Selective recognition of cysteine in its free and protein-bound states by the Zn2+ complex of a triazole-based calix[4]arene conjugate

Article abstract of DOI:10.1002/chem.201102791

Zinc-ed in: A highly fluorescent zinc complex of triazole-linked calix[4]arene (see scheme), [ZnL], for the selective recognition of Cys and other protein-bound/free thiols in the blood-serum milieu by mimicking the Zn²⁺-induced redox switch has been developed. Copyright

Full text of DOI:10.1002/chem.201102791

COMMUNICATION
DOI: 10.1002/chem.201102791
Selective Recognition of Cysteine in Its Free and Protein-Bound States by the
Zn2+ Complex of a Triazole-Based Calix[4]arene Conjugate
Rakesh Kumar Pathak, Khatija Tabbasum, Vijaya Kumar Hinge, and
Chebrolu Pulla Rao*[a]
Cysteine (Cys) is an integral part of a number of peptides
and proteins. However, when this amino acid is bound to a
metal centre as in metalloproteins, it contributes immensely
to the life processes, as also indicated by zinc enzymes.[1] In
structural motifs, such as those in mammalian transcription
factors (MTF), Cys regulates the transcription process by
coordinating to Zn2+.[2a] In the proteins, such as metallothio-
nein[2b] and albumin,[2c] Cys controls the toxic levels of metal
ions and oxidative stress in cells and in body fluids.[2d] Oxi-
dative stress[3a] induces thiol oxidation followed by release
of Zn2+ into the cytosol[3b] and the elevated concentrations
of Zn2+ is suspected to be responsible for severe infections
and diseases.[3c] Other biological thiols, such as, homocys-
teine and glutathione, also play crucial roles as powerful an-
tioxidants and detoxifiers.[3d] The deficiency of Cys results in
decreased levels of serum albumins. In order to understand
the complex relation between oxidative stress and protein-
bound/free thiols and Zn2+ content in the cytosol,[3e] it is
necessary to detect and quantify such biological entities. Al-
though there are a number of reports of Cys detection
based on different systems,[4] to our knowledge, molecular
systems that can decipher the biological relation between
Zn2+ content and protein-bound/free thiols in the cytosol
through the recognition of cysteinyl moieties, are rather
scarce. Supramolecular scaffolds based on calix[4]arene
have emerged as a recognition tool for biologically and envi-
ronmentally related ions and molecules upon appropriate
derivatisation.[5] Therefore, the development of molecular
systems based on such scaffolds for the recognition of Cys in
the presence of other biological thiols, including those pres-
ent in blood serum, is still a challenging task. Here, we
report the synthesis and characterisation of a triazole-linked
zinc complex of calix[4]arene and show its sensitive and se-
lective recognition of Cys and other reduced forms of blood
serum proteins possessing thiol moieties.
ised (Figures S01–S05 in the Supporting Information). Its
zinc complex was isolated in quantitative yield by treating L
with zinc acetate in methanol. The isolated [ZnL] complex
was characterised by elemental analysis, NMR spectroscopy
and high resolution ESI-MS (Scheme 1; Figures S04 and S06
in the Supporting Information). In order to understand the
binding behaviour of L with Zn2+, complex [ZnL] was opti-
mised (Figure 1a) by DFT by using the Gaussian G03 pack-
age (Figure S07 in the Supporting Information).
Figure 1. a) B3LYP/6-31G
ACHTUNGTREN(NUNG d,p) optimised structure of [ZnL]; b) coordina-
tion sphere around the zinc ion; bond lengths (ꢀ) and bond angles (8) in
ꢀ
ꢀ
ꢀ
the coordination sphere are: Zn N1=2.35, Zn N2=2.07, Zn N3=2.05,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Zn O1=2.00, Zn O2=1.98, O1 Zn O2=87.21, O1 Zn N1=141.60,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
O1 Zn N2=88.29, O1 Zn N3=112.42, O2 Zn N1=86.00, O2 Zn
N2=161.00, O2 Zn N3=93.42, N1 Zn N2=86.09, N1 Zn N3=
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
105.68, N2 Zn N3=105.34.
In the optimised structure of [ZnL], Zn2+ was found to be
bonded to two salicylaldimine oxygen atoms and two imine
nitrogen atoms and one pyridyl nitrogen while, the second
pyridyl nitrogen group was disposed at a distance of 5.65 ꢀ
(Figure 1b); this results in a highly distorted square pyrami-
dal geometry.
Fluorescence titrations were carried out by exciting the
corresponding solutions at 390 nm and measuring the emis-
sion spectra at 400–600 nm in a 1:2 (v/v) water/methanol
mixture at pH 7.4 for an effective HEPES buffer concentra-
tion of 10 mm. The [ZnL] complex exhibited switch on fluo-
rescence emission (lem =454 nm) unlike its precursor, L,
owing to the reversal of photoelectron transfer (PET) upon
utilisation of the lone pair of electrons on the imine moiety
by the Zn2+.
The calix[4]arene conjugate, L, was synthesised through
several steps (Scheme 1) and the compound was character-
[a] R. Kumar Pathak, K. Tabbasum, V. Kumar Hinge, Prof. C. Pulla Rao
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai-400076 (India)
Fax : (+91)22-2576-7152
E-mail: cprao@iitb.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102791.
Chem. Eur. J. 2011, 17, 13999 – 14003
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13999

Scheme 1. Synthesis of L and its zinc complex, [ZnL]. a) L2, CuSO4·5H2O and sodium ascorbate in CH2Cl2/H2O (1:1), room temperature, 12 h; b) 2-(pyr-
idin-2-yl)ethanamine, CH3OH, room temperature, 4 h; c) Zn(OAc)2·2H2O, CH3OH; R=t-butyl.
AHCTUNGTRENNUNG
Upon the titration of [ZnL] with Cys, the fluorescence of
this complex gradually decreased (Figure 2a). This can be
either attributed to the involvement of Cys in the complex
formation at the Zn2+ centre or to the removal of Zn2+ from
the complex, or both. The titrations were also carried out in
1:2 (v/v) HEPES/ethanol solution at pH 7.4 and similar
changes in the fluorescence emission were found (Figure S08
in the Supporting Information). In order to compare the
sensitivity of [ZnL] towards Cys with the other biologically
ꢀ
relevant molecules containing the SH functionality, fluores-
cence titrations were carried out between [ZnL] and mer-
captopropionic acid (MPA), cystamine (Cyst), homocysteine
(Hcy) and reduced glutathione (GSH; Figure S8 in the Sup-
porting Information). While no fluorescence quenching was
observed at low mole ratio in the case of MPA bearing one
ꢀ
ꢀ
SH and one COOH groups, considerable quenching was
noticed at higher mole ratios. Cyst, Hcy and GSH showed
significant changes as compared to MPA but to a lesser
extent as compare to that of Cys (Figure 2b, Figure S09 in
the Supporting Information). However, all the other amino
acids, including Asp and Glu possessing the side chain
ꢀ
COOH function, did not show any change in fluorescence
intensity, except for His (Figure 3a); this suggests that these
neither interact with Zn2+ nor remove the same from the
complex.
In order to determine the role of the His side chain in the
fluorescence quenching, different control experiments were
carried out by taking simple imidazole, His-OMe, Fmoc-His,
and the tripeptide, Ac-Gly-His-Gly-OMe[6] (see the Support-
ing Information). His-OMe showed little change in the fluo-
rescence of [ZnL] owing to the presence of free amine in
conjunction with the imidazole side chain, which is capable
of forming a favourable 6-membered ring. This observation
was further strengthened by the fact that in the absence of
the free amine end, in the form of Fmoc-His, fluorescence
was not quenched in spite of the presence of an imidazole
moiety. Imidazole alone did not show any fluorescence
quenching. These results further support the observations
for the tripeptide protected at both ends, Ac-Gly-His-Gly-
OMe, possessing only the side chain imidazole, which
showed no change in the fluorescence of [ZnL]. All this
clearly supports that the side chain of His alone, as present
in proteins, does not have much influence on the fluores-
cence of [ZnL], since the amine and carboxylic functions of
the free His are present in proteins (even in this tripeptide)
Figure 2. a) Fluorescence spectra obtained during the titration of [ZnL]
with Cys in aqueous methanolic (1:2 v/v) HEPES buffer (pH 7.4); inset:
the relative fluorescence intensity (I/I0) as a function of [Cys]/
ACHTUNGTRENN[UNG ZnL] mole
ratio, [ZnL]=10 mm. b) Histogram showing the ratio of fluorescence re-
ꢀ
sponse of [ZnL] with biologically relevant SH containing molecules.
14000
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 13999 – 14003

Selective Recognition of Cysteine
COMMUNICATION
on Zn2+ complex of calix[4]arene derivative that can detect
Cys even below 1 ppm.
In order to provide further support for the removal of
Zn2+ from [ZnL] by Cys, ESI MS titrations were carried out.
The spectra for the titration of Cys with [ZnL] showed
peaks at 1463 (L+Zn)+, 1400.29 (L+H)+ and 467.56
(3Cysꢀ +Zn+H2O+Na+H)+ (Figure S15 in the Support-
ing Information), in which the isotopic peak pattern pro-
vides the signature for the presence of zinc in the latter.
Similar titrations were also carried out for GSH with [ZnL]
and peaks were found at 1522 (L+Zn+Na+K)+, 1399.37
(L)+ and 674.83 for glutathione complex of Zn2+ (2GSH+
Zn)+ (Figure S16 in the Supporting Information) as the
latter exhibited the isotopic pattern for zinc. All the studies
clearly demonstrate the displacement of Zn2+ by the Cys/
GSH moiety from [ZnL], that is, [ZnL]+Cys/GSH!L+
[(Cys)/ACHTNUTRGNE(NUG GSH)Zn]; this releases the free ligand while forming
1
the zinc complex of Cys/GSH. H NMR titrations were car-
ried out to support the removal of Zn2+ from [ZnL] by
GSH. During the course of the titration, as the concentra-
tion of the GSH increased, the proton signals of bridge
ꢀ
ꢀ
ꢀ
CH2, OCH2, NCH2 and pyridyl were shifted towards
those that appear otherwise with L only (Figure S17 in the
Supporting Information). Thus, it was found that GSH dis-
places Zn2+ from the complex to liberate L. Similar studies
could not be carried out with Cys because of its poor solu-
bility in [D6]DMSO.
Figure 3. a) Histogram showing the ratio of fluorescence response of
[ZnL] in the presence of different amino acids. Inset: fluorescence colour
change in [ZnL] with and without amino acids; a control, L, was includ-
ed. b) Absorption spectra obtained during the titration [ZnL] with differ-
ent amino acids (AAs), [ZnL]=10 mm.
As [ZnL] has been found to recognise Cys among the 20
naturally occurring amino acids by displacing zinc from the
complex, its utility was extended to study the free sulfydryl
groups in proteins. For this purpose cysteine-containing pro-
teins, such as HSA and BSA, were used as model systems in
their native (nHSA/nBSA) and reduced (rHSA/rBSA)
states. The reduced forms (see the Supporting Information)
were prepared by treating the nHSA/nBSA with DTT
(10 mm) in the presence of SDS (0.1%) at 708C. The re-
duced protein samples (rHSA and rBSA) were dialysed by
using Tris buffer (5 mm, pH 7.4) to remove excess DTT. The
absence of DTT in the reduced protein samples after dialy-
sis was checked by cobalt perchlorate colour assay (Fig-
ure S18 in the Supporting Information).[7] In order to con-
firm the reduction of disulfide bonds in proteins with DTT,
the sulfhydryl contents were estimated after dialysis of these
reduced proteins with DTNB (Ellmanꢂs reagent) and found
as amide moieties (Figure S10 in the Supporting Informa-
tion). This behaviour is very different from that observed
with thiols versus Cys, in which even simple thiols at partic-
ular concentrations exhibited fluorescence quenching (Fig-
ure 2b).
Upon the titration of [ZnL] with Cys, the absorption band
of [ZnL] at about 378 nm decreased and two new bands,
namely, at approximately 325 and 425 nm, which correspond
to an increase in the absorbance of free receptor L (Fig-
ure S11 in the Supporting Information). All the other amino
acids showed no change in the band at approximately
378 nm. Further, these did not show the presence of the new
bands, which otherwise appeared in the presence of Cys
(Figure 3b). Only His (Figure 3b, and Figure S12 in the Sup-
porting Information) showed changes similar to those ob-
served for Cys (Figure S11 in the Supporting Information).
Therefore, the changes observed during the titration of Cys
can be attributed to the displacement of Zn2+ from [ZnL]
possibly by liberation of free L.
In order to check the selectivity of [ZnL] towards Cys,
competitive amino acid titrations were carried out in the
same medium with 19 other naturally occurring amino acids
along with Hcy and GSH. No significant fluorescence
change was found in the presence of these, except with His,
as expected (Figure S13 in the Supporting Information). The
ꢀ
to have about 32(ꢁ1) sulfhydryl ( SH) moieties present
(Figure S19 the in Supporting Information).[7c]
The protein reduction was again supported by measuring
the CD spectra (Figure S20 in the Supporting Information)
and comparing the data with those reported in the litera-
ture.[7] In the native state, the titrations resulted in very little
fluorescence quenching of [ZnL] at lem =454 nm, since only
one Cys residue is in the reduced form and all the others
are present as oxidised disulfides (Figure 4b). On the other
hand, a significant fluorescence intensity change of [ZnL] at
lem =454 nm was observed with the reduced protein samples
[ZnL] exhibited a minimum detection limit of 392
for Cys (Figure S14 in Supporting Information). To the best
of our knowledge, this is the first known Cys sensor based
G
ꢀ
owing to the availability of more SH groups (Figure S21 in
the Supporting Information); this can be used for the recog-
Chem. Eur. J. 2011, 17, 13999 – 14003
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14001

C. Pulla Rao et al.
Figure 4. a) Fluorescence of [ZnL] in the presence of Cys and different
proteins under UV light. b) Histogram showing the ratio of fluorescence
responses of [ZnL] with native and reduced forms of proteins including
Cys. c) Histogram showing the ratio of fluorescence responses of [ZnL]
with the samples of deproteinised blood serum: (i) TCEP alone; (ii) sam-
ples without TCEP treatment; (iii) TCEP treated samples, [ZnL]=10 mm.
Figure 5. Cartoon showing the fluorescence changes observed for [ZnL]
with native and reduced proteins.
ꢀ
nition of SH groups present in blood serum proteins (Fig-
fydryl contents, the blood serum was reduced[8] (see the Sup-
porting Information) and used as reported in the litera-
ure 4b). Titrations were also carried out with proteins con-
taining no sulfydryl groups, such as peanut agglutinin (PNA)
and soybean agglutinin (SBA), under similar DTT condi-
tions (Figure S22 in the Supporting Information). No change
in the fluorescence of [ZnL] was found; this suggests that
the absence of Cys is the cause for the non-replacement of
Zn2+ (Figure 4b). The studies were further continued by
monitoring the protein fluorescence. Protein fluorescence in
the presence of [ZnL] was completely quenched for the
band observed at lem =337 nm, while the fluorescence for
the lem =310 nm band did not show any change. This result
was the same for both nHSA/rHSA and nBSA/rBSA (Fig-
ure S23 in the Supporting Information), and was indeed ex-
pected because the accessibility of the Trp residue towards
[ZnL] is the same in both the native and reduced proteins.
However, in the presence of [ZnL], larger changes were ob-
served in the ellipticity of nHSA/nBSA, while the changes
were marginal with rHSA/rBSA (Figure S24 in the Support-
ing Information). A control sample, in the absence DTT,
clearly showed no such changes in the CD spectra.
AHCTUNGTRENNUNG
ture.[4a,e] Fluorescence intensity of [ZnL] was found to de-
crease as a function of the added reduced and deprotenised
serum sample (Figure 4c), whereas, the reducing agent
TCEP showed only a minimal change (Figure S26 in the
Supporting Information).
In summary, the present study demonstrates the synthesis
and characterisation of triazole-linked calix[4]arene conju-
gate, L, and its zinc complex, [ZnL]. The [ZnL] shows
switch on fluorescence, unlike its precursor L, owing to the
reversal of the photoelectron transfer upon utilisation of the
lone pair of electrons on imine moiety by the Zn2+. The
structure of [ZnL] was optimised by DFT calculations and
found to have a distorted square pyramidal geometry
around zinc. The highly fluorescent zinc complex [ZnL],
was found to be selective and sensitive for Cys among the
other amino acids, except His, and exhibited a minimum de-
tection limit of 392
methanol solution and 846 ppb in aqueous methanol solu-
G
tion containing blood serum. The utility of [ZnL] was shown
ꢀ ꢀ ꢀ
In solution, [ZnL] is highly fluorescent and the L is non-
fluorescent under UV light. A non-fluorescent situation was
observed when [ZnL] was treated with Cys or with reduced
by its ability to differentiate the oxidised (disulfide S S )
ꢀ
versus reduced (sulfhydryl SH) forms of Cys-containing
proteins through the recognition of their cysteinyl moiety.
The control b-sheet proteins, PNA and SBA, which are
devoid of cysteinyl moieties, did not show any change in the
fluorescence of [ZnL] under similar experimental condi-
tions. The [ZnL] was further shown to detect free biological
thiol contents in reduced blood serum samples. Therefore,
[ZnL] can be used as an in vitro receptor for the recognition
of cysteinyl moieties and the reduced form of proteins by
mimicking Zn2+ sensing elements. This can perhaps encour-
age chemists to use such zinc sensing fluorescent molecules
to elucidate the role of Zn2+ during the detoxification of the
reactive oxygen species (ROS) in biology.
ꢀ
proteins possessing SH moieties (Figure 4a); this suggests
the release of L from the complex. However, the non-Cys
proteins, namely, PNA and SBA, were fluorescent; this sug-
gests that Zn2+ was not removed from the [ZnL] and the
complex was intact (Figure 4a). The fluorescence changes
observed with reduced versus native proteins in the pres-
ence of [ZnL] are demonstrated in Figure 5.
Biological applicability of [ZnL] to sense Cys and other
sulfhydryl contents was addressed by carrying out the fluo-
rescence titrations by using blood serum. The minimum de-
tection limit of Cys was found to be 846 ppb in native blood
serum (Figure S25 in the Supporting Information). In order
to determine whether [ZnL] can recognise serum-free sul-
14002
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 13999 – 14003

Selective Recognition of Cysteine
COMMUNICATION
Z. Cao, Y. Feng, L. Long, Chem. Eur. J. 2009, 15, 5096; f) X. Chen, Y.
Zhou, X. Peng, Chem. Soc. Rev. 2010, 39, 2120; g) F. Pu, Z. Huang, J.
Ren, X. Qu, Anal. Chem. 2010, 82, 8211; h) J. F. Callan, A. P. de Sil-
vaa, D. C. Magria, Tetrahedron 2005, 61, 8551; i) Y. Fu, H. Li, W. Hu,
D. Zhu, Chem. Commun. 2005, 3189.
Experimental Section
Experimental details, synthesis, characterisation, absorption, fluores-
cence, mass spectra, and NMR spectroscopy data of all compounds is
given in Supporting Information.
[5] a) J. S. Kim, D. T. Quang, Chem. Rev. 2007, 107, 3780; b) F. Perret,
A. N. Lazar, A. W. Coleman, Chem. Commun. 2006, 2425; c) L. Muti-
hac, J. H. Lee, J. S. Kim, J. Vicens, Chem. Soc. Rev. 2011, 40, 2777;
d) F. Sansone, L. Baldini, A. Casnati, R. Ungaro, New J. Chem. 2010,
34, 2715; e) W. Zielenkiewicz, A. Marcinowicaz, S. Cherenok, V. I.
Kalchenko, J. Poznanski, Supramol. Chem. 2006, 18, 167; f) F. Perret,
N. M. Desrosiers, A. W. Coleman, Supramol. Chem. 2002, 2, 533;
g) J. P. Chinta, A. Acharya, A. Kumar, C. P. Rao, J. Phys. Chem. B
2009, 113, 12075; h) R. Joseph, B. Ramanujam, A. Acharya, C. P.
Rao, J. Org. Chem. 2009, 74, 8181; i) R. Joseph, J. P. Chinta, C. P.
Rao, J. Org. Chem. 2010, 75, 3387; j) A. Acharya, B. Ramanujam,
J. P. Chinta, C. P. Rao, J. Org. Chem. 2011, 76, 127; k) R. K. Pathak,
A. G. Dikundwar, T. N. Guru Row, C. P. Rao, Chem. Commun. 2010,
46, 4345; l) M. Kumar, R. Kumar, V. Bhalla, Org. Lett. 2011, 13, 366;
m) R. K. Pathak, S. M. Ibrahim, C. P. Rao, Tetrahedron Lett. 2009, 50,
2730; n) X. He, W. W. Yam, Org. Lett. 2011, 13, 2172; o) R. Joseph,
C. P. Rao, Chem. Rev. 2011, 111, 4658; p) R. Lalor, H. Balillie-John-
son, C. Redshaw, S. E. Matthews, A. Mueller, J. Am. Chem. Soc.
2008, 130, 2892; q) L. Baldini, M. Melegari, V. Bagnacani, A. Casnati,
E. Dalcanale, F. Sansone, R. Ungaro, J. Org. Chem. 2011, 76, 3720;
r) Y. Zhou, J. Yoon, Chem. Soc. Rev. 2012; DOI: 10.1039/
C1CS15159B.
[6] Tripeptide Ac-GHG-OMe was procured from USV Limited
(Mumbai, India) with >95% purity, as confirmed by HPLC.
[7] a) D. Bar-Or, L. T. Rael, R. Bar-Or, D. S. Slone, C. W. Mains,
N. K. R. Rao, C. G. Curtis, Clin. Chim. Acta 2008, 387, 120; b) J. Y.
Lee, M. Hirose, J. Biol. Chem. 1992, 267, 14753; c) G. L. Ellman,
Arch. Biochem. Biophys. 1959, 82, 70.
[8] Human blood serum (2.0 mL), which was obtained from consenting
adult volunteers was reduced by using TCEP (10 mm; tris-(2-carbox-
yethyl)phosphine) followed by deproteinisation with MeOH in the
presence of PPh3 (1.5 mm), followed by centrifugation for 20 min at
g=2850.
Acknowledgements
C.P.R. acknowledges the financial support from DST, CSIR and DAE-
BRNS. R.K.P. acknowledges CSIR and K.T. acknowledges UGC for fel-
lowships.
Keywords: amino acids · calixarene · biosensors · proteins ·
recognition
[1] a) J. M. Berg, Y. Shi, Science 1996, 271, 1081; b) Y. Shi, J. M. Berg,
Science 1995, 268, 282; c) I. L. Alberts, K. Nadassy, S. J. Wodak, Pro-
tein Sci. 1998, 7, 1700.
[2] a) W. Maret, Proc. Natl. Acad. Sci. USA 1994, 91, 237; b) D. Summa,
O. Spiga, A. Bernini, V. Venditti, R. Priora, S. Frosali, A. Margaritis,
D. D. Giuseppe, N. Niccolai, P. D. Simplicio, Proteins Struct. Funct.
Bioinf. 2007, 69, 369; c) D. M. Ralph, S. R. Robinson, M. S. Campbell,
G. M. Bishop, Free Radical Biol. Med. 2010, 49, 649.
[3] a) T. Finkel, N. J. Holbrook, Nature 2000, 408, 239; b) C. J. Chang, J.
Jaworski, E. M. Nolan, M. Sheng, S. J. Lippard, Proc. Natl. Acad. Sci.
USA 2004, 101, 1129; c) E. Aizenman, A. K. Stout, K. A. Hartnett,
K. E. Dineley, B. McLaughlin, I. J. Reynolds, J. Neurochem. 2002, 75,
1878; d) O. Demirkol, C. Adams, N. Ercal, J. Agric. Food Chem.
2004, 52, 8151; e) W. Maret, Biochemistry 2004, 43, 3301.
[4] a) W. Wang, J. O. Escobedo, C. M. Lawrence, R. M. Strongin, J. Am.
Chem. Soc. 2004, 126, 3400; b) S. Lim, J. O. Escobedo, M. Lowry, X.
Xu, R. Strongin, Chem. Commun. 2010, 46, 5707; c) H. Li, J. Fan, J.
Wang, M. Tian, J. Du, S. Sun, P. Sun, X. Peng, Chem. Commun. 2009,
5904; d) S. Sreejith, K. P. Divya, A. Ajayaghosh, Angew. Chem. 2008,
120, 8001; Angew. Chem. Int. Ed. 2008, 47, 7883; e) W. Lin, L. Yuan,
Received: September 7, 2011
Published online: November 14, 2011
Chem. Eur. J. 2011, 17, 13999 – 14003
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14003