A new family of bis-ureidic receptors for pyrophosphate optical sensing

Article abstract of DOI:10.1039/c3ob27244c

A new family of bis-ureidic receptors (L¹-L⁶) has been synthesised. The binding properties of L¹-L⁶ towards different anions (acetate, benzoate, glutarate, malonate, dihydrogen phosphate, hydrogen pyrophosphate, triphosphate, AMP and ADP) have been studied by means of ¹H-NMR, UV-Vis and fluorescence spectroscopies and a remarkable affinity for HPpi^3- has been observed in the case L³ (in DMSO-d₆ and DMSO-d₆-5% H₂O) which also acts as a fluorimetric chemosensor, even to the naked eye, for this anion. Theoretical calculations helped us explain the binding properties observed. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ob27244c

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A new family of bis-ureidic receptors for
pyrophosphate optical sensing†
Cite this: DOI: 10.1039/c3ob27244c
Claudia Caltagirone,*a Carla Bazzicalupi,b Francesco Isaia,a Mark E. Light,c
Vito Lippolis,a Riccardo Montis,a Sergio Murgia,a Martina Olivaria and
Giacomo Piccia
A new family of bis-ureidic receptors (L1–L6) has been synthesised. The binding properties of L1–L6
towards different anions (acetate, benzoate, glutarate, malonate, dihydrogen phosphate, hydrogen pyro-
phosphate, triphosphate, AMP and ADP) have been studied by means of 1H-NMR, UV-Vis and
fluorescence spectroscopies and a remarkable affinity for HPpi3− has been observed in the case L3 (in
DMSO-d6 and DMSO-d6–5% H2O) which also acts as a fluorimetric chemosensor, even to the naked eye,
for this anion. Theoretical calculations helped us explain the binding properties observed.
Received 19th November 2012,
Accepted 16th January 2013
DOI: 10.1039/c3ob27244c
www.rsc.org/obc
pyrophosphate, especially by means of fluorescent chemo-
sensors, has attracted the attention of chemists over the last 20
Introduction
Anion recognition and sensing has recently received consider- years.5 One of the first examples has been reported by Czarnik
able attention because of the central role played by anions in in 1994.6 Czarnik’s system is based on an anthracene deriva-
biological, industrial, and environmental processes.1
tive bearing polyamine pendant arms and it is able to bind Ppi
Colorimetric chemosensors for anions are of particular in water at pH 7 and to discriminate pyrophosphate over phos-
interest because they can allow naked eye recognition and phate. Different strategies can be used to design chemosensors
sensing, while fluorescent chemosensors offer many advan- for anions (and Ppi, in particular) involving, for example,
tages over other types of chemosensors in terms of sensitivity, excimer formation7 and the displacement approach.8 In par-
response time, and cost. Among anions, pyrophosphate (Ppi, ticular, different systems containing metal ion complexes,9 or
this acronym with omitted charges will be used throughout guanidinium10 or imidazolium11 based platforms have been
this paper when referring to pyrophosphate independently of synthesised and studied. Also neutral receptors containing pyr-
its protonation degree) is a biologically important target roles able to recognise pyrophosphate both in aqueous12 and
because it is the product of ATP hydrolysis under cellular con- non-aqueous environments13 have been successfully devel-
ditions.2 Moreover, the detection of pyrophosphate has oped. However, fewer examples of urea/thiourea containing
become important in cancer research as telomerase (a bio- receptors for pyrophosphate recognition have been reported in
marker for cancer diagnosis) activity is measured by evaluating the literature so far.14 In this context we decided to synthesise
the amount of Ppi generated in the PCR amplification of the and study a new family of bis-ureidic receptors based on 1,3-
telomerase elongation product.3 Furthermore, the high level of bis(aminomethyl)benzene and 2,6-bis(aminomethyl)pyridine.
Ppi in synovial fluids is correlated to calcium pyrophosphate In 1995 Umezawa and co-workers15 reported on bis-ureas and
dehydrate disease (CPDD), a rheumatologic disorder.4 For thioureas obtained by the reaction of 1,3-bis(aminomethyl)-
these reasons the detection and discrimination of benzene with butyl isocyanate and butyl isothiocyanate which
showed a remarkable affinity for H2PO4 (Ka = 820 M−1 for the
−
thiourea derivative) in DMSO-d6. The same group also studied
the application of the phenyl-substituted thiourea as an iono-
aDipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari,
S.S. 554 Bivio per Sestu, Monserrato (CA), 09042, Italy.
phore for an anion selective electrode with remarkable sulfate
E-mail: ccaltagirone@unica.it; Fax: +(39) 0706754456; Tel: +(39) 0706754452
bDipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della
selectivity.16 More recently Sellergren and co-workers studied
the recognition of tyrosine phosphorylated peptides by the
vinylphenyl urea derivative of 1,3-bis(aminomethyl)benzene.17
However, to the best of our knowledge, examples of colori-
metric and fluorimetric chemosensors based on 1,3-bis-
(aminomethyl)benzene or 2,6-bis(aminomethyl)pyridine plat-
forms have not been reported so far.
Lastruccia 3, Sesto Fiorentino (FI), 50019, Italy
cSchool of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK.
E-mail: M.E.Light@soton.ac.uk; Fax: +44 (0)23 8059 6805; Tel: +44 (0)23 8059 6805
†Electronic supplementary information (ESI) available: Additional information
as noted in the text including synthetic details for the preparation of L1 and L2.
CCDC 911916. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ob27244c
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potential colorimetric sensors (L2 and L5), and two potential
fluorimetric sensors (L3 and L6).
Results and discussion
We designed and successfully synthesised L1–L6 using the
Firstly, we performed anion-binding studies by means of
reaction scheme shown in Scheme 1. In the case of L1–L3 the 1H-NMR titrations in DMSO-d6. The EQNMR program19 was
commercially available 1,3-bis(aminomethyl)benzene reacted used to calculate stability constants from the 1H-NMR titration
with the suitable isocyanate (phenyl isocyanate, 2-nitrophenyl curves obtained (see ESI†) by fitting the data to a 1 : 1 binding
isocyanate, and 1-naphthyl isocyanate) in dichloromethane at model as shown in Table 1.
room temperature under nitrogen to give the three receptors
in 70–80% yield.
As shown in Table 1 the six receptors showed a medium–
low affinity for all the anions considered except for dihydrogen
In the case of L4–L6 we first synthesised the 2,6-bis(amino- phosphate (in the case of L4) and hydrogen pyrophosphate in
methyl)pyridine hydrochloridric salt by a Delepine reaction of the case of L1 and L3.
2,6-bis(bromomethyl)pyridine obtained from 2,6-bis(hydroxy-
In particular, L4–L6 immediately deprotonated (both ureidic
methyl)pyridine by bromination with HBr and PBr3.18 1,3-Bis- NHs) after the addition of 0.3 equivalents of HPpi3−. L2, on the
(aminomethyl)pyridine hydrochloridric salt was reacted with other hand, underwent a partial deprotonation of the ureidic
the suitable isocyanate (phenyl isocyanate, 2-nitrophenyl iso- NHs adjacent to the nitro group (in green in Fig. 1) in the pres-
cyanate, and 1-naphthyl isocyanate) in refluxing dichloro- ence of 0.3 equivalents of the anion. The signals of the other
methane in the presence of triethylamine to obtain L4–L6 in NHs (red) and of the phenyl CH (orange) progressively shifted
40–50% yield (see ESI†).
downfield until one equivalent of the HPpi3− anion was
In this way we built up a family of ligands comprising two added. Beyond this point also the signal of the ureidic protons
virtually “optically innocent” receptors (L1 and L4), two adjacent to the aromatic ring disappeared (Fig. 1).
In the case of L1 it was possible to follow the downfield
shift of the two NHs signals (green and red in Fig. 2) and of
the phenyl CH signal (orange) up to the addition of two
Fig. 1 1H-NMR stack plot of a DMSO-d6 solution of L2 (0.005 M) upon addition
Scheme 1 Synthesis of L1–L6.
of tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Table 1 Equilibrium constants (Ka/M−1) for the reactions of L1–L6 with the tetrabutylammonium salts of the anion considered in DMSO-d6 at 300 K. All errors are
estimated to be ≤13% (see ESI)
Anion
L1
L2
L3
L4
L5
L6
CH3COO−
C6H5COO−
Glutarate
Malonate
119
65
43
11
484
5480
88
109
10
<10
137
78
20
<10
435
>104
197
106
77
<10
698
Deprot.
121
60
15
<10
167
Deprot.
299
88
30
<10
273
Deprot.
−
H2PO4
181
HPpi3−
Partial deprot.
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Fig. 3 1H-NMR stack plot of a DMSO-d6 solution of L3 (0.005 M) upon addition
of tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Fig. 2 1H-NMR stack plot of a DMSO-d6 solution of L1 (0.005 M) upon addition
of tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Table 2 Equilibrium constants (Ka/M−1) for the reactions of L3 with tetrabutyl-
ammonium salts of hydrogen pyrophosphate, dihydrogen phosphate, acetate
and benzoate and the sodium salt of AMP in DMSO-d6 and DMSO-d6–0.5%
H2O at 300 K. All errors are estimated to be ≤14% (see ESI)
equivalents of the anion. A further excess of anion caused the
disappearance of the two NHs signals (Fig. 2).
L3 showed different behaviour compared to L1 and L2.‡ In
fact, after addition of 0.3 equivalents of HPpi3− we observed
the broadening and the downfield shift of the peak attributed
to the NHs adjacent to the naphthylic group (green in Fig. 3);
however this signal could be followed until the end of the titra-
tion experiment (over five equivalents of anion added). The
signal of the other ureidic NHs (red in Fig. 3) also shifted
upon addition of the anion without broadening. Differently to
what was observed with L1 and L2, in the case of L3 we could
not observe a shift of the phenyl CH singlet. Instead, a doublet
HPpi3−
AMP−
H2PO4
AcO−
BzO−
−
L3 (DMSO-d6)
>104
570
369
35
698
21
137
15
78
11
L3 (DMSO-d6–5% H2O)
the 1 : 1 adduct of L3 with HPpi3− was 570 M−1. This value is
however higher than those calculated for the formation con-
stants of the 1 : 1 adducts of L3 with the other anions con-
sidered under the same experimental conditions.
This interesting behaviour of L3 could be explained consid-
ering the important contribution given by the naphthylic CH
in assisting the interaction of the receptor with HPpi3−. An
additional proof of the intervention of this proton in anion
binding was brought by the X-ray crystal structure obtained for
the 1 : 2 L6/BzO− adduct obtained from L6 in the presence of
an excess of tetrabutylammonium benzoate by slow evapor-
ation of ethyl acetate (Fig. 4). In contrast to what was observed
in solution (see Table 1) a complex with a 1 : 2 stoichiometry
was isolated in the solid state where each anion interacts with
the receptor via two hydrogen bonds (N1–H901⋯O3 2.816(4);
N2–H902⋯O4 2.809(4); N4–H904⋯O5 2.831(4); N5–H905⋯O6
2.760(4) Å) and via a weaker interaction with the naphthylic
CH (C28–H28⋯O6 3.164 and C2–H2⋯O3 3.136 Å).
1
attributed by H-COSY to the naphthylic CH pointing towards
the pseudo-cavity of the receptor (blue in Fig. 3) considerably
shifted downfield upon addition of the anion.
We also performed titrations of L3 in the presence of the tri-
phosphate anion (P3O105−), AMP− and ADP− (as their sodium
salts) (Table 2).
Only with AMP− we observed significant interactions (Ka =
369 M−1) while with triphosphate and ADP− the changes in the
1H-NMR spectrum of L3 were negligible. The affinity of L3 for
pyrophosphate was also tested in a more competitive solvent
mixture, i.e. DMSO-d6–5% H2O: under these experimental con-
ditions the stability constant calculated for the formation of
‡Crystal data were collected on a Rigaku AFC12 equipped with a Saturn724+
detector mounted at the window of an FR-E+ molybdenum rotating anode gen-
In order to better understand the interactions between
erator Mo-Kα radiation (λ = 0.71075). Standard solution and refinement pro- pyrophosphate and L3, theoretical calculations were carried
cedures were followed, with the exception that the Squeeze algorithm17 was
applied to model unresolvable disordered solvent. Crystal data for L1:
out. Molecular modelling investigations on the adducts
−
formed by L1 and L3 with HPpi3−, and by L3 with H2PO4 and
ˉ
C
18H14N4O; M = 302.33, T = 100(2) K, hexagonal, space group R3, a = 32.989(7) Å,
5−
P3O10 in a 1 : 1 ligand-to-anion molar ratio have been per-
c = 7.098(2) Å, V = 6689(3) Å3, ρc = 1.351 g cm−3, μ = 0.088 mm−1, Z = 18, reflec-
tions collected: 2613, independent reflections: 2613 (Rint = 0.0000), final R
indices [F2 > 2σF2]: R1 = 0.0712, wR2 = 0.1314.
formed by means of an empirical force field method
(AMBER3),20 evaluating the atomic partial charges at the PM3
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Two main results can be derived from our molecular model-
ling investigations. First of all, the binding between the L3
ligand and the studied substrates is not only due to H-bond
interactions. Actually, the number of H-bonds established by
the receptors grows with the number of acceptor groups of the
anions and with their charge, so that the triphosphate anion
gives rise to the same or even higher number of H-bonds with
respect to the HPpi3− anion, which forms the most stable
adduct with both L1 and L3 (see Fig. 5).
5−
−
On the other hand, differently from P3O10 and H2PO4
,
Fig. 4 X-ray crystal structure of the [L6(BzO)2]2− adduct showing the main
interactions with the numbering scheme adopted. Hydrogen atoms except
those involved in hydrogen bonds and tetrabutylammonium counter ions have
been omitted for clarity.
only the HPpi3− ion is capable of fitting into the pseudo-cavity
defined by the lateral arms of the receptor. As a consequence,
entropic effects due to desolvation upon host–guest inter-
actions are supposed to play an important role in determining
the observed stability trend.
The second significant result is relative to the comparison
of the low energy conformers calculated for the HPpi3−/L1
Fig. 5 Lowest energy conformers for L3 in adduct with H2PO4 (top), HPpi3
−
−
(middle) and P3O105− (bottom).
semi-empirical level of theory21 and using an implicit simu-
lation of the solvent environment (ε = 47 Fm−1). The potential
energy surface of all the systems has been explored by means of
simulated annealing (T = 600 K, equilibration time = 10 ps, run
time = 10 ps and cooling time = 10 ps, time step = 1.0 fs). For
each studied system, 80 conformations have been sampled.
Fig. 6 Conformers found for L1 in the adduct with HPpi3− with their relative
abundances.
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Fig. 8 Changes in the fluorescence spectra of L2 (a) (1.50 × 10−4 M) and L5 (b)
(8.5 × 10−5 M) upon addition of increasing amounts of (TBA)3HPpi (2.5 × 10−2
M) in DMSO. Inset: colour change of L2 (a) and L5 (b) (both 0.005 M) upon
addition of five anion equivalents (0.075 M) in DMSO. From left to right: L2 (a)
or L5 (b), HPpi3−, AcO−, BzO−, H2PO4−, Glu2−, and Mal2−
.
Fig. 7 Conformers found for L3 in the adduct with HPpi3− with their relative
abundances.
remarkable changes. As shown in Fig. 8a upon addition of
increasing amounts of the anion we observed the disappear-
adduct with those calculated for the HPpi3−/L3 adduct. Actu- ance of the band at 277 nm and the decrease of the band at
ally, in all the conformations calculated for HPpi3−/L1 (Fig. 6) 380 nm with a concomitant hypsochromic shift of 25 nm, that
the phenyl groups are oriented in such a way that no inter- could be attributed either to the partial deprotonation
actions are observed between the aromatic protons and the observed in the 1H-NMR titration or to an interaction via
oxygen of pyrophosphate, while in the case of the HPpi3−/L3 hydrogen bonding. In fact, as has been recently reported by
adduct a CH⋯O interaction is formed in all the low energy Gunnlaugsson and co-workers,22 high concentrations (as
conformations calculated (Fig. 7).
those employed during 1H-NMR titrations) could lead to a
A UV-Vis spectroscopy study was performed in DMSO in deprotonation while under more diluted conditions an
order to evaluate the potential application of L2 and L5, both effective anion binding could be observed. Receptor L5 showed
containing two nitrophenyl units, as colorimetric sensors for only one absorption band at 345 nm (ε = 4460 mol−1
anion recognition. In DMSO L2 as a free receptor showed an cm−1 L−1). The addition of HPpi3− caused the formation of
intense band in the UV region at 277 nm (ε = 11 760 mol−1 two new bands, one intense at 284 nm (ε = 25 580 mol−1 cm−1
cm−1 L−1) and one less intense in the visible region at L−1) and the other at 490 nm (ε = 5220 mol−1 cm−1 L−1), as
380 nm (ε = 7260 mol−1 cm−1 L−1). No significant changes in shown in Fig. 8b. This latter could be probably attributed to
the UV-Vis spectrum of L2 were observed upon addition of the full deprotonation of the receptor.23 In both cases the
acetate, benzoate, glutarate, malonate and dihydrogen phos- interaction with HPpi3− could be detected by the naked eye
phate. Only in the case of HPpi3− we observed some with a colour change from yellow (free receptors) to red-orange
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Competitive studies confirmed the selectivity of L3 for
HPpi3− (see ESI†).
Similar behaviour was observed in the case of L6. This
receptor when excited at 345 nm showed an emission band at
375 nm (Φ = 0.56). Upon addition of HPPi3− we observed the
decrease of the band relative to the free receptor and the for-
mation of a band at 495 nm. Also in this case, although the
1H-NMR suggested a deprotonation event, the changes in the
fluorescence spectrum of L6 could be attributed to the binding
event (Fig. 9b).
Further experimental evidence of the binding event vs.
deprotonation was provided by recording the emission spectra
of a 1 : 1 solution of L3 and L6 and TBAOH in DMSO (see ESI†)
in which we did not observe any emission at around 500 nm.
In conclusion we synthesised a new family of bis-ureidic
anion receptors based on the two 1,3-bis(aminomethyl)-
benzene and 2,6-bis(aminomethyl)pyridine platforms. We
have demonstrated that by simply changing the substituents
in the pendant ureidic moieties both a colorimetric and a
fluorimetric selective sensing of hydrogen pyrophosphate,
even to the naked eye, can be achieved in DMSO. In particular,
in the case of the receptors bearing naphthyl group pendant
arms, both binding and optical fluorimetric selectivity is
achieved thanks to the uncommon interaction of an aromatic
CH from the fluorophore with the HPpi3− guest.
We would like to thank MIUR (Ministero dell’Istruzione
dell’Università e della Ricerca Scientifica) for financial support
(Project PRIN 2009 – 2009Z9ASCA) and Regione Autonoma
della Sardegna (Programma Master & Back) for funding a fel-
lowship to Riccardo Montis.
Fig. 9 Changes in the fluorescence spectra of L3 (a) (1.50 × 10−4 M) and L6 (b)
(8.5 × 10−5 M) upon addition of increasing amounts of (TBA)3HPpi (2.5 × 10−2
M) in DMSO. Inset: emission change of L3 (a) and L6 (b) (both 0.005 M) upon
addition of five anion equivalents (0.075 M) in DMSO. From left to right: L3 (a)
or L6 (b), HPpi3−, AcO−, BzO−, H2PO4−, Glu2−, and Mal2−
.
Notes and references
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at 345 nm. This band can be attributed to the emission of a
single fragment urea substituted naphthalene according to the
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