Development and biological assessment of fully water-soluble helical aromatic amide foldamers

Article abstract of DOI:10.1002/anie.200700301

Abiotic but bioactive: Folding aromatic amide oligomers possessing helical structures very different from those of α-peptides, β-peptides, or peptoids nevertheless show comparable abilities to penetrate into cells, as well as low toxicity and complete resistance to protease degradation. TFA ^- = trifluoroacetate. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200700301

Angewandte
Chemie
DOI: 10.1002/anie.200700301
Peptidomimetics
Development and Biological Assessment of Fully Water-Soluble
Helical Aromatic Amide Foldamers**
Elizabeth R. Gillies, FrØdØrique Deiss, Cathy Staedel, Jean-Marie Schmitter, and Ivan Huc*
Much effort is being devoted to develop oligomers that adopt
stable helical conformations and mimic the structures of
folded biopolymers.[1] For example, bio-inspired helical
oligomers based on aliphatic backbones such as b-pepti-
des[1b,c,2–5] and peptoids[1e,6–9] may adopt helical conformations
related to peptide a helices and polyproline typeI helices,
respectively. These compounds have attracted considerable
interest because of their promising biological activity in a
number of areas including the development of antibiotics[3,7]
and gene delivery systems,[4,8] and the inhibition of protein–
protein interactions.[5,9] This success was anticipated based on
Figure 1. Top view and side views of fragments shown at the same
scale of crystal structures of: a) a peptide a helix and b) a helix of an
aromatic oligoamide foldamer. Side chains (R) are shown as yellow
spheres. These structures illustrate the striking differences between
the close structural resemblance of the synthetic oligomers to
a-peptides and their improved properties, such as a decreased
susceptibility to proteolytic degradation.[10] Linear aromatic
oligomers with amphipathic arene faces also show promising
biological activity[11] and may be designed to mimic one face
of a peptide a helix.[12] However, numerous classes of helical
oligomers possess structures remote from those of peptides
and may not, a priori, feature any peptide-like biological
activity. For instance, secondary amide aromatic oligomers[1b]
developed by us[13–16] and by others,[17] for example, oligomers
of 8-amino-2-quinoline carboxylic acid, form helices that are
wider, with a smaller helical pitch, and much more stable than
those of peptides (Figure 1). These oligomers are receiving
increasing attention because of the high predictability and
tunability of their conformations. However, none has been
made highly water-soluble, and their biological potential has
remained unexplored. Here, we show that multiple ammoni-
um side chains confer helical aromatic oligoamides with high
water-solubility, and that some of their biological properties
compare favorably with those of aliphatic peptide mimics
the two motifs in terms of side-chain density, diameter, aspect ratio (at
comparable molecular weight), helical pitch, and overall chemical
composition.
even though their conformations differ considerably. Our
results provide the groundwork for promising future biolog-
ical applications of this and similar classes of molecules.
Our first attempt to prepare water-soluble oligoamides of
8-amino-2-quinoline carboxylic acid focused on amphipathic
helices bearing both hydrophilic and hydrophobic residues.[15]
These molecules allowed us to demonstrate that the stability
of the helical conformation is actually enhanced in the
presence of MeOH and H2O, but their solubility in aqueous
buffers and cell culture media proved too low to assess their
biological behavior. Thus, oligomers 1–6 (Figure 2) were
equipped with multiple cationic side chains to improve their
hydrosolubility and because such side chains are known to
assist in processes such as DNA transfection[4,8] and mem-
brane transport.[18] The multiple synthetic manipulations
carried out to control the oligomer length, side-chain
functionality, charge density, and hydrophilicity are described
in the Supporting Information. A major improvement over
previous syntheses was that monomers were activated as acid
chlorides using 1-chloro-N,N,2-trimethylpropenylamine;[19]
this makes the synthesis compatible with most usual a-
amino acid side-chain protecting groups. Thus, the tert-
butoxycarbonyl (Boc)-protected precursor of octamer 1
could be prepared on a 650-mg scale. A portion was converted
to an even longer oligomer 3, which spans almost six helical
[*] Dr. E. R. Gillies,[+] F. Deiss, Prof. J.-M. Schmitter, Dr. I. Huc
UniversitØ Bordeaux 1 – CNRS UMR5248
Institut EuropØen de Chimie et Biologie
2 rue Robert Escarpit, 33607 Pessac (France)
Fax: (+33)540-002-215
E-mail: i.huc@iecb.u-bordeaux.fr
Dr. C. Staedel
UniversitØ Bordeaux Victor Segalen 2
INSERM U869
146 rue LØo Saignat, 33076 Bordeaux (France)
[+] Current address:
Departments of Chemistry
and Chemical and Biochemical Engineering
The University of Western Ontario, London N6A 5B7 (Canada)
turns, using
a
2,6-bis(aminomethyl)pyridine spacer.[16]
Oligomers 2 and 4, which possess higher charge density,
were prepared from the deprotected methyl esters of 1 and 3,
respectively. The molecular weight of 3 (4193 Da) is com-
parable to that of a 30-residue peptide. Oligomer 4 bears 32
cationic amine groups and has a molecular weight of 6242 Da.
A polyethylene oxide (PEO) residue was introduced in 5 to
[**] This work was supported by the European Commission (Marie Curie
Postdoctoral Fellowship to E.R.G.) and by the Conseil RØgional
d’Aquitaine. We thank Katell Bathany for mass spectroscopy data.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 3. Growth of HeLa cells after 24 h incubation with cationic
~
&
^
helices 1 ( ), 2 (&), 3 ( ), 4 ( ), 5 (”) as measured by the MTS
assay. Error bars represent standard deviations on three measure-
ments. IC50 values (the concentration at which 50% growth inhibition
is observed) are in the 50–500 mgmLÀ1 range.
selective toxicity towards bacterial over mammalian cells
requires a delicate balance of hydrophobicity and cationic
charge.[11a] Alternatively, the lower aspect ratio resulting from
the derivatization of the side chains with the lysine groups
may provide increased biocompatibility.[22] The PEO-func-
tionalized octamer 5 has an IC50 greater than 500 mgmLÀ1,
which shows that 5 has significantly lower toxicity than
octamer 1, even when taking into account the contribution of
PEO to the molecular weight of 5. Thus, the high biocom-
patibility of PEO helps lower toxicity in oligocations.
Studies of the degradation of aromatic amide oligomers
were performed on oligomer 1 using proteases both unspecific
and specific to positively charged residues (proteinaseK,
pepsin, subtilisin, and trypsin) under conditions optimized for
each protease. Monitoring of a time-course experiment was
performed by HPLC with UV spectrophotometric detection
and mass spectrometry (ESI-LC-MS and MALDI-MS). After
incubation for 24 h, complete resistance to degradation (see
the Supporting Information) was observed for the four
proteases using conditions under which a reference protein
(horse heart cytochromeC was used for this control) is
completely degraded in a few hours.
As various polycations are known to be effective DNA
transfection agents, preliminary studies were carried out to
investigate the potential of 1–5 in this application. Initial
studies were carried out using HeLa and Huh7 cells and a
plasmid DNA encoding for green fluorescent protein (GFP).
Most of the evaluated helices exhibit minimal fluorescence
due to quenching by the terminal nitro groups; however, it
was found in control experiments that this fluorescence
interferes with evaluation of GFP expression as cells exposed
to helices without plasmid DNA exhibited significant fluo-
rescence in both fluorescence microscopy and flow cytometry.
Therefore, preliminary transfection experiments were also
conducted with 1, 3, and 4 using HeLa cells and plasmid DNA
encoding for luciferase. Thus far, only 4 has provided a
Figure 2. Water-soluble helical aromatic amide foldamers. Bn=benzyl,
TFA=trifluoroacetate.
further increase helix solubility and to possibly reduce helix
toxicity; a fluorescein probe was attached to octamer 6 to
monitor its fate within cells. It should also be noted that all
these oligomers exist as equimolar mixtures of interconvert-
ing right- and left-handed helices. In the case of 1, 3, 5, and 6,
these helices are enantiomers, whilst in the case of 2 and 4,
they are diastereomers owing to the chirality of the side
chains. Unlike in peptides, side-chain chirality in these
oligomers is remote from the backbone and does not favor
one or the other handedness.
To test the toxicity of this new class of molecules, the
growth of HeLa cells in the presence of cationic helices 1–5
was evaluated using the MTS assay (see the Supporting
Information for details).[20] Results are given in Figure 3.
Oligomers 1 and 3 appear to be slightly more toxic than other
derivatives, with IC50 values of approximately 100 mgmLÀ1
and 50 mgmLÀ1, respectively. This toxicity is comparable to
that of other polycations[21] such as polylysine, polyethylene-
imine (PEI), chitosan, and polyamidoamine (PAMAM)
dendrimers, which have been frequently used in applications
such as DNA transfection. The higher relative toxicity of the
longer oligomer 3 may result from the increased number of
cationic charges on the molecule or perhaps its higher aspect
ratio.[22] Lysine-functionalized oligomers 2 and 4 have IC50
values greater than 500 mgmLÀ1 and 200 mgmLÀ1, respec-
tively. Again, somewhat higher toxicity was observed for the
longer oligomer 4. The lower toxicity of these molecules
relative to 1 and 3 is notable given their higher density of
positive charges. It is possible that toxicity is decreased by
shielding of the hydrophobic aromatic backbone by the lysine
residues. Indeed, in the development of antimicrobial amphi-
pathic aryl amide oligomers, it has been demonstrated that
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Angewandte
Chemie
modest degree of transfection (about 10 times less than that of
a lipofectamine positive control), indicating that a significant
number of positive charges may be required. This is consistent
with the results of studies of PEI where a molecular weight
greater than 3000 Da was needed for stable complexation of
DNA.[23] Even longer oligomers or polymers may be a
worthwhile pursuit to achieve high transfection activity.
The significant fluorescence of cells exposed to the helices
alone indicates that they readily cross membranes. To
investigate this behavior, studies were carried out using the
fluorescein-functionalized octamer 6. The helices were found
to localize within the cytoplasm and in nucleoli within the
nucleus (Figure 4). This distribution is similar to that
Figure 5. Concentration and time-dependent cell uptake of 6 after
&
^
10 min ( ) and after 1 h ( ), and of FITC-Tat47–57 after 10 min (&) and
~
after 1 h ( ) determined by flow cytometry: a) mean fluorescence
intensity (Ifl)of cells versus concentration, b) proportion of cells
exhibiting fluorescence versus concentration.
ers and related families of foldamers. One potential advant-
age of these oligomers over peptides is that changing side
chains does not concomitantly affect helix structure or
stability. Thus, unlike in peptides, structure–activity relation-
ships may be established based on the sole effects of a side-
chain or of a main-chain modification.
Figure 4. a) Fluorescence confocal microscopy image of HeLa cells
after incubation in a 10mm solution of 6 for 1 h. b) Inset illustrates the
absence of background fluorescence in cells not exposed to 6.
Received: January 22, 2007
Published online: April 19, 2007
observed for the well-known transporter HIV Tat peptide,
oligoarginines, and analogues based on foldamers such as b-
peptides, and oligoprolines.[18] The efficiency of uptake was
quantified and compared to that of a fluorescein-labeled
Tat47–57 oligopeptide (FITC-LC-YGRKKRRQRRR-NH2).
The effects of both incubation time and concentration were
investigated. The uptake of 6 and FITC-Tat47–57 are remark-
ably similar considering the significant differences in their
structures (Figure 5). In the case of helical b-peptides and
oligoprolines,[18] it has been necessary to place the side-chain
residues in specific sequences and orientations about the helix
in order to achieve efficient cell uptake. In the case of 6,
comparable results are achieved using a simple cationic helix,
even without guanidine groups. After longer incubation
periods (1 h), the percentage of fluorescent cells is higher
following incubation with 6 than with Tat47–57. This may result
from the gradual degradation of FITC-Tat47–57, which is an a-
peptide and is therefore susceptible to proteases, in contrast
to the aromatic oligoamide helices.
We believe that the scope of abiotic foldamers is much
extended by our finding that peptides and helical aromatic
oligoamides show apparently similar behavior with respect to
cell penetration despite their important structural differences.
The results described here were obtained using unoptimized
molecules. As many possibilities exist to modify side-chain
composition and backbone constitution, our results bode well
for various biological applications of aromatic amide oligom-
Keywords: drug delivery · helical structures ·
medicinal chemistry · peptidomimetics
.
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