PEO-b-PBD Diblock Copolymers Induce Packing Defects in Lipid/Hybrid Membranes and Improve Insertion Rates of Natively Folded Peptides

Hybrid membranes assembled from biological lipids and synthetic polymers are a promising scaffold for the reconstitution and utilization of membrane proteins. Recent observations indicate that inclusion of small fractions of polymer in lipid membranes can improve protein folding and function, but the exact structural and physical changes a given polymer sequence imparts on a membrane often remain unclear. Here, we use all-atom molecular dynamics simulations to study the structure of hybrid membranes assembled from DOPC phospholipids and PEO-b-PBD diblock copolymers. We verified our computational model using new and existing experimental data and obtained a detailed picture of the polymer conformations in the lipid membrane that we can relate to changes in membrane elastic properties. We find that inclusion of low polymer fractions induces transient packing defects into the membrane. These packing defects act as insertion sites for two model peptides, and in this way, small amounts of polymer content in lipid membranes can lead to large increases in peptide insertion rates. Additionally, we report the peptide conformational space in both pure lipid and hybrid membranes. Both membranes support similar alpha helical peptide structures, exemplifying the biocompatibility of hybrid membranes.

Water Dynamics in a Peptide-appended Pillar[5]arene Artificial Channel in Lipid and Biomimetic Membranes

Peptide-appended Pillar[5]arene (PAP) is an artificial water channel that can be incorporated into lipid and polymeric membranes to achieve high permeability and enhanced selectivity for angstrom-scale separations [Shen et al. Nat. Commun.9:2294 (2018)]. In comparison to commonly studied rigid carbon nanotubes, PAP channels are conformationally flexible, yet these channels allow a high water permeability [Y. Liu and H. Vashisth Phys. Chem. Chem. Phys.21:22711 (2019)]. Using molecular dynamics (MD) simulations, we study water dynamics in PAP channels embedded in biological (lipid) and biomimetic (block-copolymer) membranes to probe the effect of the membrane environment on water transport characteristics of PAP channels. We have resolved the free energy surface and local minima for water diffusion within the channel in each type of membrane. We find that water follows single file transport with low free-energy barriers in regions surroundings the central ring of the PAP channel and the single file diffusivity of water correlates with the number of hydrogen bonding sites within the channel, as is known for other sub-nm pore-size synthetic and biological water channels [Horner et al. Sci. Adv.1:e1400083 (2015)].

Hybrid lipid/block copolymer vesicles display broad phase coexistence region

The fluidity and polar environment of ~100 nm hybrid vesicles combining dipalmitoylphosphatidylcholine (DPPC) and poly(1,2-butadiene)-block-polyethylene oxide (PBd-PEO, average molecular weight 950 g/mol) were studied upon vesicle heating using the fluorescence spectroscopy techniques of DPH anisotropy and laurdan generalized polarization (GP). These techniques indicated PBd-PEO membranes are less ordered than solid DPPC, but slightly more ordered than fluid DPPC or dioleoylphosphatidylcholine (DOPC) membranes. We find the DPH anisotropy values are less than expected from additivity of the components' anisotropies in the fluid phase mixture of DPPC and PBd-PEO, inferring that DPPC strongly fluidizes the PBd-PEO. We use transitions in DPH anisotropy and laurdan GP to create a temperature/composition phase diagram for DPPC/PBd-PEO which we find displays a significantly broader solid/fluid phase coexistence region than DPPC/DOPC, showing that DPPC partitions less readily into fluid PBd-PEO than into fluid DOPC. The existence of a broad solid/fluid phase coexistence region in DPPC/PBd-PEO vesicles is verified by Förster resonance energy transfer results and the visualization of phase separation in giant unilamellar vesicles containing up to 95% PBd-PEO and a single phase in 100% PBd-PEO vesicles at room temperature. These results add to the limited knowledge of phase behavior and phase diagrams of hybrid vesicles, and should be useful in understanding and tailoring membrane surface architecture toward biomedical applications such as drug delivery or membrane protein reconstitution.

Conformational dynamics and interfacial interactions of peptide-appended pillar[5]arene water channels in biomimetic membranes

Peptide appended pillar[5]arene (PAP) is an artificial water channel resembling biological water channel proteins, which has shown a significant potential for designing bioinspired water purification systems. Given that PAP channels need to be incorporated at a high density in membrane matrices, it is critical to examine the role of channel-channel and channel-membrane interactions in governing the structural and functional characteristics of channels. To resolve the atomic-scale details of these interactions, we have carried out atomistic molecular dynamics (MD) simulations of multiple PAP channels inserted in a lipid or a block-copolymer (BCP) membrane matrix. Classical MD simulations on a sub-microsecond timescale showed clustering of channels only in the lipid membrane, but enhanced sampling MD simulations showed thermodynamically-favorable dimerized states of channels in both lipid and BCP membranes. The dimerized configurations of channels, with an extensive buried surface area, were stabilized via interactions between the aromatic groups in the peptide arms of neighboring channels. The conformational metrics characterizing the orientational and structural changes in channels revealed a higher flexibility in the lipid membrane as opposed to the BCP membrane although hydrogen bonds between the channel and the membrane molecules were not a major contributor to the stability of channels in the BCP membrane. We also found that the channels undergo wetting/dewetting transitions in both lipid and BCP membranes with a marginally higher probability of undergoing a dewetting transition in the BCP membrane. Collectively, these results highlight the role of channel dynamics in governing channel-channel and channel-membrane interfacial interactions, and provide atomic-scale insights needed to design stable and functional biomimetic membranes for efficient separations.

Achieving high permeability and enhanced selectivity for Angstrom-scale separations using artificial water channel membranes

Synthetic polymer membranes, critical to diverse energy-efficient separations, are subject to permeability-selectivity trade-offs that decrease their overall efficacy. These trade-offs are due to structural variations (e.g., broad pore size distributions) in both nonporous membranes used for Angstrom-scale separations and porous membranes used for nano to micron-scale separations. Biological membranes utilize well-defined Angstrom-scale pores to provide exceptional transport properties and can be used as inspiration to overcome this trade-off. Here, we present a comprehensive demonstration of such a bioinspired approach based on pillar[5]arene artificial water channels, resulting in artificial water channel-based block copolymer membranes. These membranes have a sharp selectivity profile with a molecular weight cutoff of ~ 500 Da, a size range challenging to achieve with current membranes, while achieving a large improvement in permeability (~65 L m-2 h-1 bar-1 compared with 4-7 L m-2 h-1 bar-1) over similarly rated commercial membranes.