Facile synthesis of a peptidic Au(i)-metalloamphiphile and its self-assembly into luminescent micelles in water

Article abstract of DOI:10.1039/c4cc03868a

We report a short synthetic route for the preparation of a peptidic Au(i)-metalloamphiphile which, in buffered environments of physiological ionic strength, self-assembles into luminescent micellar nanostructures of 14 nm in diameter. This journal is

Full text of DOI:10.1039/c4cc03868a

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Facile synthesis of a peptidic Au(I)-metalloamphiphile
and its self-assembly into luminescent micelles in
water†
Cite this:DOI: 10.1039/c4cc03868a
Received 22nd May 2014,
Accepted 1st July 2014
Benedict Kemper,ab Yana R. Hristova,ab Sebastian Tacke,c Linda Stegemann,bd
Laura S. van Bezouwen,e Marc C. A. Stuart,e Jurgen Klingauf,c Cristian A. Strassertbd
¨
and Pol Besenius*ab
DOI: 10.1039/c4cc03868a
www.rsc.org/chemcomm
We report a short synthetic route for the preparation of a peptidic These parameters are crucial for developments as imaging labels
Au(I)-metalloamphiphile which, in buffered environments of physio- in biomedical applications.7 Molecular Au(I)-complexes are excep-
logical ionic strength, self-assembles into luminescent micellar tionally appealing due to their ability to form aurophilic interac-
nanostructures of 14 nm in diameter.
tions, some of the strongest metallophilic interactions known.8
Surprisingly however, the vast majority of luminescent molecular
Molecular self-assembly is a powerful bottom-up approach for Au(I)-complexes with short interatomic metalÁ Á Ámetal distances
the preparation of tailor-made nanomaterials, and offers distinct reported is either in the solid state or in solution based on intra-
advantages due to reduced synthetic efforts and its reversibility molecular aurophilic interactions in polynuclear complexes.8c–e,9
which ensures error correction.1 Particularly in water, controlling Rare exceptions are the report on metallophilic interactions
supramolecular interactions has become an attractive feature to in organogels by the Aida group10 and very recent work on
prepare nanosized architectures, for example rods, tubes, vesicles hydrogels by the Rodriguez and Lima labs.11 Both have shown that
or micelles.2 The preparation of supramolecular (bio)organic– in the gel state short AuÁÁÁAu distances in the supramolecular
inorganic hybrid materials promises to combine the best of two fibres lead to luminescent properties with large Stokes shifts and
worlds, the predictable self-assembly encoded in the (bio)organic long-lived emissions, which were assigned to electronic transitions
block and the functional properties embedded in the inorganic from triplet-excited states.10,11
moiety.3 We hereby report the preparation of peptidic Au(I)-
We aimed to establish a widely applicable synthetic route for
metalloamphiphiles. On the one hand, we were motivated to equipping amphiphilic peptides with molecular Au(I)-complexes
use amphiphilic peptides because they are known to be reliable under mild reaction conditions and designed a simple strategy
supramolecular synthons for the controlled self-assembly in (Scheme 1) using the well-known oligophenylalanine supra-
water.2a,f,g,4 On the other hand, water soluble molecular Au(I)- molecular synthon as reported by Gazit, Ulijn, Xu and Adams.2f,12
complexes have received a renewed focus in catalysis, lumines- First we prepared the propargyl amine functionalised Au(I)-
cence, and medicinal chemistry.5
trisulfonated-triphenylphosphane complex [H2NCH2CRCAuI-
The self-assembly of luminescent transition metal complexes (TPPTS)] 4 following a procedure reported by Laguna and
into nanostructured materials is an effective strategy for the coworkers.13 These types of Au(I)-complexes are known to be
manipulation of their photophysical properties, such as emission highly stable and soluble in water5 and were prepared in three
wavelength, excited-state lifetime and photochemical stability.3a,6
a
¨
¨
¨
Organic Chemistry Institute, Westfalische Wilhelms-Universitat Munster,
¨
Corrensstrasse 40, 48149 Munster, Germany. E-mail: p.besenius@uni-muenster.de;
Tel: +49 251 83 63928
b
¨
CeNTech, Heisenbergstrasse 11, 48149 Munster, Germany
c Department of Cellular Biophysics, Institute of Medical Physics and Biophysics,
¨
¨
¨
¨
Westfalische Wilhelms-Universitat Munster, 48149 Munster, Germany
d
¨
¨
¨
Physikalisches Institut, Westfalische Wilhelms-Universitat Munster,
¨
Heisenbergstrasse 11, 48149 Munster, Germany
e Department of Biophysical Chemistry, Groningen Biomolecular Sciences and
Biotechnology Institute, University of Groningen, Nijenborgh 7,
9747 AG Groningen, The Netherlands
Scheme 1 Synthesis of the Fmoc-protected, dipeptidic Au(I)-metallo-
amphiphile 1: (i) DMF, rt; 29% yield after purification with size-exclusion
chromatography.
† Electronic supplementary information (ESI) available: Fig. S1–S12, synthetic
procedures and full details about the instrumentation. See DOI: 10.1039/c4cc03868a
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steps starting from hydrogen tetrachloroaurate(III) with a
We obtained further evidence for the presence of nano-
88% yield (see ESI†). Complex 4 was then reacted with the structures in buffered water by monitoring the luminescence
N-hydroxysuccinimide activated fluorenylmethyloxycarbonyl after the disassembly of the aggregates. The addition of CH3CN
diphenylalanine (Fmoc-Phe-Phe-NHS) 7 to yield the peptidic is known to disrupt self-assembly in water, because it diminishes
Au(I)-metalloamphiphile Fmoc-Phe-Phe-HNCH2CRCAuI(TPPTS) the hydrophobic shielding of weak intermolecular interactions.17
1 with an acceptable overall yield of 26% over four steps At the same monomer concentration of 66.7 mM, switching from
(Scheme 1). The final reaction was performed under neutral the phosphate buffer to a 2 : 1 CH3CN :buffer mixture leads to the
conditions which avoided acidic degradation of the Au(I)- disappearance of the emission at lem = 520 nm, and appearance
complex or the deprotection of the Fmoc-protecting group in of a band at lem o 400 nm. The latter emission originates from
a basic environment. All intermediate compounds were fully the Fmoc unit in the molecularly dissolved amphiphile 1
characterised by high-resolution mass spectrometry, 1H and (see also Fig. S4, ESI†): in Fmoc-protected oligopeptides one
31P NMR spectroscopy.
generally observes quenching of the organic fluorophore
The novel Fmoc-Phe-Phe-Au(I)-metalloamphiphile 1 is highly (lem o 400 nm) and sometimes the presence of a weak excimer
soluble in aqueous buffers. For comparison, we prepared another band (lem = 400–450 nm) in the self-assembled state.18 Hence
new 1,3,5-triaza-7-phosphaadamantane (TPA) functionalised peptidic both features, the disappearance of the lower energy band at
Au(I)-complex, Ac-Phe-Phe-HNCH2CRCAuI(TPA) 12, via ligand lem = 520 nm and the simultaneous appearance of the higher
exchange of ClAuI(TPA) with Ac-Phe-Phe-HNCH2CRCH under basic energy band (lem o 400 nm) after disassembly, support the
conditions (see ESI†).13,14 However this complex was not water presence of nanostructures in buffer driven by intermolecular
soluble under neutral conditions and in millimolar concentrations, interactions and close contacts between the Fmoc groups as
most likely due to the lower hydrophilicity of the phosphane, since well as the Au(I) moieties.
the TPA ligand is only protonated in acidic water below pH 4.5.15 We
In the self-assembled state, the emission intensity of the band
therefore focussed on investigating the luminescent properties of 1 at lem = 520 nm is weak as one would expect for luminescent
(66.7 mM) in conventional phosphate buffer (10 mM, pH 7.4), under Au(I)-complexes in water.9f,11 Furthermore, we suspected that the
physiological ionic strength and at room temperature (Fig. 1A, blue highly charged trisulfonated phosphane ligands on the hydro-
curve): excitation at lexc = 340 nm leads to a broad, unstructured philic head group of the metalloamphiphile 1 would hamper the
luminescence emission band peaking at lem = 520 nm. This is self-assembly due to repulsive electrostatic interactions, and
indicative of the formation of nanostructures incorporating thereby weaken the luminescence. We have referred to systems
short AuÁ Á ÁAu distances: such a large Stokes shift is typically like these as experiencing frustrated growth,2d,17,19 whereby
observed for Au(I)-alkynyl-phosphane complexes with short intera- attractive supramolecular interactions within the hydrophobic
tomic metalÁÁÁmetal distances, reported for mononuclear Au(I)- block of an amphiphilic peptide are balanced out by repulsive
complexes in the crystalline state or for polynuclear complexes in interactions in the hydrophilic periphery. By increasing the ionic
solution, which emit from long-lived triplet states.8d,9e, f,11,16
strength from 0.1 M NaCl to 1 M NaCl, we observed that the
intensity of the emission band at lem = 520 nm increases by a
factor of three (Fig. 1A and B). The addition of NaCl simulta-
neously screens the repulsive Coulombic interactions and
increases the hydrophobic effect originating from the apolar
and aromatic moieties in the Fmoc-diphenylalanine block of
amphiphile 1 and, consequently, stabilises the self-assembly of
the highly charged monomer into nanostructures in water.17,19a,d
The increase in the emission can then also be observed by naked
eye after excitation with a 8 W standard laboratory fluorescent
lamp (Fig. 1C). We also determined the luminescence quantum
yield F = 0.01 (10 mM phosphate buffer, pH 7.4, 1 M NaCl,
293 K). Interestingly, between 0.1 M and 1 M NaCl the emission
wavelength and the excited state lifetimes are not affected, thus
indicating that rather the aggregation equilibrium but not the
morphology of the aggregates or the nature of the excited state
are influenced by the abovementioned Coulombic shielding
(vide infra).
The long excited state lifetimes indicate that the emission at
Fig. 1 (A) Emission spectra (lexc = 340 nm) of compound 1 (66.7 mM) in lem = 520 nm is originated from a triplet state (Fig. S3, ESI†).
10 mM phosphate buffer (pH 7.4) at 293 K at different ionic strengths: 0.1 M
Indeed, the exponential fits of the time-resolved luminescence
NaCl (blue)–1.0 M NaCl (red) and after addition of acetonitrile to the buffer
decay curves yield coincident values of 1.5 ms Æ 0.1 ms for both
(black);‡ (B) relative emission intensity at l = 520 nm as a function of the
ionic strength; (C) images of the luminescent Au(I)-metalloamphihile 1 in
the low and high ionic strength buffers. As recently highlighted,
it is often difficult to unambiguously attribute the emission of
Au(I)-alkynyl-phosphane complexes to excited states with a
10 mM phosphate buffer (pH 7.4) (lexc = 245 nm) with 0.1 M NaCl (left) and
1.0 M NaCl (right).
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defined character: the broad and structureless emission at observed in cryo-TEM images that the metalloamphiphile 1
lem = 520 nm can be attributed to excited states that are self-assembles in large sheet-like aggregates21 (Fig. S11, ESI†).
approximately described as a 3[s(Au–P) - p*(CRC)] metal-to- It is known that at very low ionic strength (I o 0.02 M)
ligand charge-transfer excitation, or as a metal-perturbed intra- clustering of counter ions can reduce the effective charge of
ligand excitation 3IL [p - p*(CRC)], while Au(I)ÁÁÁAu(I) interactions assemblies in solution22 which in our system is the likely cause
can contribute to the broad emission band as well.8d,9f,20 for the formation of densely packed 2D sheets. The presence of
Altogether the spectroscopic investigations reveal that the a closely packed secondary order is furthermore supported by
aqueous self-assembly of the peptidic Au(I)-metalloamphiphile photoluminescence spectroscopy: excitation at lexc = 340 nm
1 into nanostructures leads to luminescent properties with large leads to a shift of the emission at 0.1 M NaCl from lem = 520 nm
Stokes shifts and long-lived emissions, that can be assigned to to 550 nm at 0 M NaCl (Fig. S12, ESI†). This red shift in the
triplet-excited states.
luminescence emission band suggests a reduced intermolecular
Finally, we investigated the morphology of the self-assembled Au(I)Á Á ÁAu(I) distance9a,d,f from the curved micellar structure to a
nanostructures of metalloamphiphile 1 using cryogenic transmis- planar sheet-like morphology.
sion electron microscopy (cryo-TEM). We were able to identify
We like to point out that unlike the large variety of peptide
10–17 nm sized spherical objects that are most probably micellar amphiphiles reported in literature, metalloamphiphile 1 does
structures (Fig. S5, ESI†). Micelles are known to have a swollen not form rod-like materials. In intermediate to high ionic
hydrophobic core, which unfortunately reduces contrast in cryo- strength the triple charges in the phosphane ligand as head
TEM images and makes their morphological characterisation group, compared with the small, albeit very hydrophobic Fmoc-
difficult. We therefore switched to conventional TEM, using Phe-Phe-based peptide chain of the amphiphile, result in a
uranyl acetate as staining agent. The presence of spherical high packing parameter23 which drives the materials into
structures is clearly observed after depositing 2 mg mlÀ1 highly soluble micellar structures with sizes of 14 nm. Such
solutions of 1 on carbon film coated copper grids (Fig. 2), with small micelles are very rarely observed for peptidic supra-
an averaged size of 14 nm Æ 3.5 nm (Fig. S10B, ESI†). Assuming molecular materials in water.2a,f,g,4,12 This is therefore a unique
a length of 3.5 nm for the Au(I)-metalloamphiphile 1, these example where a new water soluble Au(I)-metalloamphiphile
structures are assigned to be micelles with a weakly packed self-assembles in buffered water of physiological ionic strength
peptide-based hydrophobic core that is swollen when dispersed to form luminescent and well-defined spherical nanoparticles.
in aqueous buffers, thus conforming cryo-TEM investigations.
In conclusion, we present a facile synthetic route for the
After comparing TEM images obtained from solutions of 1 in preparation of a new peptidic Au(I)-metalloamphiphile, using a
buffer and 0.1 M NaCl, with those after adding 1 M NaCl, we nucleophilic water soluble Au(I)-complex H2NCH2CRCAuI(TPPTS),
noticed that there is no apparent change in the diameter of the and a NHS activated peptide Fmoc-Phe-Phe-NHS. In buffered
micelles (Fig. S6–S10, ESI†). These results corroborate NaCl aqueous environments of medium to high ionic strength
titrations in luminescence spectroscopy: the increased ionic (0.1–1
M
NaCl), Fmoc-Phe-Phe-HNCH2CRCAuI(TPPTS) self-
strength does not lead to more densely packed micelles, since a assembles into luminescent micellar nanostructures with an aver-
reduced Au(I)Á Á ÁAu(I) distance in supramolecular structures is age diameter of 14 nm. In low ionic strength we have observed the
known to lead to a red shift in the luminescence emission formation of densely packed sheet-like morphologies. We assign the
band.9a,d, f The spectroscopic and microscopic investigations there- luminescent properties to electronic transitions from triplet-excited
fore strongly suggest that by increasing the ionic strength from states due to the large Stokes and excited state life times of 1.5 ms,
0.1 NaCl to 1 M NaCl in phosphate buffer, the formation of self- which are likely to be enhanced due to short Au(I)ÁÁÁAu(I) distances
assembled micelles in solution becomes more thermodynamically in the self-assembled nanostructures. The facile synthetic strategy is
favourable, without affecting the order in and size of the prepared fully compatible with peptide protecting group chemistry and allows
nanostructures (vide supra). Intriguingly, when the ionic strength is for the construction of more complex peptidic nanomaterials in
kept at a minimum (10 mM phosphate buffer, 0 M NaCl) we water, using our recently reported supramolecular synthons. By
adjusting the hydrophilicity and charged character of tailor-made
phosphane ligands bound to the metal complex, we aim to position
functional Au(I)-complexes into anisotropic nanostructures and
exploit applications in bioimaging, catalysis and therapeutics.
We thank U. Malkus and A. Ricker (Institute of Medical Physics
and Biophysics, Mu¨nster) for performing TEM and Dr M. Peterlechner
(Institute of Material Physics, Mu¨nster) for preliminary TEM experi-
ments. This work was supported by the DFG (SFB 858, Project B16)
[B.K., Y.R.H., P.B.]; P.B. acknowledges the ‘Fonds der Chemischen
¨
Industrie’ for a Liebig Fellowship, the ‘Nordrhein-Westfalische
Akademie der Wissenschaften und der Ku¨nste’ for a Fellowship via
the ‘Junges Kolleg’ and COST Action CM1005 (Supramolecular
Chemistry in Water). L.S. and C.A.S gratefully acknowledge the DFG
for financial support, (SFB-TRR 61, project C07).
Fig. 2 TEM image of the Au(I)-metalloamphiphile 1, deposited on carbon
coated grids from a 2 mg mlÀ1 solution in 10 mM Tris buffer, pH 7.5 and 1 M
NaCl (negative staining was performed with 2% w/v uranyl acetate).
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¨
8 (a) H. Schmidbaur, Gold Bull., 2000, 33, 3; (b) P. Pyykko, Angew.
Notes and references
‡ The luminescence emission spectra were recorded using a 405 nm
filter to filter out the fluorescence from the Fmoc moieties that is
generally observed in self-assembled Fmoc-dipeptides.18
Chem., Int. Ed., 2002, 41, 3573; (c) E. J. Fernandez, A. Laguna and
J. M. Lopez-de-Luzuriaga, Dalton Trans., 2007, 1969; (d) V. W.-W.
Yam and E. C.-C. Cheng, Chem. Soc. Rev., 2008, 37, 1806;
(e) H. Schmidbaur and A. Schier, Chem. Soc. Rev., 2012, 41, 370.
9 (a) M. J. Irwin, J. J. Vittal and R. J. Puddephatt, Organometallics, 1997,
1 (a) G. M. Whitesides and B. Grzybowski, Science, 2002, 295, 2418;
(b) J. M. Lehn, Science, 2002, 295, 2400; (c) K. S. Chichak, S. J.
Cantrill, A. R. Pease, S.-H. Chiu, G. W. V. Cave, J. L. Atwood and
J. F. Stoddart, Science, 2004, 304, 1304; (d) R. J. Wojtecki, M. A.
Meador and S. J. Rowan, Nat. Mater., 2010, 10, 14; (e) T. Aida,
E. W. Meijer and S. I. Stupp, Science, 2012, 335, 813.
´
´
16, 3541; (b) J. Barbera, A. Elduque, R. Gimenez, F. J. Lahoz,
´
J. A. Lopez, L. A. Oro and J. L. Serrano, Inorg. Chem., 1998,
37, 2960; (c) M. Enomoto, A. Kishimura and T. Aida, J. Am. Chem.
Soc., 2001, 123, 5608; (d) S.-K. Yip, E. C.-C. Cheng, L.-H. Yuan, N. Zhu
and V. W.-W. Yam, Angew. Chem., Int. Ed., 2004, 43, 4954;
(e) R. J. Puddephatt, Chem. Soc. Rev., 2008, 37, 2012; ( f ) J. C. Lima
and L. Rodriguez, Chem. Soc. Rev., 2011, 40, 5442.
2 (a) M. R. Ghadiri, J. R. Granja and L. K. Buehler, Nature, 1994,
369, 301; (b) L. Zhang and A. Eisenberg, Science, 1995, 268, 1728;
10 A. Kishimura, T. Yamashita and T. Aida, J. Am. Chem. Soc., 2004,
127, 179.
11 (a) R. Gavara, J. Llorca, J. C. Lima and L. Rodriguez, Chem. Commun.,
2013, 49, 72; (b) E. Aguilo, R. Gavara, J. C. Lima, J. Llorca and
L. Rodriguez, J. Mater. Chem. C, 2013, 1, 5538.
¨
(c) S. Forster and M. Antonietti, Adv. Mater., 1998, 10, 195; (d) K. J. C.
van Bommel, C. van der Pol, I. Muizebelt, A. Friggeri, A. Heeres,
A. Meetsma, B. L. Feringa and J. van Esch, Angew. Chem., Int. Ed.,
`
2004, 43, 1663; (e) D. M. Vriezema, M. C. Aragones, J. A. A. W.
Elemans, J. J. L. M. Cornelissen, A. E. Rowan and R. J. M. Nolte,
Chem. Rev., 2005, 105, 1445; ( f ) R. V. Ulijn and A. M. Smith, Chem.
Soc. Rev., 2008, 37, 664; (g) H. Cui, M. J. Webber and S. I. Stupp,
Biopolymers, 2010, 94, 1; (h) R. Chapman, M. Danial, M. L. Koh,
K. A. Jolliffe and S. Perrier, Chem. Soc. Rev., 2012, 41, 6023; (i) H. Frisch,
J. P. Unsleber, D. Lu¨deker, M. Peterlechner, G. Brunklaus, M. Waller
and P. Besenius, Angew. Chem., Int. Ed., 2013, 52, 10097.
3 (a) C. A. Strassert, C.-H. Chien, M. D. Galvez Lopez, D. Kourkoulos,
D. Hertel, K. Meerholz and L. De Cola, Angew. Chem., Int. Ed., 2011,
50, 946; (b) M. J. Mayoral Munoz and G. Fernandez, Chem. Sci., 2012,
3, 1395; (c) S. Sengupta, D. Ebeling, S. Patwardhan, X. Zhang, H. von
12 (a) M. Reches and E. Gazit, Science, 2003, 300, 625; (b) V. Jayawarna,
M. Ali, T. A. Jowitt, A. F. Miller, A. Saiani, J. E. Gough and R. V. Ulijn,
Adv. Mater., 2006, 18, 611; (c) Z. Yang, G. Liang, L. Wang and B. Xu,
J. Am. Chem. Soc., 2006, 128, 3038; (d) D. J. Adams, M. F. Butler,
W. J. Frith, M. Kirkland, L. Mullen and P. Sanderson, Soft Matter,
2009, 5, 1856.
13 S. Sanz, L. A. Jones, F. Mohr and M. Laguna, Organometallics, 2007,
26, 952.
14 E. Vergara, E. Cerrada, A. Casini, O. Zava, M. Laguna and P. J. Dyson,
Organometallics, 2010, 29, 2596.
15 Z. Assefa, B. G. McBurnett, R. J. Staples, J. P. Fackler Jr., B. Assmann,
K. Angermaier and H. Schmidbaur, Inorg. Chem., 1995, 34, 75.
16 N. J. Long and C. K. Williams, Angew. Chem., Int. Ed., 2003, 42, 2586.
17 (a) P. Besenius, G. Portale, P. H. H. Bomans, H. M. Janssen,
A. R. A. Palmans and E. W. Meijer, Proc. Natl. Acad. Sci. U. S. A.,
¨
Berlepsch, C. Bottcher, V. Stepanenko, S. Uemura, C. Hentschel,
H. Fuchs, F. C. Grozema, L. D. A. Siebbeles, A. R. Holzwarth, L. Chi
and F. Wu¨rthner, Angew. Chem., Int. Ed., 2012, 51, 6378;
(d) R. Charvet, Y. Yamamoto, T. Sasaki, J. Kim, K. Kato, M. Takata,
A. Saeki, S. Seki and T. Aida, J. Am. Chem. Soc., 2012, 134, 2524;
(e) M. R. Reithofer, K.-H. Chan, A. Lakshmanan, D. H. Lam,
A. Mishra, B. Gopalan, M. Joshi, S. Wang and C. A. E. Hauser, Chem.
Sci., 2014, 5, 625.
4 (a) J. D. Hartgerink, E. Beniash and S. I. Stupp, Science, 2001, 294,
1684; (b) L. C. Palmer, Y. S. Velichko, M. Olvera de la Cruz and
S. I. Stupp, Philos. Trans. R. Soc., A, 2007, 365, 1417; (c) I. W. Hamley,
Angew. Chem., Int. Ed., 2007, 46, 8128; (d) A. M. Smith, R. J. Williams,
C. Tang, P. Coppo, R. F. Collins, M. L. Turner, A. Saiani and
R. V. Ulijn, Adv. Mater., 2008, 20, 37; (e) X. Zhao, F. Pan, H. Xu,
M. Yaseen, H. Shan, C. A. E. Hauser, S. Zhang and J. R. Lu,
Chem. Soc. Rev., 2010, 39, 3480.
¨
2010, 107, 17888; (b) M. von Groning, I. de Feijter, M. C. A. Stuart,
I. K. Voets and P. Besenius, J. Mater. Chem. B, 2013, 1, 2008.
18 (a) S. Fleming, S. Debnath, P. W. J. M. Frederix, N. T. Hunt and
R. V. Ulijn, Biomacromolecules, 2014, 15, 1171–1184; (b) Y. M. Abul-
Haija, S. Roy, P. W. J. M. Frederix, N. Javid, V. Jayawarna and
R. V. Ulijn, Small, 2014, 10, 973.
19 (a) H. Dong, S. E. Paramonov, L. Aulisa, E. L. Bakota and J. D.
Hartgerink, J. Am. Chem. Soc., 2007, 129, 12468; (b) F. Versluis,
H. R. Marsden and A. Kros, Chem. Soc. Rev., 2010, 39, 3434;
(c) A. Ghosh, M. Haverick, K. Stump, X. Yang, M. F. Tweedle and
J. E. Goldberger, J. Am. Chem. Soc., 2012, 134, 3647; (d) C. Schaefer,
I. K. Voets, A. R. A. Palmans, E. W. Meijer, P. van der Schoot and
P. Besenius, ACS Macro Lett., 2012, 1, 830.
5 Y. R. Hristova, B. Kemper and P. Besenius, Tetrahedron, 2013, 69, 10525.
6 (a) K. Wang, M.-a. Haga, H. Monjushiro, M. Akiba and Y. Sasaki,
Inorg. Chem., 2000, 39, 4022; (b) P. C. Griffiths, I. A. Fallis,
T. Chuenpratoom and R. Watanesk, Adv. Colloid Interface Sci.,
20 (a) D. Li, X. Hong, C.-M. Che, W.-C. Lo and S.-M. Peng, J. Chem. Soc.,
Dalton Trans., 1993, 2929; (b) V. W.-W. Yam, S. W.-K. Choi and
K.-K. Cheung, Organometallics, 1996, 15, 1734; (c) V. W.-W. Yam,
K.-L. Cheung, S.-K. Yip and K.-K. Cheung, J. Organomet. Chem., 2003,
´
´
2006, 122, 107; (c) A. Guerrero-Martınez, Y. Vida, D. Domınguez-
´
Gutierrez, R. Q. Albuquerque and L. De Cola, Inorg. Chem., 2008,
´
´
681, 196; (d) M. Ferrer, A. Gutierrez, L. Rodrıguez, O. Rossell,
J. C. Lima, M. Font-Bardia and X. Solans, Eur. J. Inorg. Chem.,
2008, 2899; (e) X. He, E. C.-C. Cheng, N. Zhu and V. W.-W. Yam,
Chem. Commun., 2009, 4016.
47, 9131; (d) C. A. Strassert, M. Mauro and L. De Cola, in Advances in
Inorganic Chemistry, ed. R. van Eldik and G. Stochel, Academic Press,
2011, vol. 63, p. 47; (e) M. Mauro, G. De Paoli, M. Otter, D. Donghi,
G. D’Alfonso and L. De Cola, Dalton Trans., 2011, 40, 12106; 21 J. T. van Herpt, M. C. A. Stuart, W. R. Browne and B. L. Feringa,
( f ) Z. Luo, X. Yuan, Y. Yu, Q. Zhang, D. T. Leong, J. Y. Lee and
J. Xie, J. Am. Chem. Soc., 2012, 134, 16662.
7 (a) V. Fernandez-Moreira, F. L. Thorp-Greenwood and M. P. Coogan,
Chem. – Eur. J., 2014, 20, 3077.
22 (a) T. Odijk, Biophys. Chem., 1991, 41, 23; (b) P. van der Schoot,
Langmuir, 1997, 13, 4926.
Chem. Commun., 2010, 46, 186; (b) M. Mauro, A. Aliprandi, D. Septiadi, 23 J. N. Israelachvili, D. J. Mitchell and B. W. Ninha, J. Chem. Soc.,
N. S. Kehr and L. De Cola, Chem. Soc. Rev., 2014, 43, 4144.
Faraday Trans. 2, 1976, 72, 1525.
Chem. Commun.
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