## 4 SMA Polymer Derivatives  
The fields enabled by native nanodiscs are evolving through the emergence of new polymers to improve performance and enable new applications. A growing family of polymers that spontaneously fragment membranes into discs have been designed and tested (Figure 2). A systematic comparison of twelve SMA polymer types and conditions for solubilization of intact photosynthetic systems from spinach thylakoid membranes indicated those that best retain the activity and subunit composition based on chlorophyll fluorescence emission spectra, native gel electrophoresis, and immunoblotting. This narrowed the field to five that were most efficient in this case, including SMA 1440, XIRAN 25010, XIRAN 30010, SMA PRO10235, and SMA 1735, and suggests room for further optimization. Improvements to simplify separation and detection of the multiple transmembrane assemblies were found including detachment of peripheral membrane proteins by varying buffer conditions [33]. Specific derivatives have been synthesized to enhance functionality and operational range of polymers under a wider pH range and cation concentrations. This allows production of stable disks of different sizes, allowing larger memteins to be solubilized and stored. The maleic anhydride group can be modified, allowing addition of various moieties. An ethanolamine group was added to a SMA(1.3:1) polymer having an M\({}_{\text{n}}\) of 1.6 kDa, the resulting SMA-EA polymer solubilizes liposomes over a wide range of pH, temperature, salt, and cation levels to produce nanodiscs of diameters between 10 and 50 nm [34; 35]. The solution state NMR spectra of \({}^{15}\)N labelled cytochrome \(b5\) protein in SMA-EA discs display well-resolved signals indicating a folded state. When lanthanide ions such as Yb\({}^{3+}\) are added the SMA-EA discs align with external magnetic fields, allowing transmembrane helix tilt angles to be measured using solid-state NMR. An ethylenediamine group was attached to the SMA(1.3:1) to produce the zwitterionic polymer SMA-ED, which solubilizes vesicles below pH 5 and over pH 7 [36]. This polymer can be dehydrated to yield the maleimide form SMAd-A, which also solubilizes DMPC vesicles at pH values below 6 where its amine group is positively charged. Both ethylenediamine-modified forms are effective solubilizers in high salt and divalent cation concentrations. These studies extend the range of conditions for solubilizing membranes by SMA-type polymers and allow virtually any membrane to be turned into nanodiscs. Positively charged versions of SMA are also effective at solubilizing membranes into nanodiscs. The addition of a quaternary functional group to SMA(1.3:1) yields a cationic version that fragments vesicles into nanodiscs with diameters in the range of 10-30 nm depending on the ratio of added DMPC. This SMA-QA polymer is an effective solubilizer at pH values between 2 and 10 and tolerates high metal concentrations [37]. The incorporation of a positively charged group in styrene maleimide polymers yields nanodiscs from membrane at pH values below 7.8 and even in high concentrations of divalent cations [38]. Their discs have diameters of 6 nm and are stable up to 80 \({}^{\circ}\)C. The ZipA protein plays a role in bacterial cell division and purified well in SMI-based nanodiscs with yields that are only slightly reduced compared to those in the case of SMALPs. The human adenosine A\({}_{2\text{A}}\) receptor solubilized in SMI nanodiscs is able to bind ligand normally. The availability of both anionic SMA and cationic SMI copolymers to solubilize membranes free of detergent allows comparative proteomic studies of acidic or basic membranes. Moreover, the activities of these variants can be compared to minimize undesirable nonspecific electrostatic interactions. The addition of a phosphatidylcholine (PC) lipid headgroup to form zwitterionic \"zSMA\" copolymers has been explored [39]. These lack MA groups and are not technically SMA polymers but still solubilize membranes into nanodiscs of diameters 10-30 nm.