Synthesis of a silk-inspired peptide-oligothiophene conjugate

Article abstract of DOI:10.1039/b415454a

The first example of an oligothiophene-peptide conjugate, which was obtained by solid-phase acylation of a resin-bound silk-inspired oligopeptide sequence with a carboxylic acid functionalized regioregular tetra(3- hexylthiophene) derivative, is reported.

Full text of DOI:10.1039/b415454a

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C O M M U N I C A T I O N
Synthesis of a silk-inspired peptide–oligothiophene conjugate†
Harm-Anton Klok,*a Annette Ro¨sler,b Gu¨nther Go¨tz,c Elena Mena-Osteritzc and
Peter Ba¨uerle*c
a
´
Ecole Polytechnique Fe´de´rale de Lausanne, Institut des Mate´riaux, Laboratoire des Polyme`res,
Baˆtiment MX-D, CH-1015, Lausanne, Switzerland. E-mail: Harm-Anton.Klok@epfl.ch
b Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany
c Department of Organic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, D-89081,
Ulm, Germany. E-mail: Peter.Baeuerle@chemie.uni-ulm.de
Received 6th October 2004, Accepted 10th October 2004
First published as an Advance Article on the web 8th November 2004
The first example of an oligothiophene–peptide conjugate,
which was obtained by solid-phase acylation of a resin-
bound silk-inspired oligopeptide sequence with a carboxylic
acid functionalized regioregular tetra(3-hexylthiophene)
derivative, is reported.
oligothiophene conjugate GAGAG-4T described here can also
undergo directed hydrogen bonding interactions and represents
a first model compound whose self-organization is driven by
different competitive intermolecular interactions, which may
lead to unexpected and novel 2D- and 3D-nanoscale structures
and offers the possibility to fine-tune materials properties.
Introduction
Oligo- and polythiophenes are an attractive class of organic
(semi)conducting materials which have received considerable
attention for the development of organic electronic devices, such
as light emitting diodes (LEDs),1 field-effect transistors (FETs)2
or solar cells.3 Functionalization of oligo- and polythiophenes
with receptor moieties that can selectively recognize and bind
specific guests offers the possibility to transduce these events
into an electronic signal and makes these materials attractive
candidates for the development of novel sensor devices.4 Of
particular interest are polythiophenes containing side chains
functionalized with specific nucleotide sequences,5 saccharides,6
or amino acids.7 Since the performance of organic semicon-
ductors, in particular regioregular head-to-tail coupled poly(3-
alkylthiophenes) (HT-P3HT), has been found to be strongly
influenced by their supramolecular organization and packing on
a molecular level,8 such motifs may not only be able to recognize
and sense a variety of biologically relevant target molecules, but
may also be powerful auxiliaries to control intermolecular forces
and to consequently manipulate the organization of the hybrid
materials. In particular, the conjugation of well-defined a-helical
and b-strand peptide sequences with synthetic polymers is an
attractive strategy that allows enhanced structural control at the
nanometer level and to synergistically combine the properties of
the two constituent components.9
In this communication, we describe the synthesis of a
diblock-oligomer, combining a regioregular head-to-tail cou-
pled oligo(3-alkylthiophene) (HT-O3AT) and an oligopep-
tide sequence. We chose as the conjugated oligomer part
the corresponding tetramer, head-to-tail coupled tetra(3-
hexylthiophene) (4T) which is functionalized by the pentapep-
tide sequence glycine-(L-alanine)-glycine-(L-alanine)-glycine
(GlyAlaGlyAlaGly, GAGAG) inspired by Bombyx mori (silk-
worm) silk.10 AlaGly has been identified as the most important
repeat sequence in the crystalline b-sheet domains that is
dominated by multiple hydrogen bonding and provides the
strength of the silk fiber. On the other hand, on substrates
such as graphite, b-alkylated oligothiophenes typically form
highly organized, lamellar-type 2D-assemblies. This process
is driven by relatively weak and unspecific van der Waals
interactions of the alkyl side chains.11 In contrast, the peptide–
Results and discussion
The synthesis of the peptide–thiophene conjugate GAGAG-4T
was realized by the activated amide coupling of two individually
prepared building blocks, i.e., the amino-terminated pentapep-
tide sequence GAGAG and the head-to-tail coupled tetra(3-
hexylthiophene) carboxylic acid (4T-COOH). The synthesis of
the semiconducting tetrameric building block was performed
according to a recently developed strategy with which series
of HT-O3ATs up to dodecamers were prepared serving as
structurally defined model compounds for the above mentioned
ꢀ
ꢀ
11b
corresponding polydisperse HT-P3AT. First, 3,4 -dihexyl-2,2 -
bithiophene-5-carboxylic acid 112 is protected and transformed
in 88% yield to benzyl ester 2, which is subsequently selectively
iodinated at the free a-position using a mercuration/iodination
protocol to give a-iodobithienyl carboxylic acid benzyl ester 3
in 92% yield. Suzuki-type C–C cross coupling of 3 and 3,4ꢀ-
dihexyl-2,2ꢀ-bithiophene-5-boronic acid propane-1,3-diyl ester
4
12 gave the benzyl ester-capped quaterthiophene 5 in 44%, which
was hydrolyzed to target molecule 4T-COOH in 73% yield after
recrystallization (Scheme 1).
Amino-terminated GAGAG was synthesized using 9-
fluorenylmethoxycarbonyl (Fmoc) solid-phase peptide synthe-
sis (SPPS) on a p-benzyloxybenzyl alcohol (Wang) resin.13
Peptide synthesis was performed via in situ activation of the
appropriate Na-Fmoc protected a-amino acids as 1-hydro-
xybenzotriazole (HOBt) esters using N-[(1H-benzotriazol-1-
yl)(dimethylamino)methylene]-N-methylmethaminium hexaflu-
orophosphate N-oxide (HBTU)/HOBt in the presence of 2
eq. diisopropylethylamine (DIPEA). Then, in a second step,
the resin-bound peptide was swollen in a 9 : 1 (v/v) mix-
ture of dichloromethane (DCM) and N,Nꢀ-dimethylformamide
(DMF) and treated with 4 eq. 4T-COOH, 4 eq. benzotriazol-1-
yloxytris(pyrrolidino)phosphonium hexafluorophosphate (Py-
BOP) and 8 eq. DIPEA to couple the quaterthiophene car-
boxylic acid to the N-terminal amine group of the peptide. The
crude product was released from the support using 90% (v/v)
trifluoroacetic acid (TFA, aq.) (Scheme 2). After purification, the
hybrid molecule GAGAG-4T was obtained in 24% yield with a
purity >95%, as estimated from reversed-phase (RP) HPLC.
The structure of GAGAG-4T was confirmed by 1H- and
13C-NMR spectroscopy, MALDI-TOF mass spectrometry and
1
FTIR spectroscopy. Fig. 1 shows the H-NMR spectrum of
† Electronic supplementary information (ESI) available: Synthetic pro-
cedures for all compounds, as well as 13C-NMR, RP-HPLC and
MALDI-TOF mass spectrometry characterization of GAGAG-4T. See
http://www.rsc.org/suppdata/ob/b4/b415454a/
GAGAG-4T. Comparison of the integrals of the a-CH protons
of the L-alanyl residues of the pentapeptide (labelled “A” in
Fig. 1) with the aromatic CH protons of the quaterthiophene
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 4 1 – 3 5 4 4
3 5 4 1
©

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Scheme 1 (i) Toluene, rt, 15 h; (ii) CH2Cl2–HOAc (95 : 5), 0 ◦C–rt, 18 h; (iii) THF, 5 mol% Pd-cat., reflux, 8 h; (iv) THF, reflux, 3 h.
Scheme 2 (i) Fmoc solid-phase peptide synthesis; (ii) 4T-COOH, ByBOP, DIPEA (DCM–DMF 9 : 1 (v/v)); (iii) 90% TFA (aq.).
moiety (“D” and “E”) indicates a successful coupling of the
4T-COOH building block with the peptide sequence and the
absence of residual free peptide or oligothiophene precursor.
The ATR-FTIR spectrum of GAGAG-4T shown in Fig. 2
also contains features of both the pure pentapeptide and the
quaterthiophene, thus providing additional support for the
structure of the target compound. More interestingly, although
there is some unfavourable overlap with bands of the 4T-COOH
precursor, the ATR-FTIR spectrum of GAGAG-4T reveals
shoulders of ∼1630 cm−1 and 1680 cm−1 at the major carbonyl
band. These shoulders, which appear as signals in the spectrum
of GAGAG, represent the amide I absorption band and suggest
an antiparallel b-sheet organization of the peptide segments.14
First experiments concerning the electronic properties of the
peptide–oligothiophene conjugate GAGAG-4T are under way,
as well as detailed investigations of the self-organization proper-
ties on substrates. The optical and electrochemical properties
of the hybrid molecule in dichloromethane solution (kmax
=
a
390 nm, Ep = 0.63 V vs. Fc/Fc+) are quite comparable to
those of the parent quaterthiophene carboxylic acid 4T-COOH
(kmax = 393 nm, E◦1 = 0.45 V vs. Fc/Fc+).12c The peptide residue
obviously exerts a certain electron withdrawing effect onto the
3 5 4 2
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 4 1 – 3 5 4 4

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Fig. 1 1H-NMR spectrum (250 MHz, DMSO-d6) of GAGAG-4T.
Fig. 2 ATR-FTIR spectra of GAGAG (black curve), 4T-COOH (red curve), and GAGAG-4T (blue curve).
12c
conjugated p-system, which can be seen in a positive shift of the
first oxidation potential.
of carboxylic acid groups via hydrogen bonding (Fig. 3c). X-
Ray structure analyses of this compound reveal polymorphism
and two different 3D arrangements in the crystal are found.
Similar to the 2D-crystalline monolayer, both structures are
dominated by a mutual interplay of different intermolecular
interactions, which is the dimerization of the carboxylic acid
groups via hydrogen bonding and the interdigitation of the alkyl
In contrast, a great difference between the two compounds
was noted with respect to their 2D-ordering which might be
due to the tendency of the peptide sequence in GAGAG-
4T to form b-sheets via multiple hydrogen bonding. As a
preliminary result, we note that scanning tunnelling microscopy
(STM) investigations on the hybrid molecule at the solid/liquid
interface reveal completely novel features and superstructures,
which explicitly differ from those of the parent compound 4T-
COOH. At first glance, long linear strands of 3.5 to 4.0 nm in
12c
side chains via van der Waals forces. The striking difference in
STM structures observed for 4T-COOH and GAGAG-4T clearly
illustrates the effect of directed hydrogen bonding interactions
on the two-dimensional supramolecular organization of these
compounds.
˚
width and 3 A in height are formed on the surface (Fig. 3a). More
experiments are necessary to deduce a structural model of how
the molecules are arranged in the strands which explains the
observed features. In contrast, in case of the quaterthiophene
carboxylic acid 4T-COOH, a lamellar long-range ordering in
various domains due to interdigitation and van der Waals
interactions of the hexyl side chains is observed, which is typical
Conclusions
In this communication we have described the first example of
a peptide–oligothiophene conjugate composed of a head-to-tail
coupled tetra(3-hexylthiophene) and a silk-inspired GAGAG
pentapeptide sequence. The basic synthetic strategy involved
acylation of the resin-bound peptide sequence with a carboxylic
8b
11b
for head-to-tail coupled oligo- and polythiophenes (Fig. 3b).
The lamellae consist of dimers, which are formed due to pairing
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 4 1 – 3 5 4 4
3 5 4 3

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Fig. 3 (a) STM height image of GAGAG-4T on HOPG (62 × 100 nm2, bias voltage = −600 mV, sample is negative, tunnel current = 68 pA);
(b) STM height image of 4T-COOH on HOPG (100 × 100 nm2, bias voltage = −348 mV, sample is negative, tunnel current = 33 pA); (c) calculated
model of the molecular arrangement of 4T-COOH.
acid functionalized quaterthiophene derivative. This synthetic
strategy is very flexible and may be easily adapted to other
peptide sequences or other conjugated oligomers. FTIR ex-
periments indicate that the peptide sequence retains its ability
to form antiparallel b-sheet structures after conjugation to the
oligothiophene. In conjunction with preliminary STM results,
this suggests that short peptide sequences may indeed act
as auxiliaries that can influence the nanoscale structure, and
ultimately, properties of organic semiconducting materials.
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Acknowledgements
This work was partially supported by the Deutsche Forschungs-
gemeinschaft (Emmy Noether Program, H.-A.K.). We would
like to acknowledge preparative help of Dipl.-Chem. Maria
Leute (University of Ulm).
9 G. W. M. Vandermeulen and H.-A. Klok, Macromol. Biosci., 2004,
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O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 4 1 – 3 5 4 4