X-ray and neutron reflectivity studies of styrene-maleic acid copolymer interactions with galactolipid-containing monolayers.

Styrene-maleic acid (SMA) copolymers have recently gained attention for their ability to facilitate the detergent-free solubilization of membrane protein complexes and their native boundary lipids into polymer-encapsulated, nano-sized Lipid Particles, referred to as SMALPs. However, the interfacial interactions between SMA and lipids, which dictate the mechanism, efficiency, and selectivity of lipid and membrane protein extraction, are barely understood. Our recent finding has shown that SMA 1440, a chemical derivative of SMA family with a functionalized butoxyethanol group, was most active in galactolipid-rich membranes, as opposed to phospholipid membranes. In present work, we have performed X-ray reflectometry (XRR) and neutron reflectometry (NR) on the lipid monolayers at the liquid-air interface following by the SMA copolymer adsorption. XRR and Langmuir $\Pi - A$ isotherms captured the fluidifying effect of galactolipids, which made SMA copolymers easily infiltrate into the lipid membranes. NR results revealed the detailed structural arrangement of SMA 1440 copolymers within the membranes and highlighted the partition of the butoxyethanol group into the lipid tail region. This work allows us to propose a possible mechanism for the membrane solubilization by SMA.