Triiodide Anion as a Magnesium-ion Transporter for Low Overpotential Battery Cycling in Iodine-Containing Mg(TFSI)2 Electrolyte.

Magnesium iodide (MgI2) solid-electrolyte interface (SEI) layers have previously been shown to protect Mg metal anodes from passivation through products formed during Mg(TFSI)2 electrolyte decomposition (TSFI = trifluorosulfonimide). MgI2 formed in situ from small quantities of I2 added to the electrolyte shows a drastic decrease in the overpotential for magnesium deposition and stripping. In this work, a MgI2 SEI layer was created in an ex situ fashion and then the electrochemical characteristics of this MgI2 SEI layer were probed both alone and with small quantities of I2 or Bu4NI3 additives to identify the electroactive species. Chronopotentiometry (CP) and cyclic voltammetry (CV) show that the MgI2 SEI alone is insufficient for low overpotential magnesium cycling. I(3d) XPS data show that I3- is formed within the SEI layer, which can serve as the electroactive species when ligated with Mg2+ for low overpotential (<50 mV at 0.1 mA cm-2 current density) cycling. Moreover, Raman shifts at 110 and 140 cm-1 are consistent with I3- formation, and these signatures are observed before and after CP experiments. The Mg0 deposition curves in the CV with additives are consistent with diffusive species. Finally, electrochemical impedance spectroscopy (EIS) shows that there is a large decrease in the charge-transfer resistance within the SEI when either I2 or Bu4NI3 additives are used, which supports a solvating effect that facilitates magnesium deposition and stripping.

Lipid-nanodiscs formed by paramagnetic metal chelated polymer for fast NMR data acquisition.

Lipid-nanodiscs have been shown to be an exciting innovation as a membrane-mimicking system for studies on membrane proteins by a variety of biophysical techniques, including NMR spectroscopy. Although NMR spectroscopy is unique in enabling the atomic-resolution investigation of dynamic structures of membrane-associated molecules, it, unfortunately, suffers from intrinsically low sensitivity. The long data acquisition often used to enhance the sensitivity is not desirable for sensitive membrane proteins. Instead, paramagnetic relaxation enhancement (PRE) has been used to reduce NMR data acquisition time or to reduce the amount of sample required to acquire an NMR spectra. However, the PRE approach involves the introduction of external paramagnetic probes in the system, which can induce undesired changes in the sample and on the observed NMR spectra. For example, the addition of paramagnetic ions, as frequently used, can denature the protein via direct interaction and also through sample heating. In this study, we show how the introduction of paramagnetic tags on the outer belt of polymer-nanodiscs can be used to speed-up data acquisition by significantly reducing the spin-lattice relaxation (T1) times with minimum-to-no alteration of the spectral quality. Our results also demonstrate the feasibility of using different types of paramagnetic ions (Eu3+, Gd3+, Dy3+, Er3+, Yb3+) for NMR studies on lipid-nanodiscs. Experimental results characterizing the formation of lipid-nanodiscs by the metal-chelated polymer, and their increased tolerance toward metal ions are also reported.

Metal-Chelated Polymer Nanodiscs for NMR Studies.

Paramagnetic relaxation enhancement (PRE) is commonly used to speed up spin lattice relaxation time (T1 ) for rapid data acquisition in NMR structural studies. Consequently, there is significant interest in novel paramagnetic labels for enhanced NMR studies on biomolecules. Herein, we report the synthesis and characterization of a modified poly(styrene-co-maleic acid) polymer which forms nanodiscs while showing the ability to chelate metal ions. Cu2+ -chelated nanodiscs are demonstrated to reduce the T1 of protons for both polymer and lipid-nanodisc components. The chelated nanodiscs also decrease the proton T1 values for a water-soluble DNA G-quadruplex. These results suggest that polymer nanodiscs functionalized with paramagnetic tags can be used to speed-up data acquisition from lipid bilayer samples and also to provide structural information from water-soluble biomolecules.

Polymer nanodiscs: Advantages and limitations.

There is considerable interest in the development of membrane mimetics to study the structure, dynamics and function of membrane proteins. Polymer nanodiscs have been useful as a membrane mimetic by not only providing a native-like membrane environment, but also have the ability to extract the desired membrane protein directly from the cell membrane. In spite of such great potential, polymer nanodiscs have their disadvantages including lack of size control and instability at low pH and with divalent metals. In this review, we discuss how these limitations have been overcome by simple modifications of synthetic polymers commonly used to form nanodiscs. Recently, size control has been achieved using an ethanolamine functionalization of a low molecular weight polymer. This size control enabled the use of polymer-based lipid-nanodiscs in solution NMR and macro-nanodiscs in solid-state NMR applications. The introduction of quaternary ammonium functional groups has been shown to improve the stability in the presence of low pH and divalent metal ions, forming highly monodispersed nanodiscs. The polymer charge has been shown to play a significant role on the reconstitution of membrane proteins due to the high charge density on the nanodisc's belt. These recent developments have expanded the applications of polymer nanodiscs to study the membrane proteins using wide variety of techniques including NMR, Cryo-EM and other biophysical techniques.

Hydrophobic Functionalization of Polyacrylic Acid as a Versatile Platform for the Development of Polymer Lipid Nanodisks.

Polymer nanodisks have shown great potential as membrane mimetics that enable the study of functional membrane protein structural biology and also have a wider application in other fields such as drug delivery. To achieve these research goals, the ability to have a cheap, simple, fully customizable platform for future nanodisks technology applications is paramount. Here, a facile functionalization of polyacrylic acid (PAA) with varying hydrophobic groups that form nanodisks at different sizes is successfully demonstrated. The study shows that the choice of hydrophobic group can have a noticeable effect on the polymer solubilization properties and polymer-induced perturbation to the encased lipid bilayer. Due to this robust, tunable chemical synthesis method, PAA is an exciting platform for the future optimization of the hydrophobic, hydrophilic, or direct purposed functionalizations for polymer nanodisks.

Effect of polymer charge on functional reconstitution of membrane proteins in polymer nanodiscs.

Although there is a growing interest in using polymer lipid-nanodiscs, the polymer charge poses limitations for studies on membrane proteins. Here, we demonstrate the functional reconstitution of a large soluble-domain containing positively-charged ∼57 kDa cytochrome-P450 and negatively-charged ∼16 kDa cytochrome-b5 in lipid-nanodiscs, and the role of the polymer charge for high-resolution studies on membrane proteins.

Formation of pH-Resistant Monodispersed Polymer-Lipid Nanodiscs.

Polymer lipid nanodiscs are an invaluable system for structural and functional studies of membrane proteins in their near-native environment. Despite the recent advances in the development and usage of polymer lipid nanodisc systems, lack of control over size and poor tolerance to pH and divalent metal ions are major limitations for further applications. A facile modification of a low-molecular-weight styrene maleic acid copolymer is demonstrated to form monodispersed lipid bilayer nanodiscs that show ultra-stability towards divalent metal ion concentration over a pH range of 2.5 to 10. The macro-nanodiscs (>20 nm diameter) show magnetic alignment properties that can be exploited for high-resolution structural studies of membrane proteins and amyloid proteins using solid-state NMR techniques. The new polymer, SMA-QA, nanodisc is a robust membrane mimetic tool that offers significant advantages over currently reported nanodisc systems.

Styrene maleic acid derivates to enhance the applications of bio-inspired polymer based lipid-nanodiscs.

Membrane mimetics are essential to study the structure, dynamics and function of membrane-associated proteins by biophysical and biochemical approaches. Among various membrane mimetics that have been developed and demonstrated for studies on membrane proteins, lipid nanodiscs are the latest developments in the field and are increasingly used for various applications. While lipid-nanodiscs can be formed using an amphipathic membrane scaffold protein (MSP), peptide, or synthetic polymer, the synthetic polymer based nanodiscs exhibit unique advantages because of the ability to functionalize them for various applications. In addition to the use of synthetic polymers to extract membrane proteins directly from the cell membranes, recent advances in the development of polymers used for nanodiscs formation are attracting new attention to the field of nanodiscs technology. Here we review the developments of novel polymer modifications that overcome the current limitations and enhance the applications of polymer based nanodiscs to a wider variety of biophysical techniques used to study membrane proteins. A summary of the functionalization of poly(Styrene-co-Maleic Acid), SMA, polymers developed by our research and their advantages are also covered in this review article.

pH Tunable and Divalent Metal Ion Tolerant Polymer Lipid Nanodiscs.

The development and applications of detergent-free membrane mimetics have been the focus for the high-resolution structural and functional studies on membrane proteins. The introduction of lipid nanodiscs has attracted new attention toward the structural biology of membrane proteins and also enabled biomedical applications. Lipid nanodiscs provide a native lipid bilayer environment similar to the cell membrane surrounded by a belt made up of proteins or peptides. Recent studies have shown that the hydrolyzed form of styrene maleic anhydride copolymer (SMA) has the ability to form lipid nanodiscs and has several advantages over protein or peptide based nanodiscs. SMA polymer lipid nanodiscs have become very important for structural biology and nanobiotechnological applications. However, applications of the presently available polymer nanodiscs are limited by their instability toward divalent metal ions and acidic conditions. To overcome the limitations of SMA nanodiscs and to broaden the potential applications of polymer nanodiscs, the present study investigates the tunability of SMA polymer nanodiscs by systematically modifying the maleic acid functional group. The two newly developed polymers and subsequent lipid nanodiscs were characterized using solid-state NMR, FT-IR, TEM, and DLS experiments. The pH dependence and metal ion stability of these nanodiscs were studied using static light scattering and FTIR. The reported polymer nanodiscs exhibit unique pH dependent stability based on the modified functional group and show a high tolerance toward divalent metal ions. We also show these tunable nanodiscs can be used to encapsulate and stabilize a polyphenolic natural product curcumin.