Role of amphiphilicity in the design of synthetic mimics of antimicrobial peptides with gram-negative activity

Article abstract of DOI:10.1021/ml300307b

Two new series of aryl SMAMPs (synthetic mimics of antimicrobial peptides) with facially amphiphilic (FA) and disrupted amphiphilic (DA) topologies were designed and synthesized to directly assess the role of amphiphilicity on their antimicrobial activity

Full text of DOI:10.1021/ml300307b

Letter
pubs.acs.org/acsmedchemlett
Role of Amphiphilicity in the Design of Synthetic Mimics of
Antimicrobial Peptides with Gram-Negative Activity
Hitesh D. Thaker,† Alper Cankaya,† Richard W. Scott,‡ and Gregory N. Tew*,†
†Department of Polymer Science & Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States
‡PolyMedix Inc., 170 North Radnor-Chester Road, Suite 300, Radnor, Pennsylvania 19087, United States
S
* Supporting Information
ABSTRACT: Two new series of aryl SMAMPs (synthetic mimics of antimicrobial
peptides) with facially amphiphilic (FA) and disrupted amphiphilic (DA) topologies were
designed and synthesized to directly assess the role of amphiphilicity on their antimicrobial
activity against Gram-positive and Gram-negative bacteria in closely related structures. The
FA SMAMPs displayed broad spectrum antimicrobial activity against both Gram-positive S.
aureus and Gram-negative E. coli, whereas the DA SMAMPs, which contained a polar amide
bond in between the hydrophobic moieties, only exhibited activity toward S. aureus with increasing hydrophobicity. The integy
moment (IW) was used to quantify the amphiphilicity of the SMAMPs and confirmed that it is critical for the design of SMAMPs
with Gram-negative activity.
KEYWORDS: antimicrobial peptides (AMPs), synthetic mimics of antimicrobial peptides (SMAMPs), amphiphilicity, hydrophobicity,
facially amphiphilic, disrupted amphiphilic, Suzuki coupling, integy moment (IW), Volsurf software, Gram-negative, Gram-positive
he design of novel antibacterial agents with reduced
activity of AMPs is not fully understood. Gellman and co-
Tbacterial resistance has become imperative due to the workers studied the antimicrobial activity of several β-peptides
and found that the non-amphiphilic scrambled peptide was
inactive, confirming that a facially amphiphilic (FA) helix is
necessary for their antibacterial activity.8 Similarly, in a different
study, increasing the amphiphilicity and helicity of cecropin A-
melittin hybrids improved the activity and lowered the toxicity
as compared to the natural peptide, melittin.9 On the other
hand, an increase in the amphiphilicity of some peptides
(measured in terms of the hydrophobic moment) has been
shown to increase hemolytic activity.10−12
Focus has more recently turned to the interfacial partitioning
of both AMPs and SMAMPs into the membrane and their
ability to disrupt lipid organization within the bilayer.13,14
Wimley has further proposed that the peptide must have
disrupted amphiphilicity; i.e., the peptide must have imperfect
segregation of polar and hydrophobic groups with the
hydrophobic region broken up by the presence of at least
one polar residue (lysine or arginine).13 Several studies have
been reported that support this theory.11,15,16 For example, the
incorporation of a lysine residue into the hydrophobic region of
a peptide, D1 (V13), led to a significant reduction in toxicity
(32-fold) and a 3-fold enhancement in the antimicrobial activity
against Gram-negative bacteria.11 In another study, the pore-
forming ability of melittin was reduced when a charged residue
in the hydrophobic region was replaced by leucine (hydro-
phobic).15
emergence of antibiotic-resistant bacteria like methicillin-
resistant Staphylococcus aureus (MRSA) and vancomycin-
resistant Enterococcus (VRE).1 In the past two decades,
significant research on naturally occurring antimicrobial
peptides (AMPs) has demonstrated their potential as
therapeutic candidates.2 AMPs constitute an essential compo-
nent of the innate immune system of all multicellular
organisms. AMPs have multiple biological functions including
direct broad spectrum activity against a variety of pathogens
and modulation of the immune response.3 One plausible
mechanism of action involves interaction with and permeabi-
lization of the bacterial cell membrane, leading to cell death.4
This unique cell membrane interaction of AMPs makes
bacterial resistance evolution difficult, giving them a strong
advantage over conventional antibiotics.5 However, their
clinical development has been hampered due to their toxicity,
poor bioavailability, and low proteolytic stability.4,6 This has
driven the development of synthetic mimics of antimicrobial
peptides (SMAMPs), with the goals of overcoming the
problems associated with AMPs while retaining the essential
characteristics that are vital for antimicrobial activity.6,7
One of the main design parameters in the development of
SMAMPs is facial amphiphilicity, since most AMPs fold into
amphiphilic structures upon interaction with the bacterial cell
membrane.3 Natural AMPs typically have both cationic charges
and hydrophobic groups that segregate into the amphiphilic
structure. The positive charge, ranging from +2 to +9, is the
driving force for the electrostatic interaction between AMPs
and the negatively charged bacterial membrane, while the
hydrophobic residues help insertion into the hydrophobic
core.3 However, the influence of amphiphilicity on the overall
Previously, our research group designed several aromatic
oligomers based on arylamide,17,18 urea,19 and phenylene-
Received: October 1, 2012
Accepted: February 6, 2013
Published: March 27, 2013
© 2013 American Chemical Society
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ACS Medicinal Chemistry Letters
Letter
ethynylene (PE)20 backbones. These aromatic oligomers were
FA with broad spectrum antimicrobial activity and selectivity.
In the case of arylamides and ureas, rigidifying the
conformation via hydrogen bonding led to a more FA topology,
which improved the antimicrobial activity. The PE oligomers
adopted a FA topology at the oil−water interface via proper
placement of the amine groups and rotation around the single
bonds.21 However, to the best of our knowledge, there are no
reports evaluating the role of amphiphilicity and/or imperfect
hydrophobicity on the antimicrobial activity of oligomeric
SMAMPs. Herein, we report two new series of aryl SMAMPs
based on Suzuki coupling, designed to directly evaluate the role
of amphiphilicity on their activity. The first series of SMAMPs
is FA, and the second series has a disrupted amphiphilic (DA)
topology, with a polar amide linker incorporated in the
hydrophobic region to disrupt the amphiphilicity (Figure 1).
Table 1. Antimicrobial and Hemolytic Activities of the
SMAMPs with FA Topologies
a
Measured by HPLC using a C8 column with a gradient of 1%
b
acetonitrile/min starting with 100% water. Literature values
(reference 22 and references therein).
Table 2. Antimicrobial and Hemolytic Activities of the
SMAMPs with DA Topologies
Figure 1. Design of aryl SMAMPs with pendant aromatic groups. (a)
SMAMPs with a FA topology and (b) SMAMPs with a DA topology.
R1 and R2 represent the pendant aromatic groups (see Tables 1 and 2
for detailed structures). Blue and green colors represent the
hydrophilic and hydrophobic regions, respectively.
The structure−activity relationship (SAR) studies of these
series of SMAMPs reveal that an amphiphilic topology is critical
for broad spectrum activity, especially against Gram-negative
bacteria.
Previous SAR studies of aryl SMAMPs synthesized via Suzuki
coupling showed that the addition of a pendant aromatic ring
(SMAMP 1) significantly improved the activity of an otherwise
inactive SMAMP (R1 = H) toward Gram-negative E. coli.22
Moving forward with this strategy, a new series of SMAMPs
were designed with different pendant aromatic groups of
increasing hydrophobicity directly attached to the central ring
(Table 1, SMAMPs 2−5), thus retaining their FA structure. To
gain insight on the role of amphiphilicity on the antimicrobial
activity, a second series were developed, where an amide moiety
was used to append the pendant hydrophobic aromatic groups
(Table 2, SMAMPs 6−9). The polar amide group functioned as
a disruptive linker (hydrophilic patch) that broke up the
hydrophobic region, resulting in a DA topology. The synthesis
and characterization of all the SMAMPs are described in the
Supporting Information. The antimicrobial activity of these
SMAMPs was determined by their minimum inhibitory
concentration (MIC) against four pathogens (Tables S1, S2
a
Measured by HPLC using a C8 column with a gradient of 1%
acetonitrile/min starting with 100% water.
in the Supporting Information), including both Gram-negative
and Gram-positive bacteria, and the toxicity toward human red
blood cells (RBCs) was evaluated in terms of the HC50 (the
concentration that causes 50% hemolysis of the RBCs). The
relative hydrophobicity of the SMAMPs was measured by
reverse-phase HPLC retention time (Rt).
The antimicrobial and the hemolytic activities of the FA
SMAMPs are summarized in Table 1. In general, all of the
compounds had antimicrobial activities higher than the well-
known AMP MSI-78, which is currently in phase III clinical
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ACS Medicinal Chemistry Letters
Letter
trials as a topical antibiotic.23 All of the FA SMAMPs 2−5
showed improved antimicrobial activity against S. aureus as
compared to SMAMP 1. The substitution of the pendant
phenyl group with a methyl-phenyl group (SMAMP 2) resulted
in a 16-fold increase in activity against S. aureus with an MIC of
0.78 μg/mL. While this modification also led to a 2-fold
decrease in the HC50, it still resulted in a very high selectivity
toward S. aureus (selectivity ∼350). The incorporation of a
pendant indole ring in SMAMP 3 yielded the highest selectivity
in the FA series against S. aureus, with selectivity values of ∼400
for S. aureus and ∼100 for E. coli, along with very low toxicity
(HC50 = 634 μg/mL). This may be due to the unique
membrane affinity of the indole ring, as observed with
tryptophan.24 Further increases in hydrophobicity with the
tert-butyl phenyl and bis(trifluoromethyl)phenyl pendant
groups (SMAMPs 4 and 5) did not alter the antimicrobial
activity but led to increased hemolytic potency, indicating that
the SMAMPs had become less selective for both S. aureus and
E. coli. This suggests that increasing the hydrophobicity beyond
a particular threshold only increases hemolytic activity, without
any further improvement in the antimicrobial activity.
of the hydrophobic moment, but this is not possible in the case
of small molecule SMAMPs. For this purpose, VolSurf was
used, which converts the 3D molecular interaction fields of a
molecule into a set of simple descriptors.25 One of the
descriptors is the integy moment (IW), which is a vector
pointing from the center of mass of the molecule to the center
of the hydrophilic region. A higher IW value is indicative of
higher segregation of the hydrophilic and hydrophobic regions.
Thus, IW is a convenient parameter for quantifying the
amphiphilicity of a molecule. The IW values for both the FA
and DA series of SMAMPs (in their charged state) were
determined using the VolSurf software (Table S3 in the
Supporting Information).
A plot of antimicrobial potency against E. coli versus IW is
shown in Figure 2. In the DA SMAMPs (red dots), the
The DA SMAMPs 6−9 were synthesized using four different
pendant groups: benzene, indole, naphthalene, and phenyl-
benzene. Table 2 summarizes the antimicrobial and hemolytic
activities of the SMAMPs with DA topologies. The SMAMPs 6
and 7 containing benzene and indole rings, respectively, were
synthesized as a direct comparison to their FA analogues,
SMAMPs 1 and 3. In the case of SMAMP 6, the use of a polar
amide linker to connect the two benzene rings significantly
reduced the overall hydrophobicity (Rt = 26.1 min) in
comparison to its FA analogue, SMAMP 1 (Rt = 28.8 min).
This resulted in a complete loss of activity of SMAMP 6 against
both S. aureus and E. coli. Similarly, SMAMP 7 was more
hydrophilic and had a lower activity as compared to FA
SMAMP 3. The use of a pendant indole ring in the DA
SMAMP 7 improved the activity against S. aureus by 4-fold in
comparison to SMAMP 6, but no change in activity was
observed against E. coli.
Increasing the hydrophobicity of the SMAMPs in the DA
series to match the hydrophobicity range of the FA SMAMPs
required the use of more bulky and hydrophobic aromatic
residues. For this purpose, naphthalene and phenylbenzene
groups were used. SMAMP 8, with an amide linker and a
naphthalene pendant ring, had a retention time of 30.8 min,
which is comparable to the FA SMAMPs 2 and 3. Despite
identical hydrophobicities by HPLC, SMAMP 8 showed no
activity against E. coli, while SMAMP 3 had an MIC of 6.25 μg/
mL. The hydrophobicity was further increased by the addition
of a phenylbenzene group (SMAMP 9 with Rt = 33.2 min),
which improved the MIC against S. aureus (1.56 μg/mL) while
remaining inactive against E. coli. Plots of antimicrobial potency
(1/MIC) of the FA and DA SMAMPs against both E. coli and
S. aureus as a function of the HPLC retention time (see Figure
S9 in the Supporting Information) suggest that in the case of S.
aureus while amphiphilicity is important, it is the overall
hydrophobicity that determines their activity. However, for
Gram-negative E. coli, antimicrobial activity was more sensitive
to changes in amphiphilicity than S. aureus, since none of the
DA SMAMPs were active against E. coli.
Figure 2. Plot of the E. coli antimicrobial potency vs the IW for the
SMAMPs in the DA series (red dots) and the FA series (black
squares).
presence of the amide linker was enough to disrupt the
amphiphilicity, as evidenced by the lower IW values (less than
0.24) that remained constant even upon the addition of bulky
hydrophobic groups. On the other hand, the more potent FA
SMAMPs (black squares) all had IW values greater than 0.24.
These results indicate that a threshold value of amphiphilicity
exists, above which activity against Gram-negative E. coli can be
achieved.
While SMAMPs likely kill bacteria via multiple mechanisms,
similar to AMPs, membrane interactions seem to be critical.14
This increased importance of the FA topology in killing Gram-
negative bacteria is consistent with the requirement for the
generation of negative Gaussian curvature leading to pore
formation in bacterial membranes.26 The ability to induce
negative Gaussian curvature requires both positive and negative
mean curvature components.27 The SMAMPs contribute both
the negative-inducing components via cationic amines and the
positive-inducing component via insertion of hydrophobic
residues.14 The Gram-negative E. coli lipid membrane is richer
in negative intrinsic curvature lipid phosphatidylethanolamine
(PE ∼ 80%) as compared to the Gram-positive S. aureus lipid
membrane. Thus, it is probable that disruption of Gram-
negative bacteria requires more efficient insertion of hydro-
phobic components. Our aryl SMAMPs 2 and 8, for example,
have four amines and similar overall hydrophobicities as
measured by HPLC (Rt = 30.2 and 30.8 min, respectively);
however, the hydrophobicity of the DA SMAMP 8 is disrupted
by the presence of the amide linker. The high energy penalty of
Further assessment of a correlation between the antimicro-
bial activity and the amphiphilicity of these SMAMPs required
the quantification of their amphiphilicity. The amphiphilicity of
natural as well as synthetic AMPs is usually measured in terms
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2 kcal/mol required for the insertion of a polar amide moiety
into the hydrophobic core limits the penetration ability of the
DA SMAMPs.28 This reduces the volume of the hydrophobic
groups that is inserted into the membrane resulting in less
positive curvature generation, which is needed for pore
formation and a broad range of barrier disruption events.
This is reflected in the difference in activities of the DA and FA
SMAMPs 8 and 2 against E. coli (MIC > 50 vs 3.13 μg/mL,
respectively).
In summary, the SAR studies of the FA and DA SMAMPs
described here reveal the significance of topologically
homogeneous amphiphilicity for attaining antimicrobial activity,
especially against Gram-negative E. coli. The FA SMAMPs had
broad spectrum activity against both S. aureus and E. coli,
whereas the insertion of a disruptive amide linker in the DA
SMAMPs led to a complete loss of activity against E. coli. The
IW values of the SMAMPs provided an efficient approach to
quantify the amphiphilicity of these small molecules, and this
method can aid in the design of potent and selective SMAMPs
with broad spectrum activity that are potentially useful for
therapeutic applications.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, HPLC purity data, broad spectrum
antimicrobial activity, and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tew@mail.pse.umass.edu.
Funding
This work has been supported by the NIH (AI-074866 and
U01 AI-082192). Mass spectral data were obtained at the
University of Massachusetts Mass Spectrometry Facility, which
is supported, in part, by the National Science Foundation. This
work also used shared facilities supported from the MRSEC on
Polymers at UMass (DMR-0820506).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We acknowledge Melissa Lackey and Michael Lis for their
invaluable comments on the early drafts.
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Anguita, J.; Tew, G. N. Synthetic mimics of antimicrobial peptides with
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Designing antimicrobial peptides: form follows function. Nat. Rev.
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