The SMART model: Soft Membranes Adapt and
Respond, also Transiently, in the presence of
antimicrobial peptides
†
Burkhard Bechinger*
Biophysical and structural studies of peptide–lipid interactions, peptide topology and dynamics have changed our view on how
antimicrobial peptides insert and interact with membranes. Clearly, both the peptides and the lipids are highly dynamic, change
and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a con-
sequence, the peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences
preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar
to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay inter-
calated at the membrane interface where they cause pronounced disruptions of the phospholipid fatty acyl packing. At increasing
local or global concentrations, the peptides result in transient membrane openings, rupture and ultimately lysis. Depending on
peptide-to-lipid ratio, lipid composition and environmental factors (temperature, buffer composition, ionic strength, etc.), the
same peptide sequence can result in a variety of those responses. Therefore, the SMART model has been introduced to cover
the full range of possibilities. With such a view in mind, novel antimicrobial compounds have been designed from amphipathic
polymers, peptide mimetics, combinations of ultra-short polypeptides with hydrophobic anchors or small designer molecules.
Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.
Keywords: magainin; alamethicin; membrane topology; local disorder; membrane pore; membrane macroscopic phase; carpet model;
toroidal pore; hydrophobic mismatch; cecropin; peptide–lipid interactions; PGLa; peptaibol; equlibria
Introduction
Q1 In view of a worldwide re-emergence of infectious diseases and a
rapid increase in pathogens that are multi-resistant to commercially
available antibiotics, new strategies to fight such infections have to
be developed. When bacteria become resistant, more costly
second-line antibiotics are used, but even these drugs become inef-
fective over time [1]. As a consequence, the development of novel
antibiotics is needed to counteract the steady decline of approved
pharmaceuticals where alternative agents with completely novel
mechanisms of action are desirable. A promising strategy is based
on copying the mechanism of action of natural compounds such
as antimicrobial peptides (AMPs). These effector molecules of in-
nate immunity provide a first line of defence against a multitude
of pathogenic microorganisms and are capable to keep in check
many invaders of higher organisms [2,3]. They work, e.g., by physi-
cal interference with the barrier function of bacterial membranes,
and there is good evidence that the latter are indeed the primary
target of a large number of AMPs. Notably, when compared with
antibiotics that interact with specific receptors, bacteria are less
likely to develop resistance to AMPs interfering with the lipid bi-
layer [4].
Therefore, naturally occurring AMPs are valuable template struc-
tures to develop new concepts for efficient pharmaceutical com-
pounds with increased efficiency. Indeed, recent research efforts
following this strategy have come up with small amphipathic mol-
ecules, pseudopeptides and even polymers that all exhibit potent
antimicrobial activities (e.g. [5–9]). In order to achieve this goal, a
number of membrane-active peptides, many available from natural
sources, some by design, have been studied by biophysical and
biochemical methods. These peptides included early on
alamethicin and melittin [10], and later magainins [11], cecropins
and designed peptides [12–14] (cf Table T11 for sequence informa-
tion). By combining the insights from a variety of biophysical stud-
ies on different sequences, a comprehensive view on polypeptide
lipid interactions and the mechanisms of membrane perme-
abilization and pore formation have been obtained. Up to this
day, unexpected structural and dynamic features of membrane-
associated polypeptides are discovered [15,16], and a picture
emerges with multiple equilibria that govern their membrane inter-
actions and conformations (Figure F11).
Here, some of the biophysical data from a selected set of
peptides will be reviewed, which consolidate in a view of how the
peptides interact with membranes in a highly dynamic manner.
* Correspondence to: Burkhard Bechinger, Faculté de chimie, Institut le Bel, 4, rue
Blaise Pascal, 67070 Strasbourg, France. E-mail: bechinge@unistra.fr
†
Q13Contribution to the Special Issue dedicated to the Naples Workshop on Antimicro-
bial Peptides 2014
Université de Strasbourg/CNRS, UMR7177, Institut de Chimie, 4, rue Blaise Pascal,
67070 Strasbourg, France
Abbreviations: Aib, α-aminobutyric acid; AMP, antimicrobial peptide; DMPC, 1,2-
dimyristoyl-sn-glycero-3-phosphocholine; DMPG, 1,2-dimyristoyl-sn-glycero-3-
phospho-(1′-rac-glycerol); DOPC, 1,2-dioleoyl-sn-glycero-3-phosphocholine; DOPG,
1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol); GUV, giant unilamellar vesicle;
IP, in-plane; LUV, large unilamellar vesicle; NMR, nuclear magnetic resonance; PC,
phosphatidylcholine; PE, phosphatidylethanolamine; PG, phosphatidylglycerol;
PS, phosphatidylserine; POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine;
TM, transmembrane.
J. Pept. Sci. 2014 Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.
Review
Received: 17 October 2014 Revised: 21 November 2014 Accepted: 26 November 2014 Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/psc.2729
1
Journal Code Article ID Dispatch: 09.12.14 CE:
P S C 2 7 2 9 No. of Pages: 10 ME:
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Not only the conformation of the polypeptide chains undergoes
profound changes but also the lipid bilayers adapt and respond
to the insertion of peptides into their hydrophobic core and/or in-
terface. The examples that will be discussed include the very hydro-
phobic peptaibols from fungi and some of the much more
amphipathic cationic sequences that are frequently found in am-
phibians. Whereas the schemes presented in Figure 1 provide a
common description of their membrane interactions, the very de-
tails and their preferred topology and aggregation state is dictated
by the physicochemical properties of the peptides and the lipids
under investigation. In the following, biophysical investigations on
hydrophobic peptaibol sequences and amphipathic linear cationic
AMPs shall be reviewed before a model will be presented unifying
different sequences and experimental conditions. The paper starts
by presenting data on the hydrophobic peptides such as
alamethicins, which served for a long time as the paradigm for
peptide–membrane interactions and formed our ‘classical view’
on how proteins and peptides interact with lipid membranes.
Hydrophobic Sequences with a Strong Pro-
pensity for Transmembrane Alignments:
Alamethicin as an Example
Alamethicin is composed of 20 residues and the best-investigated
member of the peptaibol family, peptides of fungal origin that en-
compass α -aminobutyric acid (Aib) residues at high abundance [17]
(Table 1). In the presence of alamethicin and other peptaibols,
voltage-dependent conductance changes have been measured
with well-defined ohmic resistance, opening times and frequency
[18–22]. Therefore, at a time when the structures of membrane pro-
teins were largely unknown, alamethicins served as a paradigm for
large voltage- or ligand-gated channel proteins (reviewed, e.g., in
[23]). The alamethicin pore has been modelled as a ‘transmem-
brane helical bundle’ in which the individual helices are grouped
with their more hydrophilic side facing the water-filled pore. The
measured conductivities of the water filled openings created in this
manner agree reasonably well with theoretical predictions where
the smallest conducting oligomers are made of three, four or five
subunits (reviewed in [10,23]). The higher conductance states have
been explained by the assembly of increased order oligomers.
Therefore, it has been suggested that the peptides follow a series
of equilibria where they transfer from the aqueous environment
to an interface-associated -
Q2, a membrane inserted state and the for-
mation of oligomers [20,24–27] (Figure 1(A)).
Whereas the initial formation and the decay of the lowest con-
ductance state are characterized by high activation energies (50
and 120 kJ/mol, respectively), further addition and subtraction of
subunits occur quickly on a millisecond time scale. More recently,
molecular dynamics calculations of membrane-bound alamethicin
suggest that the macromolecular arrangement of the helix bundle
is less regular and more asymmetric than the first pictural views
suggested [28].
Indeed, oriented solid-state NMR spectroscopy shows a strong
propensity of alamethicins for transmembrane (TM) alignments in
DMPC and POPC membranes [29–31]. This is consolidated by
solution-state NMR experiments where a preference for helical
Table 1. Sequences of peptides discussed in this paper
melIttin GIGAV LKVLT TGLPA LISWI KRKRQ Q-NH
2
Magainin 2 GIGKF LHSAK KFGKA FVGEI MNS-NH
2
PGLa GMASK AGAIA GKIAK VALKA L-NH
2
cecropin A KWKLF KKIEK VGQNI RDGII KAGPA VAVVG QATQI AK-NH
2
PMAP-23 RIIDL LWRVR RPQKP KFVTV WVR
LAH4 KKALL ALALH HLAHL ALHLA LALKK A-NH
2
Alamethicin (F50/7) Ac-Aib-Pro-Aib-Ala-Aib-Aib-Gln-Aib-Val-Aib-Gly-Leu-Aib-Pro-Val-Aib-Aib-Gln-Gln-Phl
Ampullosporin A Ac-Trp-Ala-Aib-Aib-Leu-Aib-Gln-Aib-Aib-Aib-Gln-Leu-Aib-Gln-Lol
Antiamoebin Ac-Phe-Aib-Aib-Aib-Iva-Gly-Leu-Aib-Aib-Hyp-Gln-Iva-Hyp-Aib-Pro-Phl
Trichonin GA IV nOct-Aib-Gly-Leu-Aib-Gly-Gly-Leu-Aib-Gly-Ile-Lol
Zervamicin IIB Ac-Trp-Ile-Gln-Iva-Ile-Thr-Aib-Leu-Aib-Hyp-Gln-Aib-Hyp-Aib-Pro-Phl
The one-letter code is used for peptides made from conventional amino acids only. The peptaibols sequences are given by the three-letter code with the
following non-standard residues: Aib, a-aminoisobutyric acid; Iva,
D-isovaline; Hyp, trans-4-hydroxy-L-proline; Phl, L-phenylalaninol; Lol, L-leucinol; Ac-,
acetyl-; n-Oct, n-octanyl; NH
2
,thecarboxyamidetermini.
Biography
Burkhard Bechinger obtained his PhD
in 1989 in the Department of Biophys-
ical Chemistry at the Biocenter of the
University of Basel in Switzerland on
the study of electrostatic interactions
within lipid bilayers. Thereafter, he
became postdoctoral research associ-
ate at the University of Pennsylvania
(1990–1993) where he developed
solid-state NMR spectroscopy for the
investigation of membrane-associated
polypeptides. As a result of this work, it was discovered for
the first time that cationic amphipathic peptides orient paral-
lel to the membrane surface. He started his own research
group in 1993 at the Max Planck Institute of molecular Physi-
ology, Dortmund Germany and as the head of an Indepen-
dent Junior Research Group at the MPI of Biochemistry in
Martinsried, Germany (1995–2001). During this time period,
he and his team investigated histidine-containing flipper pep-
tides that change membrane topology as a function of pH
and have potent antimicrobial and nucleic acid transfection
properties. Since 2001, he is a full professor at the chemistry
department of the University of Strasbourg, France where
his team designs and studies peptides with different biologi-
cal activities as well as membrane proteins using NMR spec-
troscopy and a variety of other biophysical approaches.
Bechinger
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conformations with a flexible hinge region around proline-14 has
been observed [32,33].
A more detailed analysis reveals a quite dynamic picture where
the peptides exhibit a high degree of conformational flexibility
and structural details that are a function of the physical state of
the lipid, the lipid–peptide ratio, the presence of TM potentials
and other factors (reviewed, e.g., in [10,23]). It has even been possi-
ble to define peptide-to-lipid ratios and membrane lipid composi-
tions, including the ones close to that of bacterial membranes,
where the alamethicin helices predominantly align parallel to the
membrane surface [16,34]. Finally, related arrangements of TM he-
lices have been revealed for many membrane proteins including
potassium channels, the acetylcholine receptor, the Influenza pro-
ton channel or the phospholamban pentamer [35–37].
It is thought that alamethicin initially adsorbs and intercalates to
the membrane interface, a configuration which is gated into a se-
ries of subsequent equilibria (Figure 1(A)), where the realignment
of the helix dipole by TM electric fields can help in the insertion pro-
cess (reviewed in [10,23]). TM orientations have been observed for
other hydrophobic polypeptide sequences [14,36,38–40], including
the membrane anchors of colicins [41] or Bcl-xL [42]. Interestingly,
the membrane topology of the latter is controlled by a sensitive
equilibrium between in-plane and TM configurations, which may
be important to allow for the reversible association of these pro-
teins, which are of about 20 kDa in size.
Interesting insight into the interaction contributions that deter-
mine the membrane alignment of hydrophobic membrane se-
quences is obtained when closely related peptaibols that carry
fewer residues than alamethicin are investigated. Indeed, shorter
peptaibols (e.g. Table 1) still exhibit single channel conductances
that resemble that of the longer sequence. When studied in more
detail, zervamcin IIa (16 residues) tends to form relatively large olig-
omeric assemblies from which the exchange of monomers occurs
at an order of magnitude increased rate when compared with
alamethicin [21]. In a related manner, alamethicin causes rather
high conductance levels when reconstituted into thick membranes
[43]. Indeed, under conditions of hydrophobic mismatch, consider-
able membrane deformations occur [14,44,45], which can probably
be alleviated by assembly of the peptides into larger structures. In
contrast, antiamoebin (16 residues) does not exhibit single-channel
conductivities under conditions where other peptaibols produce
such conductivities [22]. In a similar manner, the pore-forming ac-
tivities of trichogin GA IV (11 residues extended by n-octanoyl) are
much reduced in membranes of average thickness but can be re-
stored in thinner membranes [45].
Interestingly, when investigated by solid-state NMR, zervamicin
IIa (16 residues) and ampullosporin A (15 residues; Table 1) are
found to predominantly orient parallel to the surface of POPC mem-
branes [31,46]. When reducing the membrane thickness by
reconstituting the peptides into PC bilayers made from C10:0 or
C12:0 fatty acyl chains, these shorter peptaibols insert into a TM
configuration indicating that hydrophobic mismatch energies be-
come an important contribution to membrane topology. In the
thicker membranes, the short peptides impose strain onto the
membrane and disrupt the regular lipid packing shifting the equi-
librium to the in-planar states (Figure 1(A)). Whereas favourable en-
ergy contributions for TM insertion arise by moving hydrophobic
residues from the membrane interface or from the aqueous phase
into the region of the hydrocarbon chains, unfavourable interac-
tions are associated with such lipid-derived energy contributions.
Even though these and other energy contributions have been
discussed previously, including the removal of charges from side
chains, oligomerization or polar interactions within the hydropho-
bic environment (e.g. reviewed in [47]), it should be noted that
overall large favourable and large unfavourable energy contribu-
tions are involved in the insertion process, which makes the predic-
tion of the membrane topology rather difficult. This may also
explain why anticipating the membrane topology by molecular dy-
namics simulations remains a difficult task, and the comparison
with experimental data remains essential to validate such calcula-
tions [48–51].
This short summary of a wealth of biophysical investigations on
peptaibols reveals that even for these peptides that are considered
paradigms for TM helical bundle formation, the situation is much
more complex than was initially expected. Notably, these seem-
ingly ‘well-understood’ sequences can deviate from the classical
view of a stable TM helical bundles. Therefore, it is not surprising
that the situation becomes even more complex when cationic am-
phipathic peptides are considered where additional electrostatic in-
teractions and upon membrane insertion profound changes in the
peptide physicochemical properties along the polar/apolar inter-
face occur.
Figure 1. Schematic diagram showing the equilibria governing membrane insertion and interactions of (A) hydrophobic sequences such as alamethicin and
(B) charged amphipathic peptides with a high hydrophobic moment such as magainins.
THE SMART MODEL FOR AMPs
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The Membrane Interactions of Strongly
Cationic Amphipathic AMPs
Magainins and cecropins were among the first sequences that
founded the family of linear cationic amphipathic sequences
[3,52] (Table 1). Peptides of this class have meanwhile been identi-
fied in many species where they establish an efficient and highly re-
active defence mechanisms against a wide variety of infections
[3,52]. A number of observations point to a direct interaction of
these peptides with phospholipid membranes of bacteria and fungi
where they often disrupt the bilayer integrity. These peptides in-
hibit the growth of sensitive microorganisms, exhibit cell killing ac-
tivities and/or enter into the cell interior [53]. In addition, they have
been found to modulate the immune response of the host organ-
isms. To take into account their extended functionality, the term
‘host defence peptides’ has been introduced [54–56].
When the linear cationic AMPs are considered, magainins and its
derivatives are probably the ones whose membrane interactions
have been investigated most extensively using a wide variety of
biophysical experiments (e.g. [57–61]). Upon addition to preformed
bilayers, magainins and cecropins have been found to often lyse
the membranes. However, in some experiments, discrete multi-
level conductances are also observed [62–64]. Unlike the
alamethicin channels, those recorded in the presence of magainins
or cecropins are less well defined, erratic and characterized by large
variations.
Interesting insights into the membrane interactions of magainin
2 have also been obtained when the kinetics of pore formation and
the size of the openings have been investigated by fluorophore re-
lease experiments using giant unilamellar vesicles (GUVs). These
vesicles are large enough (typically several micrometres) such that
it is possible to follow single events and by repeating the experi-
ment many times analyse theses in a statistical manner [65]. After
addition of magainin, it takes one or a few minute before the re-
lease of fluorophore sets in, which then empties the vesicle within
about 30 s [65]. These data indicate that the rate-limiting step is
pore formation rather than the membrane permeation through a
pore and in agreement with an all-or-none mechanism that has
been obtained when analysing calcein release from LUV suspen-
sions [66]. Pores form when the ratio of membrane-associated
magainin 2 over lipid reaches about 0.7 mol% in DOPC/DOPG
GUVs, with only a small dependence on the DOPC/DOPG ratio
[67]. Membrane openings occur in two stages, starting with an ini-
tial rapid release of fluorophore where very large pores form tran-
siently. These have been associated with the unbalance in tension
between the outer and inner monolayers when magainin associ-
ates with the outside of the GUV. Once the membrane ruptures
and lipids and magainin re-equilibrate between the two bilayer
leaflets, smaller pores form, thus slowing down the leakage of the
fluorophore [68]. When the size of these openings is tested, even
the persistent pores are found to be quite large with a cut-off of
the hydrodynamic radius for the exiting molecules in the 3 nm
range (corresponding to proteins of MW >20 kDa) [68].
Magainins and cecropins are about 20–40 residues in length,
highly charged (Table 1) and, therefore, soluble in aqueous solution
where they exhibit a high degree of random coil conformations
[69]. When associating with the membrane, helical conformations
are induced (Figure F22), a process that provides about half the Gibbs
free energy of membrane association (about 0.8 kcal/mol per res-
idue undergoing the transition [74,75]). Solid-state NMR and ori-
ented CD spectroscopies indicate that they reversibly associate
with the membranes with the helix axis oriented parallel to the
membrane surface [11]. This alignment of magainins [58,76,77],
their analogues [78–80] and of a considerable number of other
linear cationic AMPs [81–85] contrasts that of the much more
hydrophobic alamethicin. However, an interfacial alignment is in
agreement with the large hydrophobic moment obtained when
these charged amphipathic sequences adopt helical conformations
(Figure 2). Indeed, molecular modelling calculations visualize how
magainin 2 and the designer peptide LAH4 (Table 1) could cause
the formation of membrane lipidic pores without the need to adopt
a well-defined assembly of TM domains or of peptide–peptide con-
tacts [50,86,87]. This behaviour can be rationalized by the molecular
shapes of lipids and the peptide [88,89]. When inserted into the
membrane, one would expect that the charged residues reside in
the membrane interface, whereas the hydrophobic side chains lo-
calize below the lipid carbonyls (Figure F33).
Whereas the topology of magainin 2 parallel to the membrane
surface appears stable in solid-state NMR and fluorescence experi-
ments and has been reproduced in a wide variety of lipid bilayers
[11,58], the close relative PGLa changes its tilt angle when
reconstituted in di-saturated membranes at higher peptide-to-lipid
ratio (Figure 1(B)). The change in tilt by about 30° has been rational-
ized by homodimer formation [90], but this hypothesis still needs to
be proven experimentally [91]. The tendency of PGLa to insert into
PC or PC/PG membrane made of di-saturated fatty acyl chains is
further enhanced in thin membranes or in the presence of
magainin 2 [92,93]. However, in bilayers made of palmitoyl-oleoyl-
phospholipids, which are thought to better represent the thickness
and fatty acyl composition of natural membranes, stable in-planar
alignments are also obtained for PGLa even in the presence of
magainin 2 [11,92,94] (cf below).
The association of magainin 2 is characterized by a partitioning
coefficient in the 1000 M
1
range, but the apparent partitioning
Figure 2. Helical wheel analysis and hydrophobic moment calculations of the amphipathic helical domains of magainin 2, PGLa, LAH4 and cecropin A using
the Heliquest program [70]. The structures were obtained in DPC micelles [57,71,72] or for cecropin A in 15% hexafluoro isopropanol [73]. For LAH4, the
structural data obtained at pH 4.1 were used. The hydrophobic moments were 0.52, 0.38, 0.46 and 0.59, respectively. In the case of LAH4, this value was
obtained when virtually replacing the histidines by lysines, in order to mimic histidines in its charged state at the low pH of the experiment.
Bechinger
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constant is increased by two to three orders of magnitude in the
presence of acidic phospholipids [95,96]. This is due to the accumu-
lation of positively charged ions and macromolecules next to the
negatively charged surface. In this manner, the binding equilibrium
appears increased, but when the much higher local concentration
next to the surface, rather than the peptide concentration in bulk
solution, is taken into account, very similar partitioning coefficients
are obtained [96]. Therefore, negatively charged lipopolysaccha-
rides and anionic lipids at the outer surface of bacterial cells en-
hance the local concentrations in the proximity of the membrane
and are probably an important determinant for the selectivity of
the peptides for bacterial over eukaryotic cells. This ‘electrostatic re-
sponse’ also explains why tumour cells, where the PS content of the
outer monolayer is increased, are more susceptible to the action of
these peptides when compared with healthy cells [96–100]. Within
the same concept, the changes in the electrostatic potential during
the association of cationic peptides and the concomitant release of
peripheral membrane proteins have been suggested to be a deter-
minant of antimicrobial activities [101].
It seems that the membrane penetration depth and topology are
also modulated by the presence of negatively charged lipids [102]
and that segregation of domains enriched in cationic peptide and
acidic phospholipids occurs [103,104]. Recently, it has also been
demonstrated that the antimicrobial designer peptide LAH4
(Table 1) and other amphipathic sequences distribute unevenly in
mixed PC/PS membranes probably by adopting a mesophase-like
arrangement on the membrane surface [105]. This phenomenon
is still under investigation, but the resulting increase in concentra-
tion and organization could have important implications for the
mechanism of action and the selectivity of AMPs.
When the highly cationic amphipathic peptides adopt an ar-
rangement parallel to the membrane surface, the hydrophobic re-
gion is localized about 10 Å above the bilayer centre [58]. This
causes the lipid fatty acyl chains to move underneath the helical do-
main (Figure 3), concomitant with a disordering of the fatty acyl
chain packing [106] and membrane thinning [61,107]. Ultimately,
the formation of membrane openings [66] and macroscopic phase
transitions of the peptide–lipid assembly occur [89] (Figure F44).
Solid-state NMR measurements in the presence of magainins and
other amphipathic peptides have indeed monitored changes in
the order parameter at the lipid bilayer interior that agree with such
a model [106,108]. The bilayer disruptions extend far beyond the
immediate proximity of the membrane-inserted peptides and have
been estimated to cover a radius of approximately 50 Å [109,110].
In many ways, the peptide interacts with the lipid membranes in
a manner analogous to a detergent [89]. Whereas at high peptide
concentrations both peptides and detergents cause the disruption
of the bilayer integrity as suggested by the ‘carpet model’ [111], a
variety of supramolecular arrangements can be obtained at lower
concentrations [112–114] including ‘aggregates’ of undefined mo-
lecular structure [115]. Therefore, ‘detergent-like’ should not be
confounded with ‘always lytic’. In contrary, it should be kept in mind
that at lower detergent (peptide) concentrations, these molecules
may even result in more stable lipid bilayer arrangement, for exam-
ple, when the inverted cone-shaped structure of a detergent com-
pensates for the strain imposed by a cone-shaped PE lipid
[108,116,117] (Figure 3(B)). Depending on lipid composition, pep-
tide concentration, pH, temperature and other environmental fac-
tors, a stochastic and transient rupture and closure of the
membrane is possible (Figure 3(C)–(E)), which can explain the elec-
trophysiological traces, which look (only) on first glance like those
expected when well-defined channel structures form [62–64].
Recent publications had shown that other important physico-
chemical properties of the membranes change upon association
of AMPs. For example, a recent study showed that the lateral and
rotational diffusion of membrane components is reduced in the
presence of the AMP alamethcin or the cationinc PMAP-23 se-
quence (Table 1) [118]. As contacts and exchange between mem-
brane components are essential for proper functioning, this can
have detrimental effects on the viability of the cells.
Finally, it should be considered than many peripheral membrane
proteins and components associate through electrostatic interac-
tions. With a high density of cationic AMPs [53], screening the neg-
ative charges causes the release of many of these proteins, thereby
interfering with cell viability [101].
Synergistic Enhancement of the Activities of
AMPs
Interestingly, the efficiency of some antimicrobial compounds is
considerably potentiated when applied in combination. Such syn-
ergistic enhancements have been observed for naturally occurring
Figure 3. The effects an in-plane oriented peptide (coloured yellow and
ochre) has on the bilayer packing. (A) An isolated amphipathic helix (seen
along the helix axis) in a PC membrane. (B) The smaller head group of PE
lipids partially compensate for the membrane-disruptive effects of the
helix. (C, D and E) Higher local concentrations of peptides cause the
transient opening of the membrane. During such events, the peptide can
diffuse from one side to the other without changing the alignment
relative to the membrane normal (C) or by transiently adopting a different
orientation (D, E). The cylindrical shape of PC lipids is sketched in panel A,
the cone shape of PE and the inverted cone shape of the membrane-
inserted amphipathic helix, respectively, in panel B.
THE SMART MODEL FOR AMPs
J. Pept. Sci. 2014 Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jpepsci
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