Synthesis and self-assembly of oligo(p-phenylenevinylene) peptide conjugates in water

Article abstract of DOI:10.1002/chem.201002495

Fluorescent hydrogels: Peptide hybrids composed of an oligo(p- phenylenevinylene)-based τ-amino acid and a β-sheet-forming sequence reversibly self-assemble into fluorescent hydrogels upon a change in pH (see graphic). This study highlights how the combination of peptides and π-conjugated oligomers could be a fruitful approach towards the control of self-assembled structures.

Full text of DOI:10.1002/chem.201002495

DOI: 10.1002/chem.201002495
Synthesis and Self-Assembly of Oligo(p-phenylenevinylene) Peptide
Conjugates in Water
Miriam Mba,[a] Alessandro Moretto,[a] Lidia Armelao,[b] Marco Crisma,[c]
Claudio Toniolo,[a] and Michele Maggini*[a]
Molecular self-organization is a useful approach to pre-
pare soft and flexible, functional micro- and nanoarchitec-
tures. A practical principle to self-assemble structures is
based on simple p–p stacking of p-conjugated oligomers.
Self-assembly (SA) of functionalized oligo(p-phenyleneviny-
lene)s (OPVs) gives organogels with interesting photophysi-
cal properties and potential applications in light-emitting
diodes,[1] light-harvesting systems,[2] or thermal imaging.[3]
However, organized, robust molecular structures are diffi-
cult to obtain by p–p stacking alone.[4] Therefore, a variety
of promoters that enable the establishment of additional
noncovalent interactions, such as directional hydrogen
bonds, have received increased attention.[5] Among them,
peptide amphiphiles, made of a p-conjugated unit and hy-
drophilic peptide sequences, have been considered because
of their strong tendency to form well-defined secondary
structures and to self-assemble in water.[6,7]
Herein, we report the synthesis and SA characteristics of
two OPV peptide conjugates (OPV-1 and OPV-2, Scheme 1)
in which a new OPV-based w-amino acid has been incorpo-
rated into two different b-sheet-forming sequences by solid-
phase synthesis.[8] In these systems, a reversible SA can be
triggered by pH changes. In particular, the ornithine-rich
OPV-1 self-assembles at basic pH, whereas for OPV-2,
Scheme 1. Synthesis of peptides OPV-1 and OPV-2. Reagents and condi-
which contains glutamic acid residues, SA occurs at acidic
tions: a) NaH, THF, 258C, 6 h, 78%; b) PdCAHTGNUTRNEN(UNG OAc)2, triethylamine, triACHTUNGTRENNUNG(o-
pH.
tolyl)phosphine, DMF, 908C, microwave, 15 min, 63%; c) trifluoroacetic
acid, CH2Cl2, 258C, 2 h, then trimethylsilyl chloride, diisopropylethyl
amine, 9-fluorenylmethylchloroformate, 24 h, 87%.
[a] Dr. M. Mba, Dr. A. Moretto, Prof. C. Toniolo, Prof. M. Maggini
Department of Chemical Sciences, University of Padova
Via Marzolo 1, 35131 Padova (Italy)
The Boc–OPV–OtBu (Boc=tert-butyloxycarbonyl) amino
acid derivative 1 was synthesized by a two-step procedure as
illustrated in Scheme 1. A Horner–Emmons Wittig coupling
reaction of tert-butyl(4-formylbenzyl)carbamate (2)[9] and
phosphonate 3 gave the trans-stilbene 4 in 78% yield. In
turn, phosphonate 3 was prepared starting from 4-iodo-2,5-
dimethoxy benzaldehyde. A subsequent microwave-assisted
Heck coupling reaction of 4 with (4-vinylphenyl)acetic acid
tert-butyl ester (5) afforded OPV amino acid 1 in 63% iso-
Fax : (+39)049-8275239
E-mail: michele.maggini@unipd.it
[b] Dr. L. Armelao
ISTM-CNR, INSTM, Department of Chemical Sciences
University of Padova, Via Marzolo 1, 35131 Padova (Italy)
[c] Dr. M. Crisma
ICB, CNR, Padova Unit, Department of Chemical Sciences
University of Padova, Via Marzolo 1, 35131 Padova (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002495.
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Chem. Eur. J. 2011, 17, 2044 – 2047

COMMUNICATION
lated yield as a bright yellow solid (see the Supporting Infor-
mation for synthetic details and characterization of the
products). Crystals suitable for X-ray diffraction analysis
were grown from an n-hexane/ethyl acetate mixture. The 3D
structure of 1, illustrated in Figure 1, confirmed the trans–
Figure 1. X-ray diffraction structure of the Boc–OPV–OtBu amino acid
derivative 1 with heteroatom numbering.
trans configuration of the two OPV C=C bonds.[15] The cen-
tral phenyl ring of 1 is laterally tilted by 15.2(2) and 12.7(2)8
relative to the terminal phenyl rings (which are coplanar
within 4.5(2)8). The Boc and OtBu protecting groups lie out-
side of the plane defined by the OPV moiety. In an antipar-
Figure 2. Absorption (A), emission (B), and CD (C) spectra of OPV-1 in
acidic (nonassembled a) and basic aqueous solutions (assembled c).
Absorption (D), emission (E), and CD (F) spectra of OPV-2 in basic
(nonassembled a) and acidic (assembled c) aqueous solutions. Each
inset of panels A and D shows a solution (left) and a gel (right) irradiat-
ed at 365 nm.
allel b-sheet conformation, the intrastrand N(i)···NACTHNUGRTNEUNG(i+5) dis-
tance is about 20.8 ꢁ. Because the distance from the N ter-
minus to the C terminus in 1 is 21.2 ꢁ, this OPV amino acid
might serve as a replacement for five a-amino acid units in
a b strand.
Simultaneous removal of Boc and OtBu groups of 1 with
trifluoroacetic acid followed by protection of the N terminus
with 9-fluorenylmethoxycarbonyl (Fmoc) gave Fmoc–OPV–
OH 6 in 87% yield (Scheme 1). Standard Fmoc-mediated
solid-phase syntheses afforded peptides OPV-1 and OPV-2
in 49 and 54% isolated yields, respectively, after semiprepar-
ative HPLC purifications (see the Supporting Information
for details).
A stable hydrogel formed when a 13 mm solution of OPV-
1 was allowed to stand at pH 8 with 1n aqueous NaOH.
Lower OPV-1 concentrations did not yield any gelation. A
similar behavior was observed upon addition of 1n HCl to a
13 mm solution of OPV-2. Although gelation takes place in
both cases, gels of OPV-1 seemed more robust than those
obtained from OPV-2. The process is fully reversible and
original peptide solutions could be restored by changing the
pH to acidic and basic values for the OPV-1 and OPV-2
gels, respectively.
The FTIR absorption spectra of both gels (see the Sup-
porting Information) show the strong amide I band, typical-
ly seen for b-sheet structures, at 1627 and 1614 cmÀ1 for
OPV-1 and OPV-2, respectively.[10] On the other hand, both
peptide conjugates exhibit a single broad band at 1654 cmÀ1
before gelation. The UV absorption spectrum of OPV-1 in
an acidic aqueous solution (Figure 2A) has two maxima at
264 and 335 nm that redshift, to 292 and 345 nm, respective-
ly, upon gelation. The absorption band of OPV-2 at 265 nm
shows a 30 nm redshift upon gelation. In contrast, no shift
was observed for the OPV-2 absorption band at 335 nm
(Figure 2D). Strong emission was seen for both peptide con-
jugates (Figure 2B and E) that partially quenches upon ge-
lation. This quenching effect is stronger for assembled OPV-
2 than for OPV-1. According to Kashasꢂs exciton theory,[11]
the observed redshift in the UV/Vis absorption spectra of
peptide gels along with the residual emission may indicate
the formation of J-type aggregates of the OPV moiety.
The changes recorded for OPV-1 and OPV-2 upon gela-
tion are apparent in the UV absorption spectra (Figure 2A
and D, 28 and 30 nm bathochromic shifts of the bands at
264 and 265 nm, respectively) but relatively weak in the
case of the emission spectra (Figure 2B and E, 8 nm redshift
for OPV-1 and no shift for OPV-2) if compared to previous-
ly reported gel-forming OPVs,[12] suggesting a different elec-
tronic structure of the aggregates and, therefore, a different
geometry.[11] In such cases it has been proposed to address
these assemblies as pseudo-J aggregates.[12a]
Circular dichroism (CD) spectra of OPV-1 and OPV-2 in
water, at acidic and basic pH values, respectively, are almost
flat in the OPV absorption region (>250 nm), indicating un-
specific structures in solution (Figure 2C and F). Upon pH
change, a strong negative band emerges at around 230 nm,
for both OPV-1 and OPV-2, along with enhanced CD sig-
nals in the region of the OPV absorption that, interestingly,
exhibits Cotton effects of opposite signs. Although slightly
longer than expected, the 230 nm wavelength indicates a b-
sheet structure for the assembled peptides.[13] In both CD
spectra the bisignate excitonic bands that coincide with the
UV absorption maxima of OPV are lacking. These bands are
typical for chiral helical OPV assemblies and the lack of
them has been observed for other pseudo-J aggregates.[12,14]
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M. Maggini et al.
These findings may indicate the absence of helical structures
or the coexistence of different chiral assemblies.
TEM analysis of the OPV-1 gels (Figure 3A) shows nano-
tape 1D structures of lengths up to 1 mm and an average
width of 7 nm with some interweaving fibers that form left-
handed double helices (inset of Figure 3A). In contrast, a
spongy network texture was observed for the OPV-2 gel
(Figure 3B).
Acknowledgements
Financial support from the University of Padova, through Progetti Strate-
gici bando 2008-HELIOS, and the “Scientific equipment for research” in-
itiative (Oxford Gemini diffractometer) is gratefully acknowledged. M.C.
thanks A. Dolmella for his help with X-ray diffraction data collection
and processing.
Keywords: aggregation · gels · oligo(p-phenylenevinylene)s ·
peptides · self-assembly
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Self-Assembly of Oligo(p-phenylenevinylene) Peptides
COMMUNICATION
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Received: August 29, 2010
Published online: January 7, 2011
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