lipids (lipid-only SMALPs) [25,26]. Adding SMA to a suspension of lipids at pH 6.5 or above leads to spontaneous and immediate clearing of the suspension due to the formation of SMA/lipid particles of ∼10 nm diameter.…”