• MP cell-free in vitro synthesis, using FSs as a mild acceptor medium in which to let MPs fold
as their hydrophobic segments emerge from the ribosome (ibid., Line C).
• MP insertion into preformed lipid bilayers, the rationale there being that an FS solution above
its CMC is able to keep MPs (or membrane-inserting toxins) water-soluble while respecting
the integrity of the bilayer (ibid., Line D).
• Small-angle neutron scattering. An interesting observation indeed is that the contrast-match
point (see Chap. 9, Box 9.2) of F 6 diGlu (Fig. 3.25B) and F 6 diGluM (Abla et al. 2012) is very
close to that of hydrogenated, unlabeled proteins. Appropriately adjusting the D 2 O content of
the solution therefore permits to cancel the contribution of such a protein along with that of
the surfactant, leaving the experimenter free to analyze the conformational changes of a
deuterated protein interacting with the hydrogenated one (Table 3.3, Line E).
Because FSs are highly tensioactive and do not mix easily with hydrogenated surfactants, a
monolayer of FSs at the air-water interface is essentially impenetrable to detergents. This has been
exploited before to adsorb detergent-solubilized MPs under monolayers of appropriately tagged
fluorinated lipids (Held et al. 1997; Vénien-Bryan et al. 1997; Lebeau et al. 2001; Fotinou et al.
2013) as well as to improve particle distribution in cryo-EM studies of ND-embedded or digitoninsolubilized MPs (Efremov et al. 2015; Gatsogiannis et al. 2016; Zhang and Chen 2016; Johnson and
Chen 2017). FSs have proven useful as additives for MP crystallization (Efremov et al. 2010; Sobolev
et al. 2013; Yoon et al. 2013; Shimada et al. 2017). Nitriloacetate-derivatized FSs have been
synthesized for the same application (Petkova et al. 2007; Dauvergne et al. 2008; Kreutz et al.
2009). Newton black films (two air/water surfactant monolayers sandwiching a thin layer of aqueous
solution, as in soap bubbles) of C 6 F 13 -SO-THAM and/or a nickel-bearing version thereof have been
used to organize into two-dimensional layers a polyhistidine-tagged MP kept soluble by a
hydrogenated detergent, whose arrangement was studied by X-ray reflectivity (Petkova et al. 2007).
(H)FS-trapped MPs could potentially be induced to form three-dimensional crystals, either in
(H)FS solution or following transfer to lipidic mesophases, but this application remains to be validated
(for discussions, see Breyton et al. 2009; Popot 2010; Durand et al. 2014). However, crystallization of
a cytochrome c oxidase/cytochrome c complex has recently been achieved using a mixture of
decylmaltoside and fluorinated octylmaltoside, where the use of hydrogenated detergents alone had
been unsuccessful (Shimada et al. 2017).
As stated above (§ 3.5.1), with rare exceptions, such as perfluoro-octanoate (Shepherd and
Holzenburg 1995; Ramjeesingh et al. 1999), FSs are unable to disperse biological membranes and
extract MPs. A fluorinated derivative of octylmaltoside (F 6 OM; Fig. 3.25F) has recently been
described that is able to slowly solubilize POPC vesicles (Frotscher et al. 2015). The mixed micelles
appear to be cylindrical, exhibiting large sizes when examined by dynamic light scattering. Extraction
of MPs was not reported. Transfer to F 6 OM was shown to facilitate refolding of the β-barrel MP outer
membrane phospholipase A (OmpLA) from its urea-denatured form (Frotscher et al. 2015). As
mentioned above, an analog of F 6 OM with a phosphocholine polar head, F 6 OPC, was found to be
devoid of the solubilizing properties of F 6 OM, underlining the critical role of the polar head in this
process (Frotscher et al. 2015; Vargas et al. 2015).
The present section is by no means intended to present an exhaustive view of the properties and
applications of (H)FSs. For more details about their physical-chemical properties and more in-depth
discussions of their advantages and drawbacks for various applications, the reader is referred to earlier,
more detailed reviews (Breyton et al. 2009; Popot 2010; Durand et al. 2014).
132
3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions
as their hydrophobic segments emerge from the ribosome (ibid., Line C).
• MP insertion into preformed lipid bilayers, the rationale there being that an FS solution above
its CMC is able to keep MPs (or membrane-inserting toxins) water-soluble while respecting
the integrity of the bilayer (ibid., Line D).
• Small-angle neutron scattering. An interesting observation indeed is that the contrast-match
point (see Chap. 9, Box 9.2) of F 6 diGlu (Fig. 3.25B) and F 6 diGluM (Abla et al. 2012) is very
close to that of hydrogenated, unlabeled proteins. Appropriately adjusting the D 2 O content of
the solution therefore permits to cancel the contribution of such a protein along with that of
the surfactant, leaving the experimenter free to analyze the conformational changes of a
deuterated protein interacting with the hydrogenated one (Table 3.3, Line E).
Because FSs are highly tensioactive and do not mix easily with hydrogenated surfactants, a
monolayer of FSs at the air-water interface is essentially impenetrable to detergents. This has been
exploited before to adsorb detergent-solubilized MPs under monolayers of appropriately tagged
fluorinated lipids (Held et al. 1997; Vénien-Bryan et al. 1997; Lebeau et al. 2001; Fotinou et al.
2013) as well as to improve particle distribution in cryo-EM studies of ND-embedded or digitoninsolubilized MPs (Efremov et al. 2015; Gatsogiannis et al. 2016; Zhang and Chen 2016; Johnson and
Chen 2017). FSs have proven useful as additives for MP crystallization (Efremov et al. 2010; Sobolev
et al. 2013; Yoon et al. 2013; Shimada et al. 2017). Nitriloacetate-derivatized FSs have been
synthesized for the same application (Petkova et al. 2007; Dauvergne et al. 2008; Kreutz et al.
2009). Newton black films (two air/water surfactant monolayers sandwiching a thin layer of aqueous
solution, as in soap bubbles) of C 6 F 13 -SO-THAM and/or a nickel-bearing version thereof have been
used to organize into two-dimensional layers a polyhistidine-tagged MP kept soluble by a
hydrogenated detergent, whose arrangement was studied by X-ray reflectivity (Petkova et al. 2007).
(H)FS-trapped MPs could potentially be induced to form three-dimensional crystals, either in
(H)FS solution or following transfer to lipidic mesophases, but this application remains to be validated
(for discussions, see Breyton et al. 2009; Popot 2010; Durand et al. 2014). However, crystallization of
a cytochrome c oxidase/cytochrome c complex has recently been achieved using a mixture of
decylmaltoside and fluorinated octylmaltoside, where the use of hydrogenated detergents alone had
been unsuccessful (Shimada et al. 2017).
As stated above (§ 3.5.1), with rare exceptions, such as perfluoro-octanoate (Shepherd and
Holzenburg 1995; Ramjeesingh et al. 1999), FSs are unable to disperse biological membranes and
extract MPs. A fluorinated derivative of octylmaltoside (F 6 OM; Fig. 3.25F) has recently been
described that is able to slowly solubilize POPC vesicles (Frotscher et al. 2015). The mixed micelles
appear to be cylindrical, exhibiting large sizes when examined by dynamic light scattering. Extraction
of MPs was not reported. Transfer to F 6 OM was shown to facilitate refolding of the β-barrel MP outer
membrane phospholipase A (OmpLA) from its urea-denatured form (Frotscher et al. 2015). As
mentioned above, an analog of F 6 OM with a phosphocholine polar head, F 6 OPC, was found to be
devoid of the solubilizing properties of F 6 OM, underlining the critical role of the polar head in this
process (Frotscher et al. 2015; Vargas et al. 2015).
The present section is by no means intended to present an exhaustive view of the properties and
applications of (H)FSs. For more details about their physical-chemical properties and more in-depth
discussions of their advantages and drawbacks for various applications, the reader is referred to earlier,
more detailed reviews (Breyton et al. 2009; Popot 2010; Durand et al. 2014).
132
3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions