Topochemical polymerization in supramolecular polymers of oligopeptide-functionalized diacetylenes

Article abstract of DOI:10.1002/anie.200600610

(Figure Presented) Diacetylene macromonomers functionalized with a β-sheet-forming oligopeptide and a hydrogenated poly(isoprene) coil segment self-assembled into supramolecular polymers with a double-helical, tubular topology and a diameter of a few nanometers. Subsequent "1D topochemical polymerization" affords poly(diacetylene)s with a conjugated backbone and a high degree of biochemically relevant functionalization, as well as a defined hierarchical structure.

Full text of DOI:10.1002/anie.200600610

Angewandte
Chemie
Supramolecular Polymers
DOI: 10.1002/anie.200600610
Topochemical Polymerization in Supramolecular
Polymers of Oligopeptide-Functionalized
Diacetylenes**
Eike Jahnke, Ingo Lieberwirth, Nikolai Severin,
Jürgen P. Rabe, and Holger Frauenrath*
New generations of synthetic polymers targeting applications
at the interface of optoelectronics and the biosciences must
“speak the language” of biomaterials and mimic their
functioning. They must provide the chemical functionality
to interact with biological systems on the molecular level and
exhibit a similar ability to form hierarchical structures.[1]
Supramolecular self-assembly has proven to be a powerful
tool for the preparation of materials with structural features
on the nanometer scale.[2] Recently, the self-assembly of
oligopeptides through aggregation and formation of b sheets
has received increasing attention. This approach has been
prompted by research activities in the field of neurodegener-
ative diseases.[3] Notable examples include the self-assembly
of oligopeptides based on amyloids,[4] the formation of
organogels from synthetic oligopeptides and their deposition
on surfaces,[5] the aggregation of peptidomimetic molecules[6]
and PEG conjugates,[7] multiblock copolymers inspired by
spider silk,[8] as well as the use of self-assembled oligopeptides
in the manufacture of gold nanowires.[9]
Poly(diacetylene)s are optoelectronically active materials.
Topochemical polymerizations of diacetylenes are possible
whenever the required crystalline order of the monomers is
established.[10] They have, for example, been performed in
self-assembled mono- or multilayers of diynoic acids and their
salts[11] as well as along 1D lamellar structures in self-
assembled monolayers on surfaces.[12] Diacetylene-containing
[*] E. Jahnke, Dr. H. Frauenrath
ETH Zürich
Departement Materialien
Wolfgang-Pauli-Strasse 10, HCI H515, 8093 Zürich (Switzerland)
Fax: (+41)44-633-1390
E-mail: frauenrath@mat.ethz.ch
Dr. I. Lieberwirth
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Dr. N. Severin, Prof. J. P. Rabe
Humboldt-Universität zu Berlin
Institut für Physik
Newtonstrasse 15, 12489 Berlin (Germany)
[**] We would like to thank Dr. Heinz Rüegger for the solid-state NMR
spectroscopy experiments, Lars Massüger for the X-ray diffraction
studies, and Prof. A. Dieter Schlüter for his continuous support.
Financial support from the Deutsche Forschungsgemeinschaft
(Emmy Noether program, FR 1567/2-1) and the Fonds der
Chemischen Industrie (Fonds-Stipendium, E.J.) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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lipid amphiphiles with chiral polar head groups, such as
phosphatidyl cholines, amino acids, or aldonamides were
found to form tubular or helical superstructures in the
submicrometer range.[13] Related materials have been used
in sensing applications.[14] Finally, diacetylene polymerizations
have been carried out in hydrogen-bonded organogels and
supramolecular polymers.[15]
Here we describe the use of self-assembled supramolec-
ular polymers consisting of b-sheet-forming oligopeptides as
scaffolds for the topochemical polymerization of diacetylenes
in solution. Thus, we designed and prepared the macro-
monomers 1 and 2 which include 1) a hydrogenated poly-
(isoprene) segment (hPI) as a polydisperse aliphatic coil to
provide solubility in organic solvents and prevent global
ordering; 2) a tetra(l-alanine) segment to induce anisotropic
self-assembly through formation of a b sheet; 3) a diacetylene
moiety integrated directly into the hydrogen bonding array
without a spacer; and 4) in the case of 2, an NHAc end group
capable of forming hydrogen bonds to promote a parallel
arrangement of the molecules. The macromonomer 2 self-
assembled into a supramolecular polymer with a double-
helical topology which was, then, converted into the corre-
sponding poly(diacetylene) P2 by UV irradiation (Scheme 1).
Figure 1. IR spectrum of 2 and assignment of the amide I bands.
band at 1632 cmÀ1, as well as the amide II and amide III bands
were in excellent agreement with a b-sheet structure. A
detailed analysis[16] suggested that 2 may form bent or twisted
b-sheet structures b2 with a parallel arrangement of the
chains. It is worth noting that, by contrast, the IR spectra of 1
were consistent with a predominantly antiparallel packing of
the chains in the b-sheet structure, and a higher proportion of
other (unordered) structures.[16] This difference is remarkable
given the close structural relationship between 1 and 2. The
NHAc end group in 2 appears to enforce a parallel arrange-
ment of the molecules within the aggregates because, only in
this way, can the maximum number of hydrogen bonds be
achieved.
Transmission electron microscopy (TEM) images of
unstained samples of 2 as well as images obtained after
carbon shadowing (Figure 2a–d) showed remarkably straight
fibrillar features which were several micrometers long and,
upon qualitative inspection, appeared to have a uniform
diameter and height (as concluded from the widths of the
carbon shadows). Histographic analyses revealed a bimodal
width distribution with two narrow distributions centered
around maxima of 6.5(Æ1.4) nm and 8.7(Æ2.5) nm. An upper
limit for the height of the fibrils was determined to be
approximately 1.7(Æ1.3) nm. This height is about twice the
value that is typically expected for an individual b sheet.
Nevertheless, the observed fibrils are flat objects—flat
ribbons or collapsed tubes—that are preferentially adsorbed
flat on the carbon surface. Scanning force microscopy (SFM)
images of samples of 2 spin-coated onto highly oriented
pyrolytic graphite (HOPG) substrates (Figure 2e–j) showed
similar fibrillar features.[17] The apparent height of the fibrils
was 4.7(Æ0.5) nm according to a histographic analysis, and
their widths were estimated to be on the order of 5 nm.[18]
Remarkably, the fibrils were consistently found to be right-
handed double helices with a pitch of about 17.8(Æ1.7) nm
which were constituted from two flat ribbon-type substruc-
tures.[19]
Scheme 1. Self-assembly and topochemical polymerization of diacety-
lenes functionalized with a b-sheet-forming oligopeptide.
1 and 2 were prepared by anionic polymerization of
isoprene, high-pressure hydrogenation, stepwise solution-
phase peptide synthesis, and acetylene heterocoupling reac-
tions.[16] Solutions of 1 and 2 in CH2Cl2 or CHCl3 showed no
X-ray diffraction studies[16] on solid samples of 2 showed a
main meridional reflection at a spacing of 4.59 Š. Electron
diffraction studies[16] on multilayer films of 2 revealed a
doublet of reflections at 4.76 and 4.59 Š as well as additional
reflections at spacings of 8.04 and 4.25 Š. The results were in
agreement with the expected b-sheet-type aggregation. Fur-
thermore, they were similar to X-ray data reported for related
1
tendency toward gelation, but H NMR spectra as well as
solution-phase IR spectra gave a clear indication of aggrega-
tion.[16] In the IR spectrum of 2 (Figure 1), the main amide A
(nNH) band at 3283 cmÀ1, the predominating amide I (nCO
)
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Figure 3. Proposed model for the self-assembly and polymerization of
macromonomer 2; ribbons are formed from two parallel b sheets b2
(shown in cross-section) and wound into a tubular double-helical
aggregate.
embedded into a “cushion” of the coil segments and shielded
from the hydrophobic environment. In conclusion, the
observed aggregates are to be regarded as well-defined
supramolecular polymers with dimensions directly correlated
to their molecular constituents rather than as helical micellar
or tubular vesicular structures, as have been observed in the
case of diacetylene-containing lipid amphiphiles.[13] They are,
at the same time, a synthetic example of a “jelly-roll”
morphology which has been described in the context of
protein superstructures.[20]
Finally, we investigated the polymerizability of the macro-
monomers 1 and 2 in organic solution. Solutions of the
macromonomers in CH2Cl2 were degassed in a thermostated
quartz Schlenk tube, cooled to 08C, and exposed to UV
irradiation from a 250 W Ga-doped Hg light source. Solutions
of 1 turned only slightly yellow, and the UV spectra[16] showed
that the diacetylene functions were consumed probably in the
sense of an unspecific cross-linking reaction. By contrast,
solutions of 2 attained an intense purple color within less than
one minute. The UV spectra of the reaction mixtures showed
two strong bands at about 520 nm and 580 nm, which are
consistent with the formation of a poly(diacetylene) back-
bone (Figure 4). Raman spectra as well as solid state CP-MAS
13C NMR spectra provided further evidence for the successful
Figure 2. TEM images of samples of 2: a) unstained and b) after
carbon shadowing; histograms showing the c) width and d) height of
the fibrils (derived from the TEM images); e,f) SFM images of 2;
g,h) height profiles along and across the fibrils; histograms of the
i) height and j) helical pitch (derived from the SFM images).
systems,[8] and closely matched X-ray investigations on helical
amyloid fibrils.[4]
On the basis of the above results, we propose as a working
hypothesis the following model for the self-assembly of 2
(Figure 3). The observed helix dimensions translate into a
ribbon width on the order of 13–14 nm. As the extended
length of 2 is about 6.7 nm, we assume that the ribbons are
constituted of two parallel b-sheet-type aggregates b2. Thus,
the oligopeptide would form crystalline cores that would be
Figure 4. UV spectra of the UV-induced polymerization of 2 in CH2Cl2;
[2]=1 mgmLÀ1
.
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conversion into the poly(diacetylene) P2.[16] We attribute the
remarkable difference in polymerizability between 1 and 2 to
the role of the NHAc end group, which is necessary for the
formation of the required parallel b-sheet structures.
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In conclusion, we have successfully self-assembled the
diacetylene derivative 2 into aggregates with dimensions of a
few nanometers. We have, thus, used b-sheet-type hydrogen-
bonding networks to obtain a well-defined supramolecular
polymer with a double-helical topology. A “1D topochemical
polymerization” was then performed and these supramolec-
ular polymers were converted into the poly(diacetylene) P2.
The obtained poly(diacetylene) features a conjugated back-
bone, a high degree of functionalization with biochemically
relevant substituents, as well as, most importantly, a defined
hierarchical structure. It is these properties which may make
this system attractive as a platform for optoelectronic
applications at the interface with the biosciences.
Received: February 15, 2006
Revised: May 30, 2006
Published online: July 20, 2006
Keywords: b sheets · double helix · hierarchical structures ·
.
self-assembly · topochemical polymerization
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