We demonstrate a simple, yet effective, mixed-salt method to increase the room temperature ionic conductivity of lithium-doped block copolymer electrolyte membranes by suppressing the crystalline phases in the conducting block. We examined a mixed-salt system of LiClO4 and LiN(SO2CF3)2 (LiTFSI) doped into a lamellae-forming poly(styrene-b-ethylene oxide) (PS–PEO) diblock copolymer. The domain spacings, morphologies, thermal behavior, and crystalline phases of salt-doped PS–PEO samples were characterized, and the ionic conductivities of block copolymer electrolytes were obtained through ac impedance measurements. Comparing the ionic conductivity profiles of salt-doped PS–PEO samples at different mixed-salt ratios and total salt concentrations, we found that the ionic conductivity at room temperature can be improved by more than an order of magnitude when coinhibition of crystallite growth is promoted by the concerted behavior of the PEO:LiClO4 and PEO:LiTFSI phases. Additionally, we examined the influence of mixed-salt ratio and total salt concentration on copolymer energetics, and we found that the slope of the effective interaction parameter (χeff) vs salt concentration in our lamellae-forming PS–PEO system was lower than that reported for a cylinder-forming PS–PEO system due to the balance between chain stretching and salt segregation in the PEO domains.
A cationic protein isolated from the seeds of the Moringa oleifera tree has been extensively studied for use in water treatment in developing countries and has been proposed for use in antimicrobial and therapeutic applications. However, the molecular basis for the antimicrobial action of this peptide, Moringa oleifera cationic protein (MOCP), has not been previously elucidated. We demonstrate here that a dominant mechanism of MOCP antimicrobial activity is membrane fusion. We used a combination of cryogenic electron microscopy (cryo-EM) and fluorescence assays to observe and study the kinetics of fusion of membranes in liposomes representing model microbial cells. We also conducted cryo-EM experiments on E. coli cells where MOCP was seen to fuse the inner and outer membranes. Coarse-grained molecular dynamics simulations of membrane vesicles with MOCP molecules were used to elucidate steps in peptide adsorption, stalk formation, and fusion between membranes.
Membrane protein and membrane protein–mimic functionalized materials are rapidly gaining interest across a wide range of applications, including drug screening, DNA sequencing, drug delivery, sensors, water desalination, and bioelectronics. In these applications, material performance is highly dependent on activity‐per‐protein and protein packing density in bilayer and bilayer‐like structures collectively known as biomimetic membranes. However, a clear understanding of, and accurate tools to study these properties of biomimetic membranes does not exist. This paper presents methods to evaluate membrane protein compatibility with biomimetic membrane materials. The methods utilized provide average single protein activity, and for the first time, provide experimentally quantifiable measures of the chemical and physical compatibility between proteins (and their mimics) and membrane materials. Water transport proteins, rhodopsins, and artificial water channels are reconstituted into the full range of current biomimetic membrane matrices to evaluate the proposed platform. Compatibility measurement results show that both biological and artificial water channels tested largely preserve their single protein water transport rates in biomimetic membranes, while their reconstitution density is variable, leading to different overall membrane permeabilities. It is also shown that membrane protein insertion efficiency inversely correlates with both chemical and physical hydrophobicity mismatch between membrane protein and the membrane matrix.
We examine the kinetics of polymer chain exchange between mixed block copolymer−detergent micelles, a system relevant to the synthesis of protein-containing biomimetic membranes. While chain exchange between block copolymer (BCP) aggregates in water is too slow to observe, and detergent molecules exchange between micelles on a time scale of nanoseconds to microseconds, BCP chains exchange between mixed detergent−polymer micelles on intermediate time scales of many minutes to a few days. We examine a membrane-proteinrelevant, vesicle-forming, ultrashort polymer, poly(ethyl ethylene) 20 -poly(ethylene oxide) 18 (PEE 20 −PEO 18 ). PEE 20 −PEO 18 was solubilized in mixed micelles with the membrane protein compatible nonionic detergent octyl-β-D-glucoside (OG). Using cryo-TEM and small-angle neutron scattering (SANS), we demonstrate complete solubilization of the polymer into micelles. Using time-resolved SANS (TR-SANS), we provide the first direct evidence that detergents activate BCP chain exchange and determine kinetic parameters at two detergent concentrations slightly above the critical micellar concentration (CMC) of OG. We find that chain exchange increases 2 orders of magnitude when temperature increases from 35 to 55 °C and that even a 1 mg/mL increase in OG concentration leads to a noticeable increase in exchange rate. Our kinetic data are consistent with a single rate-limiting process rather than the distribution of exchange rates known to exist for BCPs in the absence of detergent, indicating a different exchange mechanism than the simple chain escape dominant in single-component micelles. Using the Arrhenius equation, we determine that at the detergent concentrations examined the activation energy for polymer chain exchange is only 2−3 times higher for PEE 20 PEO 18 than for short-chained lipids and that the activation barrier decreases with increasing OG concentration. On the basis of these results, we postulate that mixed micelles exchange BCPs through a detergent-mediated process such as the fusion and fragmentation mechanisms also known to occur in micellar systems. These findings explain the need for high detergent concentration and/or temperature to synthesize polymer/protein membranes. Further, we postulate this is a more general phenomenon applicable to mixed micelle systems containing amphiphiles with vastly different solubilities and CMCs differing by many orders of magnitude.
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