Templated Chromophore Assembly on Peptide Scaffolds: A Structural Evolution

Article abstract of DOI:10.1002/chem.201803205

The use of a template that bears pre-programmed receptor sites for selectively accommodating chromophores at given positions is an attractive approach for engineering artificial-light-harvesting systems. Indulging this line of thought, this work tackles the creation of tailored antenna architectures with yellow, red and blue chromophores, exploiting three dynamic covalent reactions simultaneously, namely disulfide exchange, acyl hydrazone, and boronic ester formations. The effect of various structural modifications, such as the chromophores as well as their spatial organization (distance, orientation, order) on the energy transfer within the antennas was studied by means of steady-state UV/Vis absorption and fluorescence spectroscopies. This systematic study allowed for a significant improvement of the energy-transfer efficiencies to a noticeable 22 and 15 % for the yellow and red donors, respectively, across the chromophores to the blue acceptor. Metadynamics simulations suggested that the conformational properties of the antennas are driven by intramolecular chromophoric stacking interactions that, upon forcing the α-helix to fold on itself, annul any effects deriving from the programming of the spatial arrangement of the receptor sides in the peptide backbone.

Full text of DOI:10.1002/chem.201803205

DOI: 10.1002/chem.201803205
Full Paper
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Self-Assembly |Very Important Paper|
Templated Chromophore Assembly on Peptide Scaffolds:
A Structural Evolution
Lou Rocard,[a] Darren Wragg,[b] Samuel Alexander Jobbins,[b] Lorenzo Luciani,[a]
Johan Wouters,[c] Stefano Leoni,[b] and Davide Bonifazi*[a]
Abstract: The use of a template that bears pre-programmed
receptor sites for selectively accommodating chromophores
at given positions is an attractive approach for engineering
artificial-light-harvesting systems. Indulging this line of
thought, this work tackles the creation of tailored antenna
architectures with yellow, red and blue chromophores, ex-
ploiting three dynamic covalent reactions simultaneously,
namely disulfide exchange, acyl hydrazone, and boronic
ester formations. The effect of various structural modifica-
tions, such as the chromophores as well as their spatial or-
ganization (distance, orientation, order) on the energy trans-
fer within the antennas was studied by means of steady-
state UV/Vis absorption and fluorescence spectroscopies.
This systematic study allowed for a significant improvement
of the energy-transfer efficiencies to a noticeable 22 and
15% for the yellow and red donors, respectively, across the
chromophores to the blue acceptor. Metadynamics simula-
tions suggested that the conformational properties of the
antennas are driven by intramolecular chromophoric stack-
ing interactions that, upon forcing the a-helix to fold on
itself, annul any effects deriving from the programming of
the spatial arrangement of the receptor sides in the peptide
backbone.
Introduction
systems (dyads) with high donor-to-acceptor ratios. On the
other hand, the covalent assembly of dyes offers a controlled
route as chromophores can be assembled into robust multidi-
mensional architectures such as multilayers,[11] dendrimers[12]
and macrocycles,[13] through either static or dynamic covalent
linkages,[11d,e,14] allowing for the efficient light collection and
energy-transfer control. Recently, the use of a pre-organized
template (e.g., proteins,[15] nucleic acids,[15a,16] micelles,[17] inor-
ganic scaffolds)[18] to organize distinct chromophores has been
widely explored for light harvesting. Considering its versatility,
this strategy appears to be very attractive for engineering mul-
tichromophoric antennas displaying controlled spatial organi-
zation and color distribution. In a recent endeavor, we report-
ed the creation of a multichromophoric architecture through
the use of a peptidic template Ac-QLA-X(disulfide)-QLAQLA-
X(hydrazide)-QLAQLA-X(diol)-QLA-NH2 1 (X=modified amino
acid), in which blue naphthalene-bisimide (B-NDI) 2, red pery-
lene-bisimide (R-PDI) 3 and yellow ethynylpyrene (Y-Py) 4 chro-
mophores could be spatially organized through chemoselec-
tive reactions (Figure 1).[19]
Energy is the biggest issue of our current time.[1] Despite the
very abundant and sustainable source of energy available from
sunlight, nowadays only a tiny fraction of the solar energy is
used by thermal collectors and solar panels to generate heat
and electricity. Artificial systems usually adsorb a fraction of
the solar spectrum, whereas natural photosystems can collect
a much larger energy fraction tailored by evolutionary selec-
tion.[2] Hence, mimicking natural-light-harvesting (LH) systems[3]
to efficiently capture, transfer, and store energy is one of the
most challenging tasks of the scientific community.[4] In this re-
spect, artificial antennas, with well-defined ordering of the
dyes including interchromophoric distances, orientations, and
ratio of donor-to-acceptor, can be developed.[5] The supra-
molecular mixing of dyes,[6] affording organogels,[7] supra-
molecular polymers,[8] co-crystals,[9] metal-organic frame-
works,[10] is a very handy approach for engineering simple LH
[a] Dr. L. Rocard, L. Luciani, Prof. Dr. D. Bonifazi
School of Chemistry, Cardiff University
Park Place, CF10 3AT, Cardiff (UK)
Specifically, the dye assembly was achieved by the simulta-
neous use of disulfide exchange, acyl hydrazone, and boro-
nate-ester-formation reactions that took place at tailored re-
ceptor sites on the peptidic scaffold and allowed for the pre-
cise spatial programming of the chromophores. Although the
proposed synthetic strategy is efficient and versatile to build
multichromophoric architecture exhibiting any absorbed or
emitted colors, the energy-transfer efficiencies (FET) within
triad 5, revealed to be as low as 5 and 5.5% for the Y-Py!B-
NDI and R-PDI!B-NDI sensitization processes, respectively
E-mail: bonifazid@cardiff.ac.uk
[b] D. Wragg, Dr. S. A. Jobbins, Dr. S. Leoni
School of Chemistry, Cardiff University
Park Place, CF10 3AT, Cardiff (UK)
[c] Prof. Dr. J. Wouters
Department of Chemistry, University of Namur (UNamur)
61, rue de Bruxelles, Namur 5000 (Belgium)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201803205.
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Figure 1. a) Simultaneous chromophoric assembly on pre-programmed peptide 1 (Ac-QLA-X(disulfide)-QLAQLA-X(hydrazide)-QLAQLA-X(diol)-QLA-NH2) and
energy-transfer efficiencies; b) chemical structure of the first generation of dyes bearing complementary sticky side.
(Figure 1). Thus, a structural modification of the system is re-
quired if one wants to enhance the FET values. Recalling the
Fçrster theory, the rate of energy transfer kT (FRET), defined by
Equation (1), depends on the interchromophoric distances r
and r0, the latter being the critical interchromophoric distance
at which the energy-transfer efficiency is equal to 50%. The
distance r0 is calculated according to Equation (2)[20]
Computational simulations of all the multichromophoric ar-
chitectures in explicit solvent were performed to rationalize
the conformational properties of the antennas. The use of mo-
lecular dynamics (MD) simulations allows for exquisite insight
into the structure of the peptide and chromophores at the
atomistic level, providing a means to generate and visualize
the conformation of the architecture, usually corresponding to
microstates associated with a particular energy basin. However,
MD simulations are carried out on the femtosecond timescale,
whereas the activated processes, such as the folding or unfold-
ing of the peptide backbone, take place over nanosecond or
even microsecond timescales. This means that unfeasibly long
MD simulations would be required before any such event can
be witnessed. Due to the coarse and complex nature of the
underlying energy landscape of a real chemical system, the
probability of transitioning from one energy basin to another
is also very small. Without some form of acceleration, only
states separated by barriers equal to or less than thermal fluc-
tuations (kBT) in the system are likely to be visited. To access
other regions of the configuration space and to facilitate the
crossing of larger barriers and the observation of critical events
within a reasonable time frame, one must apply an enhanced
sampling method.[21] One such method of acceleration is meta-
dynamics, the technique used in this work. In this technique, a
history-dependent bias potential is iteratively built upon the
underlying energy landscape. This discourages the system
from returning to previously visited configurations and enables
the systematic exploration of the configuration space. This
ꢀ ꢁ
6
1
tD
r0
r
ð1Þ
ð2Þ
kTðrÞ
¼
r0 ¼ 9:78 ꢀ 103 ½k2n-4QD JðlÞꢁ ðin ꢁÞ
1=6
in which tD is the fluorescent lifetime of the donor in the ab-
sence of the acceptor, k2 describes the transition dipole orien-
tation, n is the refractive index of the medium, QD is the quan-
tum yield of the donor fluorescence in the absence of the ac-
ceptor, and J(l) is the integral of the normalized spectral over-
lap between the donor emission and the acceptor absorption.
Considering QD, J(l), r, and k2 as the critical variables for im-
proving FET, in this work (Figure 2), we systematically studied
the effect of: 1) dyes with enhanced spectral overlap between
donor emission and acceptor absorption [J(l)] and energy-
donating chromophores with high emission quantum yields
(QD); 2) different interchromophoric order, 3) distances (r) and
4) orientations (variation of dipole orientation factor k2); 5) sol-
vent.
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Figure 2. En route towards the structural evolution proposed in this study: i) effect of the chromophores; ii) effect of the chromophoric order; iii) effect of the
interchromophoric distance; iv) effect of the dipole orientation; v) effect of the solvent.
allows for the induction and observation of structural changes,
such as the folding and unfolding of the peptide backbone,
which would not be possible by standard MD simulations
alone (see Supporting Information for the details).[22]
For the synthesis of Y-Per 8, a Pd-catalyzed Suzuki cross-cou-
pling was carried out between bromo-perylene 6[28] and 4,4’-bi-
phenyldiboronic acid (bis)pinacol ester 7 in the presence of
[Pd(PPh3)4] and K2CO3 to afford the corresponding boronic
ester in 72% yield. The subsequent removal of the pinacol pro-
tecting group by oxidative cleavage with NaIO4 and treatment
with a 1m aqueous solution of HCl, afforded the desired bor-
onic acid Y-Per 8 in 90% yield (Scheme 1).[29] As far as R-PDI 12
is concerned, monoanhydride intermediate 11 was obtained as
a solid through a mono-condensation reaction between the
perylene dianhydride (PDA) and amine 10[30] in a water/isopro-
panol mixture at 1208C for ten days, a method reported by
Mattern and co-workers.[31] The second condensation reaction
with 4-bromo-2,6-diisopropylaniline was subsequently carried
out in propionic acid at 1408C for 7 h under microwave irradia-
tions affording unsymmetrical PDI in 38% over two steps. Pd-
catalyzed Suzuki cross-coupling of the latter compound with
3,5-difluoro-4-formylphenyl boronic acid afforded final R-PDI
12 (Scheme 1). As expected, the dyes Y-Per 8 and R-PDI 12 ex-
hibited remarkably high QY of 70 and 94%, respectively, with
the emission profile of Y-Per 8 showing excellent spectral com-
plementarity with the absorption spectrum of R-PDI 12 (Fig-
ure 3b). Notably, the total absorption envelop of the dyes
shows a good matching with the solar spectrum (Figure 3a,b).
Following the self-assembly methodology previously devel-
oped by us,[19] the dyes were assembled on a-helix peptide 1
bearing the three-receptor sites every (i, i+7) residues. Multi-
chromophoric architecture 13 was prepared with dyes 1) Y-Per
8, R-PDI 3, and B-NDI 2, as well as 14 with 2) Y-Per 8, R-PDI 12,
and B-NDI 2, in anhydrous DMF over 4 h in the presence of a
catalytic amount of m-phenylenediamine (mPDA) and peptide
1 (Figure 4). After purification, the chemical identities of de-
sired antennas 13 and 14 were confirmed by MALDI-TOF anal-
Results and Discussion
Development of a new generation of chromophores
Following the photophysical characterization of the previously
engineered antenna 5, we recognized that the emission profile
of the pyrene moiety (Y-Py 4) does not best match the UV/Vis
absorption of the perylene bisimide module (R-PDI 3). Accord-
ingly, ethynylpyrene Y-Py 4 has been substituted by a perylene
chromophore (Y-Per) because the latter displays a low-energy
emission profile. The optical properties of the perylene[23] make
it a suitable candidate for our architecture because it strongly
absorbs in the blue region (e=38500 Lmolꢂ1 cmꢂ1 at 434 nm),
and it is highly fluorescent with a small Stokes shift.[24] In paral-
lel, we also decided to amend the structure of the red chromo-
phore because the R-PDI 3 showed a very low fluorescence
quantum yield (QY, <4%). Although the perylene bisimide and
its derivatives are known to be highly fluorescent (QY>
90%),[25] electron-rich N-benzyl substituent can undergo photo-
induced electron transfer to the PDI core,[25,26] thus quenching
the emission of the singlet excited state. Hence, we envisaged
an aliphatic linker group at the N-atom, in which the solubiliz-
ing triethylene glycol (TEG) chains are attached. It is worth
noting that the 2,6-diisopropylphenyl substituent was also in-
troduced to prevent parasite interchromophoric p-p stacking
within the antenna.[27] Thus, we prepared chromophores Y-Per
8 and R-PDI 12 (Scheme 1).
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Scheme 1. Synthesis of Y-Per 8 and R-PDI 12; a) 7, K2CO3, [Pd(PPh3)4], 1,4-dioxane/H2O, 858C, 3 h, 72%; b) i) NaIO4, THF/H2O (4 :1), RT, 1 h 30, ii) HCl (1m), RT,
20 h, 90%; c) i) amine 10, Et3N, iPrOH/H2O, 1208C, 10 d, ii) 5% HCl (aq), 1208C, 10 min, not isolated; d) 4-bromo-2,6-diisopropylaniline, propionic acid, micro-
wave irradiation, 1408C, 7 h, 38% over two steps; e) 3,5-difluoro-4-formylphenyl boronic acid, K2CO3, [Pd(PPh3)4], 1,4-dioxane/H2O, 808C, 20 h, 90%.
Figure 3. Normalized absorption (solid) and fluorescence (dashed) spectra of colored dyes a) 1st generation: Y-Py 4 (yellow), R-PDI 3 (red), B-NDI 2 (blue) and
b) 2nd generation: Y-Per 8 (yellow), R-PDI 12 (red), B-NDI 2 (blue) in DMF plotted against the solar spectrum.
yses (see Supporting Information). Antennas 13 and 14 were
analyzed by UV/Vis absorption spectroscopy in DMF solution.
The spectra were compared with the arithmetic sum of the
three independent dye absorptions after normalization on the
B-NDI maximum at 612 nm. Characteristic bands of the three
dyes were observed but a significant hypochromism for R-PDI-
centered bands was noticed, suggesting a non-quantitative in-
troduction of the R-PDI moiety due to steric hindrance and p-
p stacking interactions.
excitation spectra normalized at the maximum absorption
wavelength of blue chromophore unit B-NDI (lmax =612 nm).
(Y-Per!B-NDI)
(R-PDI!B-NDI)
For antenna 13, FET
and FET
were estimated
to be 7.7 and 7.7% with Method A and 7 and 7.2% with
(Y-Per!B-NDI)
Method B, respectively. Alternatively, FET
and
(R-PDI!B-NDI)
FET
values of 3.7 and 19% in antenna 14 have been
measured by Method A and these were found to be very close
to those obtained by Method B (3.2 and 17.9%). A significant
(R-PDI!B-NDI)
improvement for FET
was observed when using R-PDI
Steady-state fluorescence measurements were performed to
determine the energy-transfer (ET) efficiencies FET following
two methods (see also Supporting Information). Method A in-
volved calculating FET =QY*B-NDI/QYB-NDI, the ratio of quantum
yields of acceptor B-NDI within YRB peptide (QYB-NDI) upon se-
lective excitation of the donor units R-PDI (lexc =495 nm) or Y-
12 from 5.5 and 7.7% for antennas 5 and 13 to 19% for archi-
tecture 14. Metadynamics simulations of architecture 14
(Figure 5, see Supporting Information for free energy surface)
showed that the peptide backbone is essentially folded as an
a-helix, with some distortion at the nitrogen-terminal end. No-
tably, the PDI and NDI cores are facing to each other, whereas
the perylene moiety hardly faces the other two chromophoric
modules in any of the calculated conformations. The perylene
core seems to mostly interact with the diphenyl spacers in-
stead. The lack of any relevant face-to-face PDI/perylene and
NDI/perylene conformations is likely to be the cause of the
Per (lexc =423 nm) and QYB-NDI upon direct excitation (lexc
=
609 nm). In Method B, the calculation was performed by using
FET =ExD/AD, in which ExD and AD are the fluorescence and ab-
sorption intensities of the donor in the normalized excitation
and absorption spectra, respectively, with the absorption and
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Figure 4. Colored assembly with B-NDI 2, R-PDI 3 or 12 and Y-Per 8 on peptidic scaffold 1; a) mPDA, DMF, RT, 4 h. Middle: Absorption spectra of b) YRB pep-
tide 13 (solid black line) and c) YRB peptide 14 (solid black line) normalized with arithmetic sum (dashed line) of absorption of dyes B-NDI 2, R-PDI 3 or 12, Y-
Per 8 in DMF on B-NDI unit. Bottom: Emission (solid lines: lexc =423 nm in yellow, lexc =495 nm in red, lexc =612 nm in blue), and corresponding normalized
excitation spectra (dashed lines: lemis =540 nm in red, lemis =640 nm in blue) of peptides b) 13, c) 14.
Figure 5. Metadynamics simulations of antenna 14 (100 ns); a) side and b) top view of overlaid frames; c) most stable conformation.
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poor FRET between the perylene unit and the other chromo-
phores.
ested in studying the impact of the ordering of the chromo-
phores on both the yield of the assembly and FET. In particular,
we wanted to investigate the effect of the bidirectional (R-
!
PDI Y-Per!B-NDI vs. Y-Per!R-PDI!B-NDI) cascaded, or
Variation of the interchromophoric distance (r) and the
dipole orientation
direct energy transfers.
Thus, three newly designed pre-programmed peptides were
synthesized (Scheme 2) by solid phase peptide syntheses
(SPPS)[32] following Fmoc/tBu strategy and terminal N-capping:
In a parallel avenue, we investigated the effect of the inter-
chromophoric distance. Considering that too short interchro-
mophoric distances might prevent the quantitative linkage of
the PDI moiety and favor quenching mechanisms other than
energy transfers, we decided to prepare peptide templates fea-
turing a receptor site at every (i, i+10) and (i, i+14) residues,
corresponding to three (peptide 15) and four (peptide 16)
turns of a-helix, respectively (Figure 6). Considering that pep-
tide Ac-QLA-X(disulfide)-QLAQLA-X(hydrazide)-QLAQLA-X(diol)-
QLA-NH2 1 features a receptor site at every (i, i+7) residues
with a space distance of 1.08 nm in its folded conformation,
peptides 15 and 16 should lead to antenna with interchromo-
phoric distances of 1.62 and 2.16 nm (pitch of a-helix=
0.54 nm), respectively.[32] The longer interchromophoric dis-
tance might enhance the ET because it is known that maxi-
mum FRET efficiencies are observed for interchromophoric dis-
tances between 1 and 3 nm.[33] In addition, we were also inter-
a
27-mer: Ac-QLA-X(disulfide)-QLAQLAQLA-X(hydrazide)-
QLAQLAQLA-X(diol)-QLA-NH2 15, a 35-mer: Ac-QLA-X(disul-
fide)-QLAQLAQLAQLAQ-X(hydrazide)-QLAQLAQLAQLAQ-
X(diol)-QLA-NH2 16 and a new 21-mer: Ac-QLA-X(disulfide)-
QLAQLA-X(diol)-QLAQLA-X(hydrazide)-QLA-NH2 17. The resin
cleavage and side-chain deprotection were simultaneously per-
formed in the presence of trifluoroacetic acid (TFA)/triisopro-
pylsilane (TIS)/H2O/ethanedithiol (EDT) (94:1:2.5:2.5). The disul-
fide entities were subsequently introduced in solution by disul-
fide exchange with 2,2’-dipyridyl disulfide and diisopropylethyl-
amine (DIEA) to afford the targeted peptides. After purification
of the materials by RP-HPLC (see Supporting Information), the
chemical identity of the peptides was confirmed by ESI-MS(Q-
TOF). Simultaneous assembly of the chromophores onto pep-
tides 15, 16, and 17 gave antennas 21, 22, and 23, respective-
Figure 6. Structures of peptides a) 1, b) 15, c) 16 in which receptor sites are separated by 1.08, 1.62 and 2.16 nm, respectively; d) peptide 17 in which the
order of the diol and hydrazide receptor sites is switched.
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Scheme 2. Synthesis of the antennas; a) SPPS: i) Fmoc deprotection: 20% piperidine in DMF, RT, 3ꢂ6 min; ii) AA coupling: Fmoc-AA-OH, HATU, DIEA, DMF/N-
methyl-2-pyrrolidone (NMP), RT, 25 min; iii) Ac2O/pyridine/NMP (1:2:2), 2ꢂ15 min; iv) TFA/TIS/H2O/EDT (94:1:2.5:2.5), RT, 2 h; b) 2,2’-dipyridyl disulfide, DIEA,
DMF, 1 h; c) Simultaneous assembly with B-NDI 2, R-PDI 12 and Y-Per 8, mPDA, DMF, 4 h, RT.
ly, following the protocol described previously (Scheme 2). The
multichromophoric architectures were purified by size-exclu-
sion chromatography (SEC) and characterized by MALDI-TOF
mass spectrometry.
13 and 14. The measured energy-transfer efficiencies are listed
in Table 1. Notably, increasing the distance between the chro-
mophores did not significantly perturbate the FET values be-
(Y-Per!B-NDI)
cause FET
was determined to be around 3% for an-
(R-PDI!B-NDI)
The absorption spectra, normalized with the arithmetic sum
of the absorption spectra of the three free dyes, interestingly
displayed three different UV/Vis profiles (Figure 7, top). A good
overlapping between the two spectra was observed for anten-
na 23, whereas hypochromism for the R-PDI-centered bands
(compared to the arithmetic sum of the spectra of the dyes)
was noticed for antennas 21 and 22, similarly to the cases of
tennas 21–22, whereas FET
was 18% in the three
cases. The change of the order within antenna 23 slightly im-
proved ET efficiencies (FET(Y-Per!B-NDI) and FET(R-PDI!B-NDI) estimated
to be 7.7 and 21.9% with Method A; and 7.4 and 21.8% with
Method B) despite the bidirectional character of the ET pro-
cess.
Figure 7. Top: Absorption spectra of antennas (solid black line) a) 21, b) 22, c) 23 normalized with the arithmetic sum of dyes (dashed black line) B-NDI 2, R-
PDI 12, Y-Per 8 on B-NDI in DMF. Bottom: Emission (solid lines: lexc =423 nm in yellow, lexc =495 nm in red, lexc =612 nm in blue), and corresponding normal-
ized excitation spectra (dashed lines: lemis =540 nm in red, lemis =640 nm in blue) of antennas a) 21, b) 22, c) 23.
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Table 1. Energy-transfer efficiencies.
(Y-Per!B-NDI)
(R-PDI!B-NDI)
Entry
Peptide
Dyes distance
[nm]
Methods
FET
FET
[%]
[%]
A: FET =QY*B-NDI/QYB-NDI
B: FET =ExD/AD
3.7
3.2
2.7
2.9
2.5
2.5
7.7
7.4
4.2
4
17.9
19
17
1
2
3
4
5
6
YRB 14
1.08
1.62
2.16
1.08
1.08
1.08
A
B
A
B
A
B
A
B
A
B
YRB 21
17
19.1
18.5
21.9
21.8
18
17.4
16.6
16.2
YRB 22
RYB 23
RYB 30 (n=1)
RYB 31 (n=3)
4.7
4.6
The metadynamics calculations shown in Figure 8 clearly dis-
played that the energy profile is dominated by a minimum, in
which the three chromophores are strongly interacting to each
other in the case of antenna 21. In contrast to antenna 14, the
perylene and NDI cores are facing to each other, whereas the
PDI core is pointing in another direction. Surprisingly, when
the distance between the receptor sites was increased to
2.16 nm as in the case of 22, the strong intramolecular stack-
ing interaction between the different chromophores (with
broader spatial occupancies) led to a strong distortion of the
helical structure of the peptidic backbone. This finding sug-
gests that the main driving force ruling the conformation of
the peptide backbone is the interchromophoric stacking and
not the opposite (i.e., the peptide template).
linker. We investigated two catechol derivatives, of which one
was rigid, with only one sp3 carbon between the catechol and
the triazole (see amino acid precursor 26, Scheme 3), and a
second featuring three sp3 carbons as the linker (see amino
acid precursor 27, Scheme 3).
Accordingly, two Fmoc-protected catechol-containing amino
acids 26 and 27 were synthesized (see Supporting Information)
and used to prepare the peptides Ac-QLA-X(disulfide)-
QLAQLA-X(catechol)-QLAQLA-X(hydrazide)-NH2 28 and 29 fol-
lowing the RYB order (Scheme 3), which so far gave the best
FET results. After purification, the chromophore assembly was
performed following the standard protocol to give antennas
30 and 31, respectively. The absorption spectra of 30 and 31
normalized with the arithmetic sum of the absorption of the
dyes on the B-NDI profile are shown in Figure 10a,b (top).
On the one hand, the interchromophoric interactions annul
the effect of the distances between the receptor sites in the
antennas, in agreement with the experimental findings for
which the FET values are very similar for antennas 14, 21, and
22. On the other hand, when the R-PDI unit is placed at termi-
nus of the peptide (e.g., antenna 23), the deepest minimum in
the energy profile corresponds to a largely a-helical structure.
The folding of the flexible linker of the receptor sites led again
to a strong interchromophoric stacking, in particular between
the perylene and PDI cores in the case of 23.
If compared to RYB peptide 23, the measured energy-trans-
(R-PDI!B-NDI)
fer efficiencies for the new antennas are lower; FET
around 18 and 16% for 30 and 31, respectively; FET
is
is
(Y-Per!B-NDI)
around 4 and 5% for 30 and 31. The metadynamics analysis
for antenna 31 is depicted in Figure 11a–c. The peptide
scaffold predominantly adopts an a-helical conformation, but
the spatial occupancy of the dyes in antenna 31 is less con-
fined than in the previous antenna architectures bearing the
bicyclic diol. It is worth noting that some conformations for 31
exist, in which the Y-Per unit interacts in a face-to-face fashion
with the other chromophore cores (Figure 11). However, due
to the large flexibility of the linkers, this strategy does not
seem to correctly pre-organize the dipole orientation of the
dyes and does not dramatically enhance the energy-transfer
processes.
Replacement of the linker (orientation of the dipoles) and
solvent effects
As clearly displayed in the metadynamics simulations of all an-
tennas, the non-planar bicyclic diol anchor forces the boronic
ester to adopt an unfavorable conformation, for which the Y-
Per core points in another direction with respect to that of
chromophores R-PDI and B-NDI (Figure 5). Indeed, when look-
ing at the crystal structures of diol-based sticky side precursors
24 and 25 (Figure 9), one can easily recognize that the diol
functional group sits on a different plan than that of the linker,
thus in net contrast to the directional properties of the other
receptor sites. This results in an unfavorable pre-organization
of the architecture, preventing the chromophores to adopt
face-to-face conformations. Thus, we conjectured that the non-
planar bicyclic diol site could be replaced by a catechol-based
Alternatively, the boronic ester linkage in catechol-based an-
tennas 30 and 31 appears to be less stable than the one ob-
tained from the bicyclic diol, due to the conjugation of the cat-
echol moiety making it more Lewis acidic. We also assumed
that the presence of traces of Me2NH and water, coming from
the thermal decomposition of DMF by the extrusion of CO,
might catalyze the hydrolysis of the boronic ester linkage, es-
pecially under very dilute conditions (needed for the fluores-
cence measurements). The unleashing of free perylene dye in
(Y-Per!B-NDI)
solution directly altered the FET
values. This consider-
ation prompted us to look at an alternative solvent, such as
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Figure 8. Metadynamics simulations (100 ns); a,d,g) side and b,e,h) top view of overlaid frames; c,f,i) most stable conformation of antennas 21, 22, 23 re-
spectively.
dry N,N-dimethylacetamide (DMA). Accordingly, the assembly
of the dyes was performed on Ac-QLA-X(disulfide)-QLAQLA-
X(diol)-QLAQLA-X(hydrazide)-NH2 17 in distilled DMA to give
antenna 23 after purification by SEC.
Normalized UV/Vis absorption spectrum of 23 in DMA
shows a great correlation confirming the quantitative introduc-
tion of the chromophores as in the case of DMF (Figure 10c,
top). Finally, the fluorescence measurements were monitored
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Figure 9. Side view of the crystal structures of a) diol 24 and b) protected diol 25. Crystals were grown by slow evaporation from solutions in THF for 24 and
EtOAc for 25. Color code: grey (C), red (O) and blue (N).
Scheme 3. Synthesis of catechol based antennas; a) SPPS: i) Fmoc deprotection: 20% piperidine in DMF, RT, 3ꢂ6 min; ii) AA coupling: Fmoc-AA-OH, HATU,
DIEA, DMF/NMP, RT, 25 min; iii) Ac2O/pyridine/NMP (1:2:2), 2ꢂ15 min; iv) TFA/TIS/H2O/EDT (94:1:2.5:2.5), RT, 2 h; b) 2,2’-dipyridyl disulfide, DIEA, DMF, 1 h;
Chromophore assembly with B-NDI 2, R-PDI 12 and Y-Per 8, c) m-PDA, DMF, 4 h, RT.
in DMA and compared with those obtained in DMF (Fig-
as well as higher QY, revealed to have no major impact on the
(R-PDI!B-NDI)
(Y-Per!B-NDI)
ure 10c, bottom). Although FET
was estimated to be
FET
FET
.
However,
a
significant improvement of
(Y-Per!B-NDI)
(R-PDI!B-NDI)
22% in both solvents, FET
was determined to be
(from 5 to about 20%) was observed with highly
around 15% (Table 2), a double value compared to that record-
ed in DMF. This finding further suggests that when boronic
esters are used, one needs to carefully choose a suitable anhy-
drous solvent to avoid leakage of the chromophore in solution
as a consequence of parasitic hydrolysis.
florescent PDI (QY=94%). In a parallel avenue, we discovered
that increasing the distance between the receptor sites on the
peptide scaffold (from 1.08 to 1.62 and 2.16 nm) did not
impact the ET efficiencies and neither did replacing the bicyclic
diol site with a catechol derivative. Metadynamic analyses sug-
gested that the conformations of the antennas are driven by
intramolecular chromophoric stacking interactions. Upon forc-
ing the a-helix to fold on itself, these interactions annul any ef-
fects deriving from the programming of the spatial arrange-
ment of the receptor sides in the peptide backbone.
Conclusions
We have investigated a-helix peptides as templates to engi-
neer antennas through a simultaneous multichromophoric as-
sembly approach. Specifically, we systematically studied the
effect of 1) dyes with enhanced spectral overlap between
donor emission and acceptor absorption and energy-donating
chromophores with high emission quantum yields; 2) different
interchromophoric order, 3) distances (r) and 4) orientations
and; 5) solvents. Replacing the ethynylpyrene chromophore
bearing a perylene unit with the latter displaying better spec-
tral overlaps between donor emission and acceptor absorption
Provided that stable chromophores are used, these findings
suggest that, to engineer efficient antennas following a tem-
plate approach, one needs to select 1) templates featuring re-
strained conformational properties, 2) highly fluorescent chro-
mophores, and 3) robust linkages that are chemically inert
under the working conditions. It is with these guidelines that
our activities will progress in the field, creeping ever closer to
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Figure 10. Top: Absorption spectra of antennas: a) RYB 30 (black solid line); b) RYB 31 (black solid line) normalized on B-NDI with arithmetic sum (dashed
line) of absorption of dyes B-NDI (in blue), R-PDI (in red), Y-Per (in yellow) in DMF; c) RYB 23 (black solid line) normalized on B-NDI with arithmetic sum
(dashed line) of absorption of dyes B-NDI (in blue), R-PDI (in red), Y-Per (in yellow) in DMA. Bottom: Emission (solid lines: lexc =423 nm in yellow,
lexc =495 nm in red, lexc =609 nm in blue), and corresponding normalized excitation spectra (dashed lines: lemis =540 nm in red, lemis =640 nm in blue) of an-
tennas a) 30 and b) 31 in DMF, c) 23 in DMA.
Figure 11. Metadynamics simulations of antenna 31 (100 ns); a) side and b) top view of overlaid frames; c) most stable conformation.
the ultimate goal of developing artificial antennas with unitary
Table 2. Energy-transfer efficiencies within 23 in DMF and DMA.
light-harvesting efficiency.
(R-PDI!B-NDI)
(Y-Per!B-NDI)
Entry Peptide Solvent Methods
FET
FET
[%]
[%]
Acknowledgements
A: FET
QY*B-NDI/QYB-NDI
B: FET =ExD/AD 21.8
=
21.9
7.7
1
2
RYB 23 DMF
RYB 23 DMA
7.4
14
15
D.B. and L.R. gratefully acknowledge the EU through the ERC
Starting Grant “COLORLANDS” and ITN-ETN “PHOTOTRAIN”
projects, as well as Cardiff University for the financial support.
A
B
22
22
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Keywords: chromophores · energy transfer · metadynamics ·
peptide · self-assembly
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Manuscript received: June 25, 2018
Accepted manuscript online: August 21, 2018
Version of record online: && &&, 0000
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FULL PAPER
&
Self-Assembly
L. Rocard, D. Wragg, S. A. Jobbins,
L. Luciani, J. Wouters, S. Leoni,
D. Bonifazi*
&& – &&
Templated Chromophore Assembly on
Peptide Scaffolds: A Structural
Evolution
Designing antennas: Investigation of a-
helix peptides as templates for engi-
neering light-harvesting antennas
ous structural parameters was studied
and a significant improvement of the
energy transfer across the chromo-
phores was observed.
through a multichromophoric assembly
approach is reported. The effect of vari-
&
&
Chem. Eur. J. 2018, 24, 1 – 14
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!