8.D.1.  The Synthetic Surfactant Amphipol Polymers (SS-AP) Family 

Amphipols (APs) comprise several classes of synthetic surfactants that make it possible to handle membrane proteins in detergent-free aqueous solution as though they were soluble proteins. The strongly hydrophilic backbones of these polymers is grafted with hydrophobic chains, making them amphiphilic. Amphipols have been shown to stabilize in aqueous solution under their native state four well-characterized integral membrane proteins: (i) bacteriorhodopsin, (ii) a bacterial photosynthetic reaction center, (iii) cytochrome b6f, and (iv) matrix porin (Tribet et al. 1996; APs are amphiphilic polymers that bind to the transmembrane surfaces of a protein in a noncovalent but quasi-irreversible manner. Membrane proteins complexed by APs are in their native state, stable, and they remain water-soluble in the absence of detergent (Popot et al. 2003). APs are amphipathic polymers that adsorb onto the hydrophobic transmembrane surfaces of membrane proteins. Their usefulness, constraints, and prospects for functional and structural studies have been discussed (Popot 2010). Rajesh et al. 2011 reviewed techniques to produce functional membrane proteins and discuss amphipols, nanodisc and styrene maleic acid lipid particle (SMALP) technologies that are used to allow cell-free expression of membrane proteins. The structures and solution behaviors; the mechanisms of association with membrane proteins; and the dynamics and solution properties of the resulting complexes have been discussed (Popot et al. 2011). Amphipol-assisted folding of inclusion bodies has been useful (Banères et al. 2011). Amphipols have been used to study GPCRs (TC#1.A.14), TRP and Ryanodine channels (TC#s 1.A.4 and 1.A.3, respectively) and integral membrane proteases (Klöpfer and Hagn 2019; Barniol-Xicota and Verhelst 2020).

Aamphipols are designed to keep membrane proteins soluble in water in the absence of a free surfactant. High molecular weight (MW) congeners of amphipols are known to exhibit a high affinity for hydrophobic particles (Tribet et al. 1996).  Thus, low MW amphipols can keep soluble in their native state the four integral membrane proteins: (i) bacteriorhodopsin (BR), (ii) the photosynthetic reaction center from Rhodobacter sphaeroides R-26 (RC), (iii) the cytochrome b6f complex from Chlamydomonas reinhardtii, and (iv) the matrix porin (OmpF) from Escherichia coli.  Amphipols (APs). APs are amphiphilic polymers that bind to the transmembrane surface of the protein in a noncovalent but, in the absence of a competing surfactant, quasi-irreversible manner. Membrane proteins complexed by APs are in their native state, stable, and they remain water-soluble



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