The use of poly(styrene-co-maleic acid) (SMA) for the solubilization of lipid membranes and membrane proteins is becoming more widespread, and with this, the need increases to better understand the chemical properties of the copolymer and how these translate into membrane solubilization properties. SMA comes in many different flavors that include the ratio of styrene to maleic acid, comonomer sequence distribution, average chain length, dispersity, and potential chemical modifications. In this work, the synthesis and membrane active properties are described for 2:1 (periodic) SMA copolymers with M w varying from ∼1.4 to 6 kDa. The copolymers were obtained via an iterative RAFTmediated radical polymerization. Characterization of these polymers showed that they represent a well-defined series in terms of chain length and overall composition (F MAnh ∼ 0.33), but that there is heterogeneity in comonomer sequence distribution (F MSS ∼ 0.50) and some dispersity in chain length (1.1 < Đ < 1.6), particularly for the larger copolymers. Investigation of the interaction of these polymers with phosphatidylcholine lipid self-assemblies showed that all copolymers inserted equally effectively into lipid monolayers, independent of the copolymer length. Nonetheless, smaller polymers were more effective at solubilizing lipid bilayers into nanodiscs, possibly because longer polymers are more prone to become intertwined with each other, thereby hampering their solubilization efficiency. Nanodisc sizes were independent of the copolymer length. However, nanodiscs formed with larger copolymers were found to undergo slower lipid exchange, indicating a higher stability. The results highlight the usefulness of having well-defined copolymers for systematic studies.
illumination of vesicles containing BODIPY-conjugated lipid fluorophores significantly accelerates the rupture and SLB formation process. Tail groupconjugated BODIPY groups have a more pronounced effect than head groupconjugated BODIPYs. Other commonly used fluorophores, such as NBD and Texas Red do not cause illumination-induced acceleration regardless of the fluorescent group position. The accelerating effect of BODIPY is dependent on unsaturation in the acyl chains of the background phospholipid. Vesicles composed of saturated phospholipids (diphytanoyl-PC) are not susceptible to the influence of BODIPY-conjugates in the presence of illumination. This suggests that the acceleration of SLB formation in the presence of light may be mediated by lipid oxidation. To test this, we included an antioxidant (alpha-tocopherol) in the vesicles. When alpha-tocopherol is present, the acceleratory effect of BODIPY and light is significantly reduced. Using QCM-D we measured the mass of the SLB formed by vesicle rupture. After illumination, SLBs formed from unsaturated lipids and BODIPY-conjugated probes are ''lighter'' than those without illumination. Zeta potential measurements show that illumination of these vesicles causes them to become more negatively charged, and DLS measurements show that illumination causes the vesicles to shrink in size. This suggests photochemical reactions are altering the vesicle composition. Together our results show that excitation of BODIPY-lipid conjugates can significantly alter membrane properties, perhaps due to photosensitized lipid oxidation.
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