et al. 2012). They can directly extract glycerol-3-phosphate dehydrogenase (GlpD) from Escherichia
coli membranes, although somewhat less effectively than the most efficient detergents (Yeh et al.
2005). GlpD was markedly more stable following DPS extraction than it is in OG solution. The size of
the complexes was not reported. DPSs have been used as the recipient surfactant to keep G proteincoupled receptors (GPCRs) water-soluble during or after in vitro cell-free synthesis (Corin et al. 2011;
Wang et al. 2011). Here, also, no size analysis was reported.
One caveat, when examining the solubilizing and stabilizing properties of DPSs, is that they are
able to assemble into large structures, tubes, and vesicles, whose walls are thought to be comprised of
peptide bilayers (Fig. 3.18). Whereas this basic organization evokes that of lipid bilayers, the peptide
bilayer’s internal chemistry is likely quite different, due to the formation of hydrogen bonds between
Fig. 3.17 Molecular model of a typical “designer peptide surfactant” (DPS). The hydrophobic moiety is
generally comprised of alanine or valine residues, the hydrophilic one of lysine, arginine, or aspartate ones.
The N- and C-termini are capped by acetylation and amidation, respectively. Each DPS is approximately
2–2.5 nm long, similar to phospholipids. Color code: green, carbon; red, oxygen; blue, nitrogen; white,
hydrogen. The peptide shown was among the most efficient of those tested for stabilizing the Photosystem
I reaction center in aqueous solutions (Matsumoto et al. 2009) (From Wang et al. 2011. # 2005 National
Academy of Sciences, USA. See also Santoso et al. 2002).
Fig. 3.18 (A, B) Quick-freeze/deep-etch/platinum-coating transmission electron microscopy image of
structures formed by designer peptide surfactants (DPSs) dissolved in water. The preparations contain both
vesicles and hollow nanotubes, some of the latter branched and apparently budding. Red arrows point to
tube openings (From Vauthey et al. 2002, # 2002 National Academy of Sciences, USA). (C, D) Proposed
molecular models of nanotubes (C) and nanovesicles (D) formed by cationic DPSs at pH below their pI
values. Color code: blue, positively charged amino acid heads; green, hydrophobic tail. The peptides pack
so that the polar heads are exposed to water, sequestering the hydrophobic tails within the bilayer, much
like in lipid bilayers. The diameters of the nanostructures are about 50–100 nm. Unlike lipid bilayers, in
which the hydrophobic effects that expel the acyl chains from water are primarily responsible for the
assembly, it is proposed that the packing of DPS bilayers involves backbone-to-backbone hydrogen bonds
(Reprinted with permission from von Maltzahn et al. 2003, # 2003 American Chemical Society).
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3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions
coli membranes, although somewhat less effectively than the most efficient detergents (Yeh et al.
2005). GlpD was markedly more stable following DPS extraction than it is in OG solution. The size of
the complexes was not reported. DPSs have been used as the recipient surfactant to keep G proteincoupled receptors (GPCRs) water-soluble during or after in vitro cell-free synthesis (Corin et al. 2011;
Wang et al. 2011). Here, also, no size analysis was reported.
One caveat, when examining the solubilizing and stabilizing properties of DPSs, is that they are
able to assemble into large structures, tubes, and vesicles, whose walls are thought to be comprised of
peptide bilayers (Fig. 3.18). Whereas this basic organization evokes that of lipid bilayers, the peptide
bilayer’s internal chemistry is likely quite different, due to the formation of hydrogen bonds between
Fig. 3.17 Molecular model of a typical “designer peptide surfactant” (DPS). The hydrophobic moiety is
generally comprised of alanine or valine residues, the hydrophilic one of lysine, arginine, or aspartate ones.
The N- and C-termini are capped by acetylation and amidation, respectively. Each DPS is approximately
2–2.5 nm long, similar to phospholipids. Color code: green, carbon; red, oxygen; blue, nitrogen; white,
hydrogen. The peptide shown was among the most efficient of those tested for stabilizing the Photosystem
I reaction center in aqueous solutions (Matsumoto et al. 2009) (From Wang et al. 2011. # 2005 National
Academy of Sciences, USA. See also Santoso et al. 2002).
Fig. 3.18 (A, B) Quick-freeze/deep-etch/platinum-coating transmission electron microscopy image of
structures formed by designer peptide surfactants (DPSs) dissolved in water. The preparations contain both
vesicles and hollow nanotubes, some of the latter branched and apparently budding. Red arrows point to
tube openings (From Vauthey et al. 2002, # 2002 National Academy of Sciences, USA). (C, D) Proposed
molecular models of nanotubes (C) and nanovesicles (D) formed by cationic DPSs at pH below their pI
values. Color code: blue, positively charged amino acid heads; green, hydrophobic tail. The peptides pack
so that the polar heads are exposed to water, sequestering the hydrophobic tails within the bilayer, much
like in lipid bilayers. The diameters of the nanostructures are about 50–100 nm. Unlike lipid bilayers, in
which the hydrophobic effects that expel the acyl chains from water are primarily responsible for the
assembly, it is proposed that the packing of DPS bilayers involves backbone-to-backbone hydrogen bonds
(Reprinted with permission from von Maltzahn et al. 2003, # 2003 American Chemical Society).
122
3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions