elements that, directly or indirectly, interact with the target MPs. Their self-organizing properties are
therefore conceptually and practically vastly different from those of the synthetic polymers to which
we will now turn, which is why they have been discussed separately in Chap. 3. An interesting,
possibly intermediate case, which is currently difficult to classify, is that of amphipathic polymers
obtained by grafting poly-γ-glutamic acid with octyl and glucosyl groups, which have been used to trap
bacteriorhodopsin (BR) and a G protein-coupled receptor (GPCR) as water-soluble medium-sized
complexes (Han et al. 2014, 2017). It has not been reported whether the peptide is structured or not.
Under the assumption that it is not, these polymers have been included in the present chapter.
The term “amphipol” was coined to distinguish these particular polymers from the very vast
family of amphipathic polymers, most of which have structures that, e.g. because of their length, or of
the distribution of hydrophilic and hydrophobic moieties, are not suited to stabilizing MPs under the
form of small complexes. Amphipathic polymers are not only a fascinating object of study for physical
chemists interested in molecular self-organization, they also have hosts of practical applications, such
as for controlling the rheology of solutions, stabilizing emulsions, forming coats, delivering drugs, etc.
APols represent only a minute subset of this vast family, as exemplified by the (sobering!) observation
that, in a recent extensive review dedicated to the structure, synthesis, and applications of amphipathic
polymers (Raffa et al. 2015), only one reference out of nearly 500 bears on APols.
In the present chapter, we will review first (§ 4.2) the chemical structures of those polymers that
have been validated as bona fide APols, leaving aside those that have not been shown to form small
water-soluble complexes with MPs in their native state and therefore do not qualify as APols. For the
same reason, “blocky copolymers,” which feature long stretches of hydrophilic and hydrophobic units
(see Fig. 4.9), are not included here because, in comparative studies using as model hydrophobic,
non-membrane seed proteins called oleosins, they have been shown to form much larger complexes
than APols do (Gohon et al. 2011). Similar observations were reported upon extraction of MPs with
PreserveX™-QML (a proprietary mixture of blocky polymers), which yielded particles several
hundreds of nm large (Trubetskoy et al. 2006). The issue is not settled yet in the case of random
poly[N-(2-hydroxypropyl)-methacrylamide-co-dodecylmethacrylate] polymers, which have been
shown to keep water-soluble dimers of a synthetic peptide mimicking the transmembrane
(TM) anchor of glycophorin A (Stangl et al. 2014), forming complexes whose size and composition
have not yet been reported. In § 4.3, we will examine those studies that provide information about the
organization adopted by APols when they are dissolved in water. Finally, in § 4.4 we will review
labeled and functionalized APols. The formation and properties of MP/APol complexes will be
discussed in Chap. 5.
4.2
Amphipol Chemical Structure and Synthesis
In this section, we will examine what the basic structures of those polymers that have been proven to be
effective as APols are. One point that should be raised and stressed from the onset is that there is a very,
very long way to go from scribbling a potentially interesting chemical structure on the back of an
envelope to turning it into a routine tool for biochemists and biophysicists to use. A first synthesis and
test can be relatively easy and quick; debugging the synthesis, discovering and eliminating the pitfalls
of contaminants and side products, developing a reliable and economically viable synthesis and
purification route, assaying the new molecules in a large enough variety of conditions and on a
broad enough panel of MPs, understanding the behavior of the MP/APol complexes thus formed,
identifying their most promising applications, and convincing a good commercial partner to synthesize
and market the molecules represent a long-term, labor-intensive, and hard-to-fund project. It is therefore advisable to shave one’s many ideas with Occam’s razor and retain only a limited number of basic
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4 Chemical Structure, Synthesis, and Physical-Chemical Properties of Amphipols
therefore conceptually and practically vastly different from those of the synthetic polymers to which
we will now turn, which is why they have been discussed separately in Chap. 3. An interesting,
possibly intermediate case, which is currently difficult to classify, is that of amphipathic polymers
obtained by grafting poly-γ-glutamic acid with octyl and glucosyl groups, which have been used to trap
bacteriorhodopsin (BR) and a G protein-coupled receptor (GPCR) as water-soluble medium-sized
complexes (Han et al. 2014, 2017). It has not been reported whether the peptide is structured or not.
Under the assumption that it is not, these polymers have been included in the present chapter.
The term “amphipol” was coined to distinguish these particular polymers from the very vast
family of amphipathic polymers, most of which have structures that, e.g. because of their length, or of
the distribution of hydrophilic and hydrophobic moieties, are not suited to stabilizing MPs under the
form of small complexes. Amphipathic polymers are not only a fascinating object of study for physical
chemists interested in molecular self-organization, they also have hosts of practical applications, such
as for controlling the rheology of solutions, stabilizing emulsions, forming coats, delivering drugs, etc.
APols represent only a minute subset of this vast family, as exemplified by the (sobering!) observation
that, in a recent extensive review dedicated to the structure, synthesis, and applications of amphipathic
polymers (Raffa et al. 2015), only one reference out of nearly 500 bears on APols.
In the present chapter, we will review first (§ 4.2) the chemical structures of those polymers that
have been validated as bona fide APols, leaving aside those that have not been shown to form small
water-soluble complexes with MPs in their native state and therefore do not qualify as APols. For the
same reason, “blocky copolymers,” which feature long stretches of hydrophilic and hydrophobic units
(see Fig. 4.9), are not included here because, in comparative studies using as model hydrophobic,
non-membrane seed proteins called oleosins, they have been shown to form much larger complexes
than APols do (Gohon et al. 2011). Similar observations were reported upon extraction of MPs with
PreserveX™-QML (a proprietary mixture of blocky polymers), which yielded particles several
hundreds of nm large (Trubetskoy et al. 2006). The issue is not settled yet in the case of random
poly[N-(2-hydroxypropyl)-methacrylamide-co-dodecylmethacrylate] polymers, which have been
shown to keep water-soluble dimers of a synthetic peptide mimicking the transmembrane
(TM) anchor of glycophorin A (Stangl et al. 2014), forming complexes whose size and composition
have not yet been reported. In § 4.3, we will examine those studies that provide information about the
organization adopted by APols when they are dissolved in water. Finally, in § 4.4 we will review
labeled and functionalized APols. The formation and properties of MP/APol complexes will be
discussed in Chap. 5.
4.2
Amphipol Chemical Structure and Synthesis
In this section, we will examine what the basic structures of those polymers that have been proven to be
effective as APols are. One point that should be raised and stressed from the onset is that there is a very,
very long way to go from scribbling a potentially interesting chemical structure on the back of an
envelope to turning it into a routine tool for biochemists and biophysicists to use. A first synthesis and
test can be relatively easy and quick; debugging the synthesis, discovering and eliminating the pitfalls
of contaminants and side products, developing a reliable and economically viable synthesis and
purification route, assaying the new molecules in a large enough variety of conditions and on a
broad enough panel of MPs, understanding the behavior of the MP/APol complexes thus formed,
identifying their most promising applications, and convincing a good commercial partner to synthesize
and market the molecules represent a long-term, labor-intensive, and hard-to-fund project. It is therefore advisable to shave one’s many ideas with Occam’s razor and retain only a limited number of basic
152
4 Chemical Structure, Synthesis, and Physical-Chemical Properties of Amphipols