OmpX/A-35 complexes (Perlmutter et al. 2014) (see Chap. 5, Fig. 5.11B). One may note, however,
that F177 is located in the most distal part of an extracellular loop, whereas the side chain of W58 faces
the lumen of the β-barrel (Fig. 14.14), two positions where an interaction with the loops or tails of
A8-35 molecules adsorbed onto the TM surface is not particularly likely (at least in the crystallographic conformation). One can probably not exclude an interaction with other APol particles
(Watkinson et al. 2017). Another possibility is that the protective effect of A8-35 is indirect. In the
case of OmpX, MD data indicate that trapping with A8-35 severely restricts the mobility of the
extramembrane loops (Perlmutter et al. 2014) (see Chap. 5, Fig. 5.31), so that certain conformations
become much less accessible in APol than in detergent solution (Fig. 5.32B). One cannot exclude
that such an APol-induced stiffening (for a discussion, see Chap. 5, § 5.7) shifts the distribution of
OmpF conformers away from those in which W58 and F177 are most accessible to peroxide-derived
free radicals.
14.3.4 Can Amphipols Better Suited to ESI-MS Be Developed?
Vacuum is a strange medium for a membrane biochemist to maneuver in, when the basis of our
thinking is the way chemical groups organize with respect to water. I therefore feel highly incompetent
to put forward suggestions about possible directions for the future of APols in the field of ESI-MS.
However, my perceived incompetence has seldom prevented me from indulging in speculations, and I
hope the reader will bear with me for a few more. If there turns out to be a grain of truth in them,
perfect. If not, at least we will have had our half-page of fun.
It may seem bizarre that monomeric MPs resist better transfer from aqueous solution to vacuum
when associated to APols than to detergent (Calabrese et al. 2015; Watkinson et al. 2015), whereas the
trimeric state of DAGK is not particularly well preserved (Hopper et al. 2013). A single case is not a
demonstration, and it does not prove that A8-35 cannot be used for studying MP oligomers. One may
wonder, however, what could be done to improve the usefulness of APols in preserving the native state
of MPs during their transfer from the solution to the gas state. One can probably speculate that the
problem, if any, may lie in (i) the polymeric nature of APols and (ii) the ionic nature of A8-35’s polar
moieties. The polymeric nature of APols implies that, as compared to DDM, less entropy will be
generated when they are shed by the protein. The “cooling” effect of surfactant evaporation (Ruotolo
et al. 2008; Borysik and Robinson 2012; Reading et al. 2015) therefore ought to be less efficient with
APols than with detergents. The ionic nature of the polar moieties of A8-35 implies that they will
establish strong interactions between themselves and with the protein as the complexes are dehydrated.
The larger the molecules, the more polar contacts they will establish with their neighbors and with the
MP surface. As is the case of DDM vs. Triton X-100 (Konijnenberg et al. 2014) or octylglucoside
vs. C 8 E 4 (Reading et al. 2015), more collisional energy will probably be needed to disassemble the
APol belt than if the hydrophilic moieties were less polar. The same situation probably holds for
glucosylated NAPols, whose polar moieties can establish multiple hydrogen bonds between them upon
dehydration, much as those of DDM do (Rouse et al. 2013).
14.3 Electrospray Ionization-Mass Spectrometry Studies
643
that F177 is located in the most distal part of an extracellular loop, whereas the side chain of W58 faces
the lumen of the β-barrel (Fig. 14.14), two positions where an interaction with the loops or tails of
A8-35 molecules adsorbed onto the TM surface is not particularly likely (at least in the crystallographic conformation). One can probably not exclude an interaction with other APol particles
(Watkinson et al. 2017). Another possibility is that the protective effect of A8-35 is indirect. In the
case of OmpX, MD data indicate that trapping with A8-35 severely restricts the mobility of the
extramembrane loops (Perlmutter et al. 2014) (see Chap. 5, Fig. 5.31), so that certain conformations
become much less accessible in APol than in detergent solution (Fig. 5.32B). One cannot exclude
that such an APol-induced stiffening (for a discussion, see Chap. 5, § 5.7) shifts the distribution of
OmpF conformers away from those in which W58 and F177 are most accessible to peroxide-derived
free radicals.
14.3.4 Can Amphipols Better Suited to ESI-MS Be Developed?
Vacuum is a strange medium for a membrane biochemist to maneuver in, when the basis of our
thinking is the way chemical groups organize with respect to water. I therefore feel highly incompetent
to put forward suggestions about possible directions for the future of APols in the field of ESI-MS.
However, my perceived incompetence has seldom prevented me from indulging in speculations, and I
hope the reader will bear with me for a few more. If there turns out to be a grain of truth in them,
perfect. If not, at least we will have had our half-page of fun.
It may seem bizarre that monomeric MPs resist better transfer from aqueous solution to vacuum
when associated to APols than to detergent (Calabrese et al. 2015; Watkinson et al. 2015), whereas the
trimeric state of DAGK is not particularly well preserved (Hopper et al. 2013). A single case is not a
demonstration, and it does not prove that A8-35 cannot be used for studying MP oligomers. One may
wonder, however, what could be done to improve the usefulness of APols in preserving the native state
of MPs during their transfer from the solution to the gas state. One can probably speculate that the
problem, if any, may lie in (i) the polymeric nature of APols and (ii) the ionic nature of A8-35’s polar
moieties. The polymeric nature of APols implies that, as compared to DDM, less entropy will be
generated when they are shed by the protein. The “cooling” effect of surfactant evaporation (Ruotolo
et al. 2008; Borysik and Robinson 2012; Reading et al. 2015) therefore ought to be less efficient with
APols than with detergents. The ionic nature of the polar moieties of A8-35 implies that they will
establish strong interactions between themselves and with the protein as the complexes are dehydrated.
The larger the molecules, the more polar contacts they will establish with their neighbors and with the
MP surface. As is the case of DDM vs. Triton X-100 (Konijnenberg et al. 2014) or octylglucoside
vs. C 8 E 4 (Reading et al. 2015), more collisional energy will probably be needed to disassemble the
APol belt than if the hydrophilic moieties were less polar. The same situation probably holds for
glucosylated NAPols, whose polar moieties can establish multiple hydrogen bonds between them upon
dehydration, much as those of DDM do (Rouse et al. 2013).
14.3 Electrospray Ionization-Mass Spectrometry Studies
643