Intact cells and membrane fragments lend themselves to many functional and some structural
approaches. Among the latter are, for instance, Förster resonance energy transfer (FRET) or electron
paramagnetic resonance (EPR) studies, which can give information about conformational changes,
oligomerization, interaction with soluble proteins, ligands, etc., or atomic force microscopy, singlemolecule force spectroscopy, solid-state NMR, or some electron microscopy studies. In particularly
favorable cases, the target MP assembles into dense patches in which it is the only or the principal
protein. Some of these patches spontaneously form, or can be coaxed into forming, two-dimensional
(2D) crystals in the membrane plane, making the proteins that form them accessible to crystallographic
approaches. Such is the case, for instance, of bacteriorhodopsin (BR; see Chap. 1, § 1.6.1) and of the
nicotinic acetylcholine receptor (nAChR; ibid., § 1.6.2).
Investigations of MPs integrated in their native membrane environment are extremely precious,
because they provide benchmarks against which to gauge results obtained with more perturbed
samples, e.g. in solution. It is exceedingly rare, however, that they suffice to provide an exhaustive
understanding of the way a protein carries out its function. In particular, they seldom lead to highresolution structures permitting atomic models of the protein’s structure to be built. Furthermore, most
spectroscopic approaches requiring pure samples cannot, usually, be carried out without first extracting
the target MP from its native membrane and purifying it in aqueous solution – even if later experiments
are carried out on reconstituted membrane-based samples, such as artificial vesicles, 2D crystals, or
three-dimensional (3D) crystals formed of stacked 2D ones (see Chap. 11, § 11.2.2).
Solubilizing one’s target MP and handling it in solution is most often an obligate prerequisite to
purifying it, and this is usually where the troubles of the membrane biochemist begin.
2.2
Detergents
2.2.1
Chemical Structure
Detergents (from Latin detergere, to wipe away) are a special class of surfactants, whose distinctive
characteristic is to be endowed with the ability to solubilize fats: whereas all detergents are surfactants,
many surfactants – lipids, for instance – are non-solubilizing, and the distinction between the two
classes of surfactants should be made
1 . As noted in the introduction to Chap. 1, nearly all biological
molecules, including proteins, comprise hydrophobic and hydrophilic regions and, as a consequence,
are surface active. In the present book, however, the term “surfactant” will be restricted to relatively
small molecules, such as lipids, detergents, amphipathic peptides, or amphipols (APols), and not used
for biological macromolecules, whereas “detergent” will be reserved to those surfactants that are able
to solubilize fats. The dissociating character (“detergency”) of some surfactants can be borderline,
however, so that they are solubilizing with some membranes and/or under certain circumstances and
not under others (see § 2.3.1).
Detergents are able to make fats water-soluble because of the special arrangements their
molecules adopt when assembling in aqueous solutions. Most detergent molecules look like tadpoles:
they are comprised of one polar “head” and one hydrophobic “tail,” often an alkyl chain (Fig. 2.1)
1 There is a cultural divide between physicists and biologists about the use of these two words. Physicists shun the word
“detergent,” which they associate to laundry, and term detergents “surfactants,” without any special consideration of their
solubilizing properties, which they usually have little use for. Biologists are not always familiar with the term
“surfactant” and tend to call “detergents” even non-solubilizing surfactants. In this book, where we will constantly
deal with solubilizing and non-solubilizing surfactants, the term “detergent” will be exclusively used to designate the
former.
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2 Extracting Membrane Proteins from Their Native Environment
approaches. Among the latter are, for instance, Förster resonance energy transfer (FRET) or electron
paramagnetic resonance (EPR) studies, which can give information about conformational changes,
oligomerization, interaction with soluble proteins, ligands, etc., or atomic force microscopy, singlemolecule force spectroscopy, solid-state NMR, or some electron microscopy studies. In particularly
favorable cases, the target MP assembles into dense patches in which it is the only or the principal
protein. Some of these patches spontaneously form, or can be coaxed into forming, two-dimensional
(2D) crystals in the membrane plane, making the proteins that form them accessible to crystallographic
approaches. Such is the case, for instance, of bacteriorhodopsin (BR; see Chap. 1, § 1.6.1) and of the
nicotinic acetylcholine receptor (nAChR; ibid., § 1.6.2).
Investigations of MPs integrated in their native membrane environment are extremely precious,
because they provide benchmarks against which to gauge results obtained with more perturbed
samples, e.g. in solution. It is exceedingly rare, however, that they suffice to provide an exhaustive
understanding of the way a protein carries out its function. In particular, they seldom lead to highresolution structures permitting atomic models of the protein’s structure to be built. Furthermore, most
spectroscopic approaches requiring pure samples cannot, usually, be carried out without first extracting
the target MP from its native membrane and purifying it in aqueous solution – even if later experiments
are carried out on reconstituted membrane-based samples, such as artificial vesicles, 2D crystals, or
three-dimensional (3D) crystals formed of stacked 2D ones (see Chap. 11, § 11.2.2).
Solubilizing one’s target MP and handling it in solution is most often an obligate prerequisite to
purifying it, and this is usually where the troubles of the membrane biochemist begin.
2.2
Detergents
2.2.1
Chemical Structure
Detergents (from Latin detergere, to wipe away) are a special class of surfactants, whose distinctive
characteristic is to be endowed with the ability to solubilize fats: whereas all detergents are surfactants,
many surfactants – lipids, for instance – are non-solubilizing, and the distinction between the two
classes of surfactants should be made
1 . As noted in the introduction to Chap. 1, nearly all biological
molecules, including proteins, comprise hydrophobic and hydrophilic regions and, as a consequence,
are surface active. In the present book, however, the term “surfactant” will be restricted to relatively
small molecules, such as lipids, detergents, amphipathic peptides, or amphipols (APols), and not used
for biological macromolecules, whereas “detergent” will be reserved to those surfactants that are able
to solubilize fats. The dissociating character (“detergency”) of some surfactants can be borderline,
however, so that they are solubilizing with some membranes and/or under certain circumstances and
not under others (see § 2.3.1).
Detergents are able to make fats water-soluble because of the special arrangements their
molecules adopt when assembling in aqueous solutions. Most detergent molecules look like tadpoles:
they are comprised of one polar “head” and one hydrophobic “tail,” often an alkyl chain (Fig. 2.1)
1 There is a cultural divide between physicists and biologists about the use of these two words. Physicists shun the word
“detergent,” which they associate to laundry, and term detergents “surfactants,” without any special consideration of their
solubilizing properties, which they usually have little use for. Biologists are not always familiar with the term
“surfactant” and tend to call “detergents” even non-solubilizing surfactants. In this book, where we will constantly
deal with solubilizing and non-solubilizing surfactants, the term “detergent” will be exclusively used to designate the
former.
60
2 Extracting Membrane Proteins from Their Native Environment