Photo-responsive Bioactive Surfaces Based on Cucurbit[8]uril-Mediated Host–Guest Interactions of Arylazopyrazoles

Article abstract of DOI:10.1002/chem.201705426

A photoswitchable arylazopyrazole (AAP) derivative binds with cucurbit[8]uril (CB[8]) and methylviologen (MV²⁺) to form a 1:1:1 heteroternary host–guest complex with a binding constant of K_a=2×10³ m^?1. The excellent photoswitching properties of AAP are preserved in the inclusion complex. Irradiation with light of a wavelength of 365 and 520 nm leads to quantitative E- to Z- isomerization and vice versa, respectively. Formation of the Z-isomer leads to dissociation of the complex as evidenced using ¹H NMR spectroscopy. AAP derivatives are then used to immobilize bioactive molecules and photorelease them on demand. When Arg-Gly-Asp-AAP (AAP–RGD) peptides are attached to surface bound CB[8]/MV²⁺ complexes, cells adhere and can be released upon irradiation. The heteroternary host–guest system offers highly reversible binding properties due to efficient photoswitching and these properties are attractive for designing smart surfaces.

Full text of DOI:10.1002/chem.201705426

DOI: 10.1002/chem.201705426
Communication
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Host–Guest Systems |Hot Paper|
Photo-responsive Bioactive Surfaces Based on Cucurbit[8]uril-
Mediated Host–Guest Interactions of Arylazopyrazoles
Maike Wiemann+,[a] Rebecca Niebuhr+,[b] Alberto Juan,[a] Emanuela Cavatorta,[a]
Bart Jan Ravoo,*[b] and Pascal Jonkheijm*[a]
of spatio-temporal control.[5–7] Shinkai et al. proposed to use
Abstract: A photoswitchable arylazopyrazole (AAP) deriva-
light for manipulating self-assembled structures. Their self-
tive binds with cucurbit[8]uril (CB[8]) and methylviologen
complementary azobenzene derivative underwent cyclic oligo-
(MV2+) to form a 1:1:1 heteroternary host–guest complex
merization in E-form and lead to intramolecular cyclization
with a binding constant of Ka =2ꢀ103 mÀ1. The excellent
when irradiated with UV light and subsequent switching to Z-
photoswitching properties of AAP are preserved in the in-
isomer.[8] Feringa and co-workers used dithienylcyclopentene
clusion complex. Irradiation with light of a wavelength of
photochromic switches to tune the viscosity of solutions.[8,9]
365 and 520 nm leads to quantitative E- to Z- isomeriza-
Azobenzene isomerization processes have successfully been
tion and vice versa, respectively. Formation of the Z-
employed to enable a large functional change in biomolecules
isomer leads to dissociation of the complex as evidenced
and ligands in a number of instances.[10,11] Inclusion of azoben-
1
zene derivatives as guests in macrocyclic hosts such as cyclo-
to immobilize bioactive molecules and photorelease them
dextrins (CD) and cucurbiturils (CB) can give rise to photosensi-
using H NMR spectroscopy. AAP derivatives are then used
on demand. When Arg-Gly-Asp-AAP (AAP–RGD) peptides
tive host–guest complexation to regulate recognition and
are attached to surface bound CB[8]/MV2+ complexes,
function.[12–18] Photosensitive host–guest complexation is cur-
cells adhere and can be released upon irradiation. The
rently intensively explored on surfaces and is promising to
heteroternary host–guest system offers highly reversible
come yet a step closer to mimic natural cell-extracellular
binding properties due to efficient photoswitching and
matrix (ECM) interactions on surfaces.[1] Zhang et al. prepared
these properties are attractive for designing smart sur-
b-CD surfaces modified with azobenzene-containing propyltri-
faces.
ethoxysilane guests to tune the wettability properties of sur-
faces.[19] We have demonstrated the possibility of photospecific
protein assembly via azobenzene-functionalized ligands on b-
CD surfaces.[20] Gong and co-workers designed a-CD self-as-
External control of bioactivity on biointerfaces and coatings
has attracted considerable interest across bioanalytical and
biomedical applications.[1–4] In supramolecular chemistry, an im-
portant focus lies on the use of light to exploit and direct re-
versible control over the state of molecular assemblies and
complex structures because light does not require additional
components and light can be applied with a very high degree
sembled monolayers (SAM) to immobilize azobenzene-modi-
fied cell adhesive RGD peptides and subsequently control cell
attachment and release on this surface with UV light.[19]
Cucurbit[8]uril-mediated host–guest heteroternary com-
plexes including photoactive azobenzenes have been used in
photomodulation of the assembly of (bio)molecules on sur-
faces.[22–29] For example, Scherman et al. reported the photoin-
duced disassembly of raspberry-shaped colloids, representing
these systems as useful tools for cargo delivery.[22] We have re-
cently assembled these photoresponsive azobenzene-contain-
ing heteroternary complexes onto chips and employed them
to attract and release proteins, viruses and bacteria by photo-
isomerization.[23,24] Interestingly, when these heteroternary
complexes have been constructed with both redox- and light-
responsive elements multi stimuli-responsivity becomes avail-
able.[25]
[a] M. Wiemann,+ Dr. A. Juan, Dr. E. Cavatorta, Prof. P. Jonkheijm
Bioinspired Molecular Engineering Laboratory of the MIRA Institute for
Biomedical Technology and Technical Medicine and of the MESA and
Institute for Nanotechnology, University of Twente
P.O. Box 217, 7500 AE, Enschede (The Netherlands)
E-mail: p.jonkheijm@utwente.nl
[b] R. Niebuhr,+ Prof. B. J. Ravoo
Organic Chemistry Institute and Center for Soft Nanoscience
Westfꢀlische Wilhelms-University Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: b.j.ravoo@uni-muenster.de
[+] These authors contributed equally to this work.
Azobenzenes have a thermodynamically stable E-isomer and
a metastable Z-isomer and they can be switched from E to Z
with UV irradiation (light of wavelength lꢀ360 nm) and back
from Z to E with visible irradiation (lꢀ460 nm).[26] Unfortunate-
ly, the thermodynamic stability of the Z-isomer is low and the
overlapping absorbance bands lead to incomplete photo-
switching with a photostationary state (PSS) of about 70–
80%.[6,12,27] Increasing the half-life time, while retaining good
Supporting information and the ORCID identification numbers for the
authors of this article can be found under https://doi.org/10.1002/
chem.201705426.
ꢂ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution-NonCommercial-NoDerivs License, which permits use and distribu-
tion in any medium, provided the original work is properly cited, the use is
non-commercial and no modifications or adaptations are made.
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Figure 1. a) UV/Vis spectrum of the photo-isomerization of AAP in the presence of CB[8]/paraquat (1:1:1) at 100 mm in water with corresponding switching
cycles (see text for details). b) 1H NMR spectrum of the re-isomerization of a 1:1:1 mixture irradiated with l=520, 365 and 520 nm for 10 min ((E-Z-E) from top
to bottom) at 100 mm. c) Scheme of heteroternary inclusion complex formation and dissociation (R1: CH2CONH-(OCH2CH2)4-OH). d) Isothermal calorimetry of
1
CB[8]/paraquat with E-AAP and the corresponding 1:1 fit. e) 1H NMR titration with 100 mm CB[8]/paraquat and E-AAP. f) Job plot based on a H NMR titration.
addressability has been a major challenge and led to the
design of new derivatives, such as o-methoxy and o-fluoro azo-
benzenes or bridged azobenzenes.[28,29]
tails and characterization of AAP), a solution of E-AAP (1 mm)
was titrated to a 1:1 solution (0.1 mm) of CB[8] and paraquat
while monitoring the change in heat using isothermal titration
calorimetry (ITC, Figure 1d). An exothermic, 1:1 binding event
was observed and a binding constant between CB[8]/paraquat
and E-AAP was determined to be Ka =2.5ꢀ103 mÀ1 (Figure 1d).
Additionally, AAP concentrations in a range of 0.05 to 0.4 mm
where titrated to a 0.1 mm solution of CB[8]/paraquat in a
1H NMR titration (Figure 1e, full spectra are given in Fig-
ure S2a). On the basis of the downfield shift of the aromatic
signals of paraquat, a binding constant of Ka =2.1ꢀ103 mÀ1 was
obtained. A 1:1 binding stoichiometry was verified using a Job
plot (Figure 1 f, 1H NMR spectra are given in Figure S2b).[35]
Upon increasing the AAP concentration the aromatic signals of
paraquat as well as both methyl signals of the pyrazole unit of
AAP shifted upfield. These observations are in agreement with
CB[8] guest complexation and show that paraquat is interact-
ing with the AAP moiety inside the CB[8] cavity.[25] The ternary
complex was also identified using ESI-ToF mass spectrometry,
which showed a signal at m/z 974.4 corresponding to a doubly
charged heteroternary complex (see Supporting Information
Figure S1).
Alternatively, recent literature documents excellent switching
efficiencies and improved half-life times of Z-isomers in the
case one aryl ring in the azobenzene system is exchanged with
a five-membered nitrogen based heteroaromatic ring, so-called
azoheteroaryl photoswitches.[30–32] For example, N-substituted
arylazopyrazoles (AAP) show half-life times of 10 to 1000 days
at 258C.[31] PSS for both isomers are ꢁ98% upon irradiation
with l=365 nm for E to Z isomerization while employing l=
520 nm leads to Z to E isomerization.[30] Ravoo and co-workers
have recently shown that AAPs form photoresponsive inclusion
complexes with b-CD.[33] Light-responsive switching of these b-
CD-AAP host–guest inclusion complexes occurred more effi-
cient and with a superior thermal half-life time of the Z-isomer
compared to commonly used azobenzenes.[33] Incorporation of
the AAP in CD vesicles and nanoparticles revealed excellent
photoresponsive aggregation and dispersion.[33] Since hetero-
cyclic azo-compounds such as AAPs are relatively unknown in
host–guest chemistry and its surface-related applications, we
describe the novel use of AAP as potent photoswitchable
ligand for heteroternary CB[8]-based inclusion complexes in so-
lution and on surfaces (Figure 1). We show the potential for
fabricating photosensitive bioactive surfaces using an AAP-
modified integrin binding Arg-Gly-Asp (RGD) peptide. Our
work represents an alternative to the use of photolabile caging
groups on RGD motifs for the controlled attachment of cells.[34]
To verify CB[8]-mediated supramolecular complexation of
AAP (Figure 1c, see Supporting Information for synthetic de-
To investigate the photoisomerization of AAP in the pres-
ence of CB[8]/paraquat, 1H NMR and UV/Vis spectroscopy
measurements were performed (Figure 1a, b). Upon irradiation
with UV light (l=365 nm) the AAP shows the characteristic
changes in absorbance, which remained detectable when com-
plexed. A significant decrease and a 10 nm blue-shifted p–p*
absorbance band at l=328 nm concomitant with an increase
of and a 10 nm red-shifted n–p* band at l=430 nm confirmed
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the isomerization from the E to Z-isomer.[33] Moreover, Z to E
isomerization occurred upon irradiation with light of l=
520 nm, optically similar as observed in the case of non-com-
plexed AAP. Alternating the wavelength of irradiation between
l=520 nm and l=365 nm and following the maximum ab-
sorption of E-AAP at l=311 nm showed that AAP stably and
near-quantitatively switched between the two isomers (Fig-
ure 1a, inset). Re-isomerization was also characterized using
1H NMR (Figure 1b, full spectra are given in Figure S2c). The
methyl signals of E-AAP at d=2.55 nm and 2.41 nm reversibly
shifted to d=1.87 nm and 1.54 nm, due to formation of the Z-
isomer. Comparing the spectra of the E and Z-isomer in the ar-
omatic region around d=7.3 ppm, the signals of AAP sharp-
ened in the Z-state, indicating a disassembly of the 1:1:1 com-
plex (Figure 1b). These photo-isomerization properties of AAP,
that is, the separated excitation of the different states, are ad-
vantageous for molecular switches and photoresponsive mate-
rials and are an improvement when compared to the photo-
isomerization properties of azobenzenes. In addition, these re-
sults demonstrate that the photo-isomerization properties of
AAP are unaffected when complexed with CB[8] and paraquat.
Having established that a heteroternary, photoswitchable
complex forms in aqueous solution, we prepared self-assem-
bled monolayers (SAMs) that are modified with AAP conjugat-
ed ternary complexes (Figure 2). To introduce specific cell inter-
actions an integrin-specific binding peptide RGD was attached
to the AAP moiety (see Supporting Information for synthetic
details). In short, the carboxylic acid functionality of AAP was
modified with a tetraethylene glycol spacer bearing an azide
moiety. This AAP–azide derivative was suitable for strain-in-
duced, metal-free cycloaddition with a purified bicyclononyne-
RGD conjugate. The AAP–RGD peptide binds to CB[8] entities
through inclusion of the hydrophobic E-AAP moiety and to
cells via the integrin-binding peptide RGD.
SAMs were then prepared on gold sensors for surface plas-
mon resonance (SPR) and quartz crystal microbalance with dis-
sipation monitoring (QCM-D) with a background layer of anti-
fouling oligo(ethylene glycol) alkanethiols consisting of 1 or
0.1% maleimide groups (Mal-EG4, see Supporting Information
for details).[36] Thiolated methyl viologen (MV2+) was conjugat-
ed to the maleimide groups and acted as the first guest for
CB[8] to bind the macrocycle to the surface (for clarity only ex-
plicitly shown in QCM-D plot, Figure 3b). SPR spectroscopy
Figure 3. Concentration-dependent E-AAP assembly to a MV2+/CB[8] surface
studied by a) SPR and b) QCM-D. c) SPR response of flowing E- and Z-AAP
over MV2+/CB[8] SAMs. d) Change in SPR angle shift (square) and QCM-D
frequency (circle) vs. E-AAP concentration (1:1 Langmuir fit is shown).
and QCM-D measurements confirmed that the formed mono-
layer is efficiently binding CB[8] and AAP (Figure 3a,b and Fig-
ure S3). Contact angle measurements verified the assembly of
the surface (Figure S4). A concentration series of AAP over a
range of 0.1–1 mm was performed at
a flow rate of
100 mLminÀ1, in the presence of 50 mm CB[8], and followed by
SPR (Figure 3a) and QCM-D (Figure 3b). The binding constant
of the binary CB[8]/MV complex has been estimated K=1.3ꢀ
105 mÀ1.24 As we performed the binding studies at 50 mm of
CB[8], nearly 80% of CB[8] is expected bound to the MV2+-sur-
face. The binding constant of AAP to the binary complex of
CB[8]/MV2+ is estimated to be Ka =1.9ꢀ103 mÀ1 (Kd =518 mm,
SPR) and Ka =3.5ꢀ103 mÀ1 (Kd =283 mm, QCM-D) (Figure 3d).
These values are in good agreement with the binding con-
1
stants determined in solution using ITC and H NMR. Control
experiments confirmed negligible nonspecific interactions of
AAP–RGD and RGD with other surface components while
when the smaller host CB[7] was used, AAP showed no bind-
ing (Figure S3). Therefore, we conclude that the selective for-
mation of the AAP consisting heteroternary complex on the
surface occurred as envisioned.
Figure 2. Assembly of heteroternary complex on antifouling SAMs. After cell
adhesion, UV irradiation releases AAP–RGD and cells.
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Subsequently, binding of E- and Z-AAP isomers to the sur-
face were studied using SPR (Figure 3c). A significant change
in the SPR angle appears when flowing an irradiated (with l=
520 nm) solution of 1 mm E-AAP over a surface of CB[8]/MV2+
(Figure 3c). This change was absent when the solution was ir-
radiated with UV light (l=365 nm) indicating that the binding
of the Z-isomer to the CB[8]/MV2+ surface is negligible (Fig-
ure 3c). The binding of AAP to the surface was also photo-
modulated in flow (Figure S3c).
RGD presenting SAMs. This result indicates that efficient inte-
grin-mediated adhesion only occurred on CB[8]-mediated
AAP–RGD surfaces (inset Figure 4a). Similar results were visible
on SAMs using 0.1% maleimide groups (Table S1). We note
that specific integrin mediated cell adhesion occurred despite
the modest binding constant of AAP–RGD to the binary com-
plex. Presumably the weakly bound AAP–RGD molecules
remain close to the surface available for rebinding as we have
observed before in a cell force microscopy study.[36]
Based on the results that we formed CB[8]-mediated hetero-
ternary N-substituted arylazopyrazole complexes on the sur-
face, we then performed a set of experiments to demonstrate
that these responsive supramolecular layers can be used for
the photocontrolled cell adhesion. These experiments were
performed on monolayers presenting the AAP–RGD complexes
using the above-mentioned assembly strategy. The supra-
molecular substrates were seeded with mouse myoblast C2C12
cells for 1 h in cell culture medium. Control surfaces without
RGD, bearing just MV2+ or CB[8]/MV2+, showed limited cell ad-
hesion, whereas CB[8]-mediated AAP–RGD containing surfaces
showed increased number of adhered cells (an overview of
images is shown in Tables S1 and S2). In addition, adhered
cells on AAP–RGD surfaces were more elongated and covered
a larger area compared to all control surfaces of MV2+ and
CB[8]/MV2+ (Figure 4a). Staining in green of focal adhesion
protein vinculin revealed well-formed focal adhesions at the
ends of the red-stained actin only on the CB[8]-mediated AAP–
Having established that specific cell adhesion occurs on self-
assembled CB[8]/MV2+/AAP–RGD monolayers, we evaluated
cell detachment from the surfaces induced by photoswitching
of AAP–RGD. As control surfaces we used a maleimide–SAM to
which a cysteine-capped RGD peptide was coupled (cRGD
SAM).[36] After C2C12 cells were seeded for 1 h, parts of these
surfaces were irradiated for 10 min with UV light and dipped
once in PBS. Cells were imaged (Table S3) and counted before
and after irradiation on at least 10 spots on the entire surfaces.
Significantly less cells were counted on the supramolecular
AAP–RGD surface after partly irradiation while this was not the
case when partly irradiation was performed on the control sur-
faces (Figure 4b). This result leads to the conclusion that AAP–
RGD is switchable on surfaces. Shortening the irradiation time
to 1 min removed similar amounts of cells from the supra-
molecular surface while longer irradiation did not improve the
results.
In conclusion, we have reported a novel CB[8]-mediated
photoresponsive heteroternary complex consisting of N-substi-
tuted arylazopyrazole compounds. Complexation of the het-
eroternary complex on the surface has been studied using SPR
and QCM-D and yields binding constants that are similar to
1
the values we found in solution studies using ITC and H NMR.
We applied this new type of photoresponsive complexes for
fabricating bioactive surfaces. Cells adhered to the supramolec-
ularly immobilized arylazopyrazole-modified RGD peptide and
were removed by irradiation with UV light. This type of aryl-
azopyrazoles with improved photoswitching behavior when
compared to the commonly applied azobenzene derivatives
are of interest for constructing supramolecular assemblies in
solution and on surfaces. The design of dynamic reversible bio-
active surfaces opens possibilities for novel innovative schemes
including multi-responsive bioactivity.[25,37]
Acknowledgements
We thank Dr. J. Voskuhl for providing an intermediate com-
pound. This work was supported by the Netherlands Organiza-
tion for Scientific Research (NWO) (NWO-VIDI 723.012.106 to
P.J.).
Conflict of interest
Figure 4. a) Fluorescence microscopy image of fixed C2C12 cells seeded for
1 h on CB[8]/MV2+/AAP–RGD SAMs. Cells were stained for nucleus (blue),
actin (red) and vinculin (green), scale bar 100 mm. Inset is a magnified image
of same surface, scale bar 50 mm. b) Quantitative analysis of C2C12 cells
The authors declare no conflict of interest.
before (no UV) and after (UV) irradiation of l=365 nm of the CB[8]/MV2+
/
AAP–RGD and cRGD SAMs.
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Manuscript received: November 15, 2017
Version of record online: && &&, 0000
Chem. Eur. J. 2017, 23, 1 – 6
www.chemeurj.org
5
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Novel photo-responsive CB[8]-mediat-
ed complexes to direct cell adhesion:
Arylazopyrazoles form moderately
&
Host–Guest Systems
M. Wiemann, R. Niebuhr, A. Juan,
E. Cavatorta, B. J. Ravoo,* P. Jonkheijm*
stable, light-responsive ternary com-
plexes with CB[8] and methyl viologen.
The heteroternary complex can be used
for the photo-controlled release of cells.
&& – &&
Photo-responsive Bioactive Surfaces
Based on Cucurbit[8]uril-Mediated
Host–Guest Interactions of
Arylazopyrazoles
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Chem. Eur. J. 2017, 23, 1 – 6
www.chemeurj.org
6
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!