Tunable E- Z Photoisomerization in α,β-Peptide Foldamers Featuring Multiple (E/ Z)-3-Aminoprop-2-enoic Acid Units

Article abstract of DOI:10.1021/acs.orglett.9b01360

Systems in which an external stimulus elicits a response through some sort of modification at the molecular or supramolecular level bear potential for the development of smart materials and devices. This work describes a versatile synthetic approach suitable for the stepwise incorporation of multiple, even consecutive, units of the simplest C^α,β-unsaturated β-amino acid, (E/Z)-3-aminoprop-2-enoic acid, in peptide-based foldamers. The properties of these, including photoinduced E/Z isomerizations, were investigated.

Full text of DOI:10.1021/acs.orglett.9b01360

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tunable E−Z Photoisomerization in α,β-Peptide Foldamers
Featuring Multiple (E/Z)‑3-Aminoprop-2-enoic Acid Units
Giulia Marafon,† Marco Crisma,‡ and Alessandro Moretto*,†,‡
†Department of Chemical Sciences, University of Padova, 35131 Padova, Italy
‡Institute of Biomolecular Chemistry, Padova Unit, CNR, 35131 Padova, Italy
S
* Supporting Information
ABSTRACT: Systems in which an external stimulus elicits a response
through some sort of modification at the molecular or supramolecular
level bear potential for the development of smart materials and devices.
This work describes a versatile synthetic approach suitable for the
stepwise incorporation of multiple, even consecutive, units of the
simplest Cα,β-unsaturated β-amino acid, (E/Z)-3-aminoprop-2-enoic
acid, in peptide-based foldamers. The properties of these, including
photoinduced E/Z isomerizations, were investigated.
oldamers are oligomeric molecules based on building
blocks other than those characterizing naturally occurring
synthetic methodology based on the oxidative amidation of
conjugated olefins.8 Specifically, the reaction involved two
peptide segments, one requiring a glycinamide at the C-
terminus and the other an acryloyl group at the N-terminus.
Although successful, such a strategy suffers from limitations,
the most important being that it allows the in situ generation of
only a single ΔZβAla residue within a peptide sequence. Here,
we report a novel and more versatile synthetic approach
suitable for the stepwise incorporation of multiple ΔZβAla
units, even consecutively, in peptides. The processes of Z−E
and E−Z photoisomerization and the properties of the
resulting foldamers are also described.
F
biopolymers, able to highly populate a specific conformation,
thus giving rise to a stable and well-defined three-dimensional
(3D) architecture.1 Peptides based on β-amino acids, γ-amino
acids, and noncoded α-amino acids, aromatic oligoamides,
azapeptides, and oligoureas are among the most extensively
investigated classes of compounds in this area.2 The possibility
of triggering the switch of a foldamer between two distinct but
structurally defined states (for such systems, the term “dynamic
foldamers” has been coined)3 may offer interesting oppor-
tunities. For example, a foldamer undergoing elongation and
contraction of its end-to-end distance resulting from the
transition between two conformations, each characterized by a
different pitch, can be viewed as a molecular spring.4
Moreover, a conformational transition can convey chemical
information across the entire length of a foldamer from a
signaling unit at one end to a reporter at the other end, even at
multinanometer distances.5 In this regard, we exploited the
(E)-fumaramide/(Z)-maleamide photoswitchable system as a
linker between a chiral and an achiral helical peptide foldamer
segment to turn on−off transmission of stereochemical
information (a preferred helical screw sense) to the latter.6
However, such an approach results in the antiverse orientation
of the two peptide chains connected to the linker. We therefore
became interested in developing peptide-based foldameric
systems in which the photoswitchable unit itself would be an
amino acid.7 For this aim, we selected (E/Z)-3-aminoprop-2-
enoic acid [or (E/Z)-3-aminoacrylic acid]. Considering this β-
amino acid as the Cα,β-unsaturated analogue of β-alanine (β-
Ala) and exploiting the ΔE/ΔZ terminology commonly used
for Cα,β-didehydro analogues of protein amino acids, we
abbreviate the E and Z isomers of 3-aminoprop-2-enoic acid as
ΔEβAla and ΔZβAla, respectively. We recently succeeded in
introducing ΔZβAla into a peptide sequence through a
To achieve multiple insertions of ΔZβAla residues into a
peptide sequence, a derivative compatible with the multistep
chemical strategy involved in the solution protocol for peptide
synthesis was needed, owing to the intrinsic instability of the
unprotected amine group conjugated to the unsaturated CC
bond that does not tolerate removal of any N-protecting group.
Compound 8 [exo-3-tert-butoxycarbonylamino-7-oxabicyclo-
(2.2.1)hept-5-ene-exo-2-carboxylic acid] was selected as our
ΔZβAla precursor (Figure 1a). In this compound, the olefinic
CC bond is masked by the formal cycloaddition of furan to
Boc-ΔZβAla-OH, therefore allowing removal of the Boc group
to produce a stable amino derivative. Following our protocol,
the cycloaddition of furan (1) to maleic anhydride (2) afforded
selectively the syn (exo) cycloadduct (3), which was
subsequently converted to the racemic form of (exo,exo)
cycloadduct 5a (R = CH3) or 5b (R = CH2CH3) after a few
synthetic steps, namely, monoesterification (yielding 4a and
4b) and acyl azide formation followed by a Curtius
rearrangement carried out in the presence of tert-butyl alcohol
Received: April 18, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b01360
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Figure 1. (a) Synthesis of Boc-fmΔZβAla-OH (8) and ΔZβAla/ΔEβAla photoconversion. (b) X-ray diffraction structures of 5a, 6c, 7b, and 9(Z)a.
Intramolecular hydrogen bonds are indicated by dashed lines. (c) Chemical structures of foldamers built by incorporation of consecutively coupled
fmΔZβAla residues. (d) Chemical structures of the synthesized foldamers characterized by a row of one to four consecutive ΔZβAla residues.
and p-toluenesulfonic acid as the catalyst (70% overall yield).
By heating 5a and 5b in acetonitrile, we obtained Boc-ΔZβAla-
OR (6a and 6b) quantitatively via the retro-Diels−Alder
mechanism. Upon irradiation at 290−320 nm (UV-B), 6a and
6b were quantitatively converted to the Boc-ΔEβAla-OR
isomer (7a and 7b).
Leu. Accordingly, starting from H-Leu-OMe, through a series
of conventional peptide couplings (each involving either 8 or a
Boc-protected α-amino acid) and Boc deprotection steps, a set
of oligomers containing up to four consecutively coupled
fmΔZβAla residues were prepared. Because 8 was used in its
racemic form while all α-amino acids were of the S
configuration, each foldamer was obtained as a mixture of
diastereomers [compounds 9−12 (Figure 1c)]. However,
upon thermal treatment of these mixtures to induce retro-
Diels−Alders reaction (which proceeded with quantitative Z
stereoselectivity and yield), restoration of the ΔZβAla CC
double bond removed the chiral heterogeneity. The synthe-
sized peptide foldamers comprised a set of compounds of the
general sequence Boc-Val-(ΔZβAla)n-Leu-OMe [n = 1, 9-
(Z,Z); n = 2, 10(Z,Z); n = 3, 11(Z,Z,Z); n = 4, 12(Z,Z,Z,Z)]
characterized by a row of one to four consecutive ΔZβAla
residues flanked by one α-amino acid at each end (Figure 1d).
An analogue of 9(Z) in which Boc protection is replaced by a
trifluoroacetyl group was also synthesized, and its structure
determined by X-ray diffraction [Figure 1b, structure 9(Z)a].
The conformation of the central ΔZβAla residue closely
matches that reported above for the Boc-protected derivative
6a, giving rise to an intraresidue N−H···OC H-bond (C6
structure) internal to the tripeptide backbone. However, the
N−H groups of Val(1) and Leu(3) are oriented nearly
perpendicular to each other, and the same holds true for the
corresponding carbonyl groups. Such a disposition of the
potential intermolecular H-bonding donors and acceptors,
combined with the inaccessibility of the N−H and CO
The back-conversion was achieved in 75% yield by
irradiation at 254 nm. Finally, precursor 8 (Boc-fmΔΖβAla-
OH, where fm stands for furan masking of the olefinic double
bond) was obtained after saponification of 5a and 5b. The
stereochemical output of this series of reactions (see the
Supporting Information for NMR spectra) was verified by
single-crystal X-ray diffraction analysis of compounds 5a, 6c,
and 7b. Analysis of 5a (Figure 1b) unambiguously confirms
the syn,syn (exo,exo)-type structure of the resulting racemic
cycloadduct.9 The structure of Boc-ΔZβAla-OH (6c, obtained
after saponification of 6a and 6b) shows the occurrence of an
intramolecular N−H···OC H-bond that closes a six-atom
pseudocycle (C6 structure) (Figure 1b), made possible by the
almost exact (within 1.0°) trans, cis, and trans arrangement of
the ϕ, θ, and ψ backbone torsion angles (rotations about the
N−Cβ, CβCα, and Cα−C bonds, respectively) of ΔZβAla.
Finally, in 7b the molecule is perfectly flat in that all non-H
atoms, except two methyl carbons of the Boc group, lay on the
same plane (Figure 1b) and the value of all backbone torsion
angles is 180°. The results described above prompted us to
plan the synthesis of foldamers in which ΔZβAla would be
combined with α-amino acid residues characterized by a good
propensity to adopt the β-sheet conformation, namely, Val and
B
DOI: 10.1021/acs.orglett.9b01360
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1
Figure 2. (a) H NMR spectra of Boc-Val-(ΔZβAla)n-Leu-OMe [n = 1−4; 9(Z), 10(Z,Z), 11(Z,Z,Z), and 12(Z,Z,Z,Z), respectively] (peptide
concentration of 3 mM in CD3CN). NH protons involved in an intraresidue H-bond are highlighted in green, while α- and β-olefinic protons are
highlighted in orange and purple, respectively. (b and c) UV−vis absorption spectra and fluorescence emission (λex at 350 nm), respectively, of
Boc-Val-(ΔZβAla)n-Leu-OMe (n = 1−4). (d) 1H NMR spectra of 9(Z)−9(E) isomerization recorded under different periods of exposition during
UV-B illumination. Proton signals belonging to ΔZβAla and ΔEβAla units are highlighted. (e) TEM images recorded for 4:6 (bottom) and 2:8
(top) 9(Z)/9(E) isomeric mixtures (scale bar of 100 nm, stained samples). (f) Kinetics of 9(Z)−9(E) (UV-B, black squares) and 9(E)−9(Z) (254
nm, red circles) photoconversions (data from HPLC).
groups of ΔZβAla (already intramolecularly engaged), hampers
the possibility that intermolecular H-bonding may give rise to a
sheetlike arrangement in the crystal packing.
olefin and amide groups. Such a conjugation is expected to be
maximized if all ΔZβAla residues adopt a conformation similar
to that found for 6a and 9(Z)a (see above), i.e., a fully planar
system. The UV−vis absorption profiles of the 9(Z,Z)−
12(Z,Z,Z,Z) series suggested that we explore a set of different
wavelengths (hv as reported in Figure 2b) to induce the
photoisomerization of these foldamers to their corresponding
E isomers and for the reverse process, as well. Clearly, on the
way from the all-Z to all-E isomeric states of the -(ΔβAla)n-
oligomers, when n > 1 additional combinations of isomeric
forms are also implicated, for a total of 2n isomers. The 9(Z)
→ 9(E) isomerization was monitored by NMR spectroscopy
(Figure 2d, red trace). Experimental condition: 3 mM 9(Z) in
CD3CN, quartz NMR tube.
The 1H NMR spectra of the foldamers belonging to the Boc-
Val-(ΔZβAla)n-Leu-OMe series (n = 1−4; 2 mM in CH3CN)
are reported in Figure 2a. The NH proton of each ΔZβAla
residue is found in the low-field spectral region, indicative of its
involvement in an intraresidue (C6) H-bond. As a function of
foldamer elongation in the Boc-Val-(ΔZβAla)n-Leu-OMe
series, a remarkable red-shift of the maximum is observed in
the corresponding UV−vis absorption spectra (Figure 2b),
from 260 nm (n = 1) to 310 nm (n = 2), 337 nm (with a
shoulder at 362 nm) (n = 3), and 350 nm (with a shoulder at
370 nm) (n = 4). The increasing number of consecutive
ΔZβAla residues in the foldamer backbone seems to give rise to
a progressively expanded, conjugated π-system involving both
After irradiation under UV-B light for 5 min, the new isomer
9(E) started to form, yielding a 9(Z)/9(E) mixture in 4:6
C
DOI: 10.1021/acs.orglett.9b01360
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Figure 3. (a−c) HPLC profiles of the photoconversion process from the all-Z isomer to mainly the all-E isomer starting from 10(Z,Z), 11(Z,Z,Z),
and 12(Z,Z,Z,Z), respectively. Solvent MeOH, concentrations of 5 μM for all samples. The HPLC profiles reported at the right of each row show
the corresponding all-E−all-Z back-isomerization process.
molar ratio (determined by HPLC), which gave rise to a weak
organogel state (Figure 2d, green trace, and corresponding
TEM image). Irradiation for an additional 5 min generated a
9(Z)/9(E) mixture in a 2:8 molar ratio (by HPLC), which
turned the NMR trace in a set of broad NMR proton signals
and in a robust organogel state (Figure 2d, blue trace, and
corresponding TEM image). After addition of DMSO-d6 to the
CD3CN solution, the organogel collapsed to a liquid state, and
upon irradiation for an additional 10 min, the isomerization
process produced 9(E) in quantitative yield (Figure 2d, violet
trace). TEM analyses clearly show how the progressive
formation of ordered supramolecular fibers is related to the
incremental conversion of 9(Z) to 9(E) (Figure 2e). The latter
compound at the critical concentration of 2.4 mM (referenced
to its own concentration in the mixture) was the origin of a
dense fiber network that could entrap the solvent (acetoni-
trile). The reversible 9(E) → 9(Z) conversion was achieved by
irradiation at 254 nm (Figure 2e, experiments run in MeOH to
avoid aggregation). While the 9(Z) → 9(E) transformation
occurred quantitatively, the back process 9(E) → 9(Z) reached
a photostationary equilibrium at 77% conversion. In our view,
the strong tendency of 9(E) to self-associate into fibers, at
variance with its 9(Z) counterpart, might arise from an
extended conformation adopted by ΔEβAla (similar to that
described above for 7 in the crystal state), combined with the
availability for intermolecular H-bonding of an additional NH
group free from intramolecular interactions.
As for the higher oligomers, we found that photoconversions
from the all-Z isomer to a mixture in which the all-E isomer
was by far the most abundant component required successive
irradiation steps at different wavelengths. On the basis of the
UV−vis absorption profiles of Boc-Val-(ΔZβAla)n-Leu-OMe (n
= 1−4) reported in Figure 2b, we selected four wavelengths
(indicated by arrows in Figure 2b) that fall each in the red tail
of the absorption profile of one of the compounds: irradiation
at 350 nm, followed by UV-B, for 10(Z,Z); 365 nm, followed
by 350 nm, for 11(Z,Z,Z); and 395 nm, followed by 365 nm
and successively by 350 nm, for 12(Z,Z,Z,Z) (Figure 3a−c,
respectively). Indeed, on the way from the all-Z to the all-E
isomers, a mixture of isomers in which Z and E units are
variously combined is likely to be formed, somehow
resembling the conjugation state of some of the lower all-Z
homologues. These latter require progressively higher
irradiation energies for their Z−E conversion as the number
of consecutive Z units decreases. It is noteworthy that these
processes can be achieved in high conversion yields (as shown
in the reported HPLC profiles) when concentrations are <5
μM. These conditions are required because of the strong
aggregation tendency of 10(E,E)−12(E,E,E,E) and their
mixed intermediate species. All HPLC peaks were found to
be iso mass with the corresponding precursors. Interestingly, if
compared to their all-Z counterparts, foldamers 10(E,E),
11(E,E,E), and 12(E,E,E,E) are characterized by blue-shifted
UV−vis absorption profiles (see the Supporting Information),
accompanied by a loss of fluorescence. The back-isomerization
process was also investigated [from all-E to all-Z isomers
(Figure 3a−c, right HPLC profiles)]. High levels of conversion
were found in the case of 10(Z,Z) (at 254 nm) and 11(Z,Z,Z)
(at UV-B), while moderate conversion was achieved in the case
of 12(Z,Z,Z,Z). All compounds after reconversion to their
corresponding all-Z isomers restored the native fluorescence.
To summarize, foldamers featuring consecutive ΔZβAla/
ΔEβAla units are endowed with interesting conformational,
electronic, and supramolecular aggregation properties that can
be modulated by selective E−Z photoisomerization. The
synthetic route developed in this work offers possibilities for
the exploitation of the ΔZβAla/ΔEβAla dyad in photo-
responsive systems.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01360.
Experimental details, X-ray crystallographic data, spec-
troscopic data for new compounds, and additional
figures (PDF)
D
DOI: 10.1021/acs.orglett.9b01360
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Organic Letters
Accession Codes
Letter
(9) The X-ray diffraction structure of the endo,exo diastereomer of
5b was previously reported. See: Masesane, I. B.; Batsanov, A. S.;
Howard, J. A. K.; Mondal, R.; Steel, P. G. Beilstein J. Org. Chem. 2006,
2, 9.
CCDC 1901757−1901760 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: alessandro.moretto.1@unipd.it.
ORCID
Marco Crisma: 0000-0003-3552-4106
Alessandro Moretto: 0000-0003-1563-9053
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was funded by the CARIPARO Foundation (Ph.D.
program, Padova, Italy), the Italian Ministero dell’Istruzione,
̀
dell’Universita e della Ricerca (MIUR) (Grant PRIN
20157WW5EH), and the University of Padova (Grant P-
DiSC 06BIRD2018-UNIPD).
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