Intrinsically Photoswitchable α/β Peptides toward Two-State Foldamers

Article abstract of DOI:10.1002/anie.201806035

A simple, unsaturated, E–Z photoisomerizable β-amino acid, (Z)-3-aminoprop-2-enoic acid, has been introduced into peptide foldamers through a one-pot chemical coupling, based on Pd/Cu-catalyzed olefin oxidative amidation, between two peptide segments carrying, respectively, a -Gly-NH₂ residue at the C-terminus and an acryloyl group at the N-terminus. Reversible conversion between the Z and E configurations of the 3-aminoprop-2-enoic linkage was achieved photochemically. A crystallographic analysis on two model compounds shed light on the consequences, in terms of 3D structure and self-association properties, brought about by the different configuration of the unsaturated linkage. As a proof of concept, E–Z photoisomerization of a 3-aminoprop-2-enoic acid residue, inserted as the junction between two conformationally distinct peptide domains (one helical while the other β-sheet promoter), allowed supramolecular self-association to be reversibly turned on/off.

Full text of DOI:10.1002/anie.201806035

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Accepted Article
Title: Intrinsically photoswitchable α/β peptides toward two-state
foldamers
Authors: Alessandro Moretto, Giulia Marafon, and Marco Crisma
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806035
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10.1002/anie.201806035
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Intrinsically Photoswitchable Peptides Toward Two-State
Foldamers
Giulia Marafon,[a] Marco Crisma,[b] Alessandro Moretto*[a,b]
Abstract: A simple, unsaturated, E-Z photoisomerizable -amino
acid, (Z)-3-aminoprop-2-enoic acid, has been introduced into peptide
acids, the E and Z isomers of 3-aminoprop-2-enoic acid are
herewith abbreviated as EAla and ZAla, respectively. The
instability of EAla / ZAla derivatives carrying the free amino
group hampers their use as amino component in standard
peptide coupling reactions. Not surprisingly, peptides containing
this residue have not been reported yet. A way around the
synthetic problem may be offered by an approach based on the
oxidative amidation of conjugated olefins, reported by Kim,
Chang and coworkers, who efficiently prepared simple Z-
configured enamides by reacting primary amides with
conjugated olefins in the presence of a Pd/Cu co-catalyst
system.16 By adapting this strategy, starting from Z-Gly-NH2 (Z,
benzyloxycarbonyl) and ethyl acrylate we successfully
synthesized the (Z) dipeptide Z-Gly-ZAla-OEt 1, which was
subsequently quantitatively photo-converted to its E-isomer 2 by
irradiation at 290-320 nm (Figure 1A). Compounds 1 and 2 were
crystallographically characterized. In the structure of 1 (Figure
1B) Gly(1) is nearly fully-extended (1,1 = 156.1°,-176.0°).6 The
conformation of ZAla(2) is described by three torsion angles,
foldamers via
a one-pot chemical coupling, based on Pd/Cu-
catalyzed olefin oxidative amidation, between two peptide segments
carrying, respectively, a -Gly-NH2 residue at the C-terminus and an
acryloyl group at the N-terminus. Reversible conversion between the
Z and E configurations of the 3-aminoprop-2-enoic linkage was
achieved photochemically. A crystallographic analysis on two model
compounds shed light on the consequences, in terms of 3D-
structure and self-association properties, brought about by the
different configuration of the unsaturated linkage. As a proof of
concept, E-Z photoisomerization of a 3-aminoprop-2-enoic acid
residue, inserted as the junction between two conformationally
distinct peptide domains (one helical while the other -sheet
promoter), allowed supramolecular self-association to be reversibly
turned on/off.
The field of foldamers1 encompasses
a growing set of
conformationally controlled, oligomeric molecules able to
develop well defined 3D-architectures. Examples include
peptides based on non-coded -amino acids, - and γ-peptides,
azapeptides, oligoureas, aromatic oligoamides.2-9 Typically, a
foldamer is designed to highly populate a single conformation.
Development of foldamers able to switch their 3D-structure
between two states in a controlled way, although amenable in
principle to interesting applications, is particularly challenging.10-
related to rotations about the N-C (2 = -176.2°), C-C (2
=
1.5°), and C-C (2 = 176.9°) bonds. This arrangement allows
formation of an N-HꞏꞏꞏO=C intra-residue H-bond which closes a
6-atom pseudocycle. In the structure of 2 (Figure 1C), not only
Gly(1) is fully extended (1,1 = 178.5°,-178.8°), but also all of
the , , and  backbone torsion angles of EAla(2) are within -
179.2° and 173.2°. The packing modes of 1 and 2 are in part
similar, being characterized by layers of antiparallel molecules
(Figures 1B and 1C). However, they differ by the number of
intermolecular H-bonds. Specifically, each molecule of 1 makes
two N-HꞏꞏꞏO=C intermolecular H-bonds on one side only,
whereas on the opposite side only C-HꞏꞏꞏO and van der Waals
contacts are observed (Figure 1B; see also SI, Table S5)
because the intramolecularly H-bonded conformation adopted
by ZAla prevents its N-H group from being approached within
H-bonding distance by any other potential H-bond acceptor of
the flanking molecule. Conversely, for 2, each molecule is
connected to its neighbors by four intermolecular H-bonds, two
on each side (Figure 1C), giving rise to a flattened -sheet.
14
To this end, an appealing potential is offered by E-Z
photoisomerization of double-bond containing molecules. We
have recently shown that incorporation of a fumaramide or
maleamide unit at the N-terminus of a peptidomimetic foldamer
allows this latter to exhibit a functional response as a result of
photoisomerization.15 In this work, we expanded the photo-
induced control of conformational switches to appropriately
designed foldamers incorporating at an internal position an
unsaturated -amino acid residue, namely (E/Z)-3-aminoprop-2-
enoic acid [or (E/Z)-3-aminoacrylic acid], which can be viewed
as the C,-unsaturated analog of -alanine (-Ala). By
combining this latter abbreviation with the E/Z terminology
commonly used for C,-didehydro analogs of protein amino
Notably, attempts to carry out the olefin oxidative
amidation by replacing either the benzyloxycarbonyl N-
protecting group (with Fmoc or Boc), or Gly with other amino
acids (e.g., Ala, Leu, Val) did not afford any product, in all
probability owing to the different stability of the protecting groups
close to the reaction site and to the steric hindrance exerted by
the side chains, respectively.
[a]
G., Marafon, Prof., A., Moretto
Department of Chemical Sciences
University of Padova
via Marzolo, 1, Padova, Italy
E-mail: alessandro.moretto.1@unipd.it
Dr., M., Crisma, Prof., A., Moretto
Institute of Biomolecular Chemistry
Padova Unit, CNR
[b]
via Marzolo, 1, Padova, Italy
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (A): Synthesis of Z-Gly-ZAla-OEt (1) and its photo-conversion to Z-Gly-EAla-OEt (2). (B) and (C): Packing modes in the crystal structures of 1 and 2,
respectively. Intra- and intermolecular H-bonds are indicated by dashed lines.
Next, we explored the introduction of the unsaturated -
amino acid internally to the peptide backbone. To this aim, a set
of N-acrylamide-functionalized -amino acid ester derivatives of
the type Acr-Aaa-OR [Acr = acryloyl; Aaa = Gly, Ala, Leu, Aib (-
amino-isobutyric acid); R = methyl or ethyl] were prepared and
placed to react with Z-Gly-NH2 through olefin oxidative amidation
to afford the corresponding tripeptides of general formula Z-Gly-
ZAla-Aaa-OR. Yields decreased from excellent to poor with
increasing bulk of Aaa (Gly: 88%; Ala: 53%; Leu: 28%; Aib: 8%).
However, the Z / E stereoselectivity of product formation was
comparable in all cases ( 15:1). A representative example of
the photo-conversion of these Z-configured tripeptides to the
corresponding E-isomers and back to their Z-form is illustrated in
Figure 2 in the case of the Z-Gly-ZAla-Leu-OMe (3) / Z-Gly-
EAla-Leu-OMe (4) pair. We found by HPLC monitoring (Figure
2A, right panel) that the 3-to-4 conversion occurs quantitatively
within 20 min by irradiation at 290-320 nm, whereas the reverse
process, carried out by irradiation at 254 nm (to exploit a slight
difference in the UV-Vis absorption profiles of 3 and 4; see SI,
Figure S37), reaches a photo-stationary equilibrium in which (E)
4 and (Z) 3 are present in a 30% : 70% molar ratio. Photo-
conversions (Z to E and E to Z) were also monitored by 1H-NMR
spectrometry (Figure 2B).
As a further step, we placed a ZAla residue as the junction
between two conformationally distinct peptide domains.
Specifically, Z-(Ala-Aib)2-Ala-Gly-NH2
5 (highly folded, 310-
helical)18-21 was combined through olefin oxidative amidation
with the -amyloid22 16-20 segment derivative Acr-Lys(Boc)-Leu-
Val-Phe-Phe-OH 6, giving Z-(Ala-Aib)2-Ala-Gly-ZAla-Lys(Boc)
-
Figure 2. (A) Left: formulas of Z-Gly-ZAla-Leu-OMe (3) and Z-Gly-EAla-
Leu-OMe (4). Right: time evolution of their photo-conversions monitored by
HPLC. (B) 1H NMR spectra (CD3CN) of (Z) 3 (top), its (E) isomer 4 obtained
by irradiation at 290-320 nm (bottom), and the mixture of E/Z isomers resulting
as photo-stationary equilibrium upon irradiation of the E isomer at 254 nm
(center). Diagnostic signals are marked. 3J coupling constants between olefinic
protons are 9 Hz for (Z) 3 and 14 Hz for (E) 4.
Leu-Val-Phe-Phe-OH 7 in 64% yield (Figure 3A). Again, photo-
conversion from (Z) 7 to (E) 8 was achieved quantitatively.
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Figure 3. (A) Computer-generated model of Z-(Ala-Aib)2-Ala-Gly-ZAla-Lys(Boc)-Leu-Val-Phe-Phe-OH (7), which combines a helical domain (from 5) with a -
sheet promoting segment (from 6) The -ZAla- junction is circled. (B) Overlap of the 1H NMR spectra (DMSO-d6) of the two reactants (upper part, 5, red trace,
and 6, blue trace) and (lower part) of the resulting product (Z)
7 (violet) and its E isomer 8 (green). Diagnostic signals are marked.
1
Figure 3B compares the H NMR spectra of the two reactants 5
and 6 with that of the resulting product (Z) 7, highlighting the
disappearance of the acrylate proton signals belonging to 6 and
of the Gly-NH2 signals belonging to 5, accompanied in 7 by a
downfield shift of the Gly -CH2 signals and the onset of the
olefinic signals and of the low-field ZAla NH. The latter signal
moves upfield upon photoisomerization to (E) 8.
Notably, the Lys side-chain Boc-protection survived in 7. Its
subsequent removal afforded Z-(Ala-Aib)2-Ala-Gly-ZAla-Lys-
Leu-Val-Phe-Phe-OH 9 which, in turn, was photo-converted to
its EAla-containing isomer 10. Upon standing, the 5 mM water
solution of (E) 10 turned into a stable hydrogel, suggesting that
the E-isomeric state of the olefinic junction allows efficient -
sheet formation involving the -amyloid-derived segment of the
foldamer without interference from the helical domain (Figure
4B). Hydrogelation is not observed in the case of 9, in all
probability because the Z configuration of the olefinic junction, in
addition to sequestering the ZAla NH group from
intermolecular H-bonding, makes a kink in the overall shape of
the foldamer (Figure 4A).
According to our model (Figure 4B), 9 may thus self-associate at
best into dimers (as indirectly supported by the crystal structure
of 2), but extensive -sheet formation does not occur.
Interestingly, the hydrogel formed by 10, upon 15 min irradiation
at 254 nm, returns to a fluid state, in which the (E) 10 and (Z) 9
isomers are present in nearly equimolar amounts.
Figure 4. (A) Models of the two-domain foldamers Z-(Ala-Aib)2-Ala-Gly-
ZAla-Lys-Leu-Val-Phe-Phe-OH (Z) 9 and its E-isomer 10. (B) Schematic
representation of the self-assembly modes of (Z) 9 and (E) 10. (C) TEM
images taken at time intervals monitoring the photo-conversion of (Z) 9 to (E)
10 and back from (E) 10 to a 6:4 (E) 10 / (Z) 9 mixture (scale bars: 500 nm).
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Morphological insight on these phenomena was provided by
TEM analysis (Figure 4C). Specifically, we recorded a set of
TEM images, beginning with a sample cast from a 5 mM water
solution of 9 and then analyzing samples taken after 5, 10 and
20 min of irradiation at 290-320 nm. The increasing (E) 10 / (Z) 9
molar ratio (from 0 to 95%) resulting from the photo-conversion
gives rise to formation of fibers of increasing size. Such fibers
significantly dissociate upon partial back photo-conversion (from
95% 10 to a 6:4 mixture of 10 and 9) promoted by 254 nm
irradiation (see SI, p. S37-S40).
Keywords: chemical ligation • foldamers • peptidomimetics •
photoisomerization • supramolecular chemistry
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To summarize, we succeeded in introducing the simplest
unsaturated (E-Z photoisomerizable) -amino acid, namely (Z)-
3-aminoprop-2-enoic acid (ZAla), into peptide foldamers, via a
one-pot chemical coupling based on Pd/Cu-catalyzed olefin
oxidative amidation. A limitation to such synthetic approach, at
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seems to be allowed at the position preceding the in situ
generated ZAla unit. Higher versatility is tolerated about the
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crystallographic analyses on two model compounds. As a
specific case, the switch between the Z and E isomers of a 3-
aminoprop-2-enoic acid residue, inserted as the junction
between two different peptide domains (one helical while the
other -sheet promoter), allows supramolecular self-association
to be reversibly turned on/off. We are confident that the photo-
switchable ZAla / EAla system may provide a valuable
structural element in the design of two-state functional foldamers
for biomimetic and nanotechnological applications.10-14,23,24
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Conflict of interest
The authors declare no conflict of interest.
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10.1002/anie.201806035
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A
simple unsaturated -amino
G. Marafon, M. Crisma, A. Moretto*
acid was introduced into peptide
foldamers via a one-pot chemical
coupling between two peptide
segments carrying, respectively, a
primary amide at the C-terminus
and an acryloyl group at the N-
terminus. This novel type of
Page No. – Page No.
Intrinsically photoswitchable
peptides toward two-state
foldamers
ligation
gives
access
to
peptidomimetics and foldamers
with photochemically tunable 3D-
structure and self-association
tendency.
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