A 'naked-eye' chemosensor system for phytate

Article abstract of DOI:10.1016/j.tetlet.2003.11.125

We report the synthesis of two new anion receptors of a covalently linked 1,3,5-triarylbenzoamido-crown ether. Our results show that combined with a picrate salt they act by means of an intermolecular charge transfer process (EDA complex), as naked-eye se

Full text of DOI:10.1016/j.tetlet.2003.11.125

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1261–1265
A Ônaked-eyeÕ chemosensor system for phytate
*
ꢀ
Jeroni Morey, Maria Orell, Miquel Angel Barcelo, Pere M. Deya, Antoni Costa
ꢁ
ꢀ
and Pablo Ballester
Departament de Quꢁımica, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
Received 22 September 2003; revised 17 November 2003; accepted 26 November 2003
Abstract—We report the synthesis of two new anion receptors of a covalently linked 1,3,5-triarylbenzoamido-crown ether. Our
results show that combined with a picrate salt they act by means of an intermolecular charge transfer process (EDA complex), as
naked-eye sensors for basic anions, especially for sodium phytate in DMSO/H2O (1:1).
ꢀ 2003 Elsevier Ltd. All rights reserved.
In the field of supramolecular chemistry the use of
naked-eye anion chemosensors capable of the simulta-
neous complexation of cationic and anionic guests has
recently experienced a considerable advance due to the
great advantages the use of these sensors presents in
relevant fields such as biology, the environment or food
technology.1 These chemosensors are constructed
according to the receptor–chromophore general bino-
mial, covalently binding a specific anion receptor with
another unit, a chromophore, responsible for translating
the former receptor–anion association into an optical
signal.2;3 This colour variation can be related to either
structural or conformational changes in the receptor
structure when the complex is formed4 or even to the
formation of a charge transfer complex.5
evidence of the existence of naked-eye chemosensing
systems for the determination of phytate.10
Herein we report two new anion sensing systems,
chemosensors, based on 1,3,5-triarylbenzoamidobenzo-
15-crown-5, 6a, and 1,3,5-triarylbenzoamido-18-crown-
6, 6c, receptors (Scheme 1), which in addition to a
chromophore not covalently coordinated to the recep-
tor, that is, a picrate salt (ammonium, sodium or
potassium), act together as a colorimetric anion sensor
capable of initiating a visible to the naked eye colour
change, which is specific for basic anions and especially
for sodium phytate (Fig. 1).
Receptors 6a and 6c were synthesised from the previ-
ously reported triester.11 As illustrated in Scheme 1, tris
ester 1 was protected as the corresponding tris benzyl 2a
or tris allyl ethers 2b. Hydrolysis of triesters 2a–b
afforded the triacids 3a and 3b, respectively, which were
converted to triacyl chlorides 4a–b and coupled with
40-aminobenzo-15-crown-5 and 2-(aminomethyl)-18-
crown-6, in dichloromethane, respectively, to produce
receptors 5a, 5b, 5c and 5d in 85%, 52%, 77% and 48%
yields, respectively. Debenzylations were observed with
compounds 5a and 5c using Me3SiI/CH2Cl2 providing
Myo-Inositolhexakis(dihydrogenphosphate) dodecaso-
dium salt, a compound known as phytate, present in
blood, urine and interstitial and intracellular fluids, is
also known to be an important constituent of the human
diet6 (Fig. 1). From several studies, in vitro and in vivo,
the results prove that phytate plays an important role as
a crystallisation inhibitor of calcium salts in biological
fluids,7 becoming a clear alternative in the treatment of
calcium oxalate renal lithiasis. In addition, phytate
presents other therapeutic properties as an anticancer-
1
yields of 63% and 44% for 6a and 6c, respectively. H,
13C NMR and mass spectra were consistent with the
proposed structures.12
8
ogenic or an antioxidant agent.9 Nevertheless, to our
knowledge and despite the great interest, there is no
The receptors 6a and 6c showed a high affinity for pic-
rate salts. In the 1H NMR spectra, the proton signals of
the crown ether ethylenes of 6a and 6c shifted down-
field upon complexation with ammonium, sodium
and potassium picrate, indicating the presence of a
Keywords: Naked-eye anion chemosensor; Phytate.
* Corresponding author. Tel.: +34-971172690; fax: +34-971173496;
e-mail: jeroni.morey@uib.es
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.125

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J. Morey et al. / Tetrahedron Letters 45 (2004) 1261–1265
Figure 1. Sodium phytate. Colour change in the presence of 6a/potassium picrate in DMSO/H2O (1:1): from left to right: no anion, sodium phytate.
Scheme 1. Reagents and conditions: (i) K2CO3, RBr; (ii) NaOH, H2O/THF, reflux; (iii) SOCl2/CH2Cl2; (iv) R00NH2, CH2Cl2; (v) Me3SiI, CH2Cl2.
cation–dipole interaction between the metallic and the
Lewis basic crown ether groups. The extractabilities
([complexed]/[free + complexed] · 100%) of 6c in CH2Cl2
were estimated to be sodium picrate, 51%, ammonium
picrate, 87% and potassium picrate, 99%.
anion in the form of a tetrabutylammonium acetate,
citrate, trimesoate, isophthalate, Fꢀ or H2PO4ꢀ salt, the
solution changes from yellow to dark green in a few
seconds. No colour variation was observed for tetra-
butylammonium salts of the following anions: Clꢀ, Brꢀ,
Iꢀ, NOꢀ3 and HSOꢀ4 under the same experimental con-
ditions. In the case of insoluble salts in DMSO, such as
potassium acetate, KF, NaHPO4 and sodium phytate, a
mixture in DMSO/H2O (1:1 v/v) was employed, the
same colour variation being observed. The 6a/potassium
picrate system has proved to be very useful with both
simple monoanionic salts and more complex poly-
The chemosensor systems were prepared by mixing
equal volumes of a receptor solution, 6a or 6c, 10ꢀ3 M,
with a 10ꢀ4 M picrate salt solution and an anion solution
of 10ꢀ2 M in DMSO. Thus, for example, upon addition
to the 6a/potassium picrate system, which is initially pale
yellow due to the presence of the picrate ion, a basic

J. Morey et al. / Tetrahedron Letters 45 (2004) 1261–1265
1263
anionic salts, GMP and ADP. On the contrary, no
colour variation was observed when using the 6a and 6c/
potassium picrate systems, respectively, with ATP.
the complexation process of the receptor system 6a, by
means of the UV–vis absorption method (Fig. 2), or 6c/
potassium picrate shows two maxima: one located at
264 nm corresponding to the absorption of the receptor,
and the second at 374 nm assigned to picrate. When
treating a solution of receptors 6a and 6c, indepen-
dently, with an increasing amount of NaOH (1.0 M) in
DMSO the disappearance of the 264 nm band due to the
receptor and the appearance of a new band due to
phenolate, caused by the deprotonation of receptor 6a/
6c located at about 320 nm, was observed. In this way,
when adding aliquots of a sufficiently basic anion in
DMSO or a DMSO/H2O mixture, a gradual decrease of
the maximum at 264 nm (phenol) and the gradual
appearance of a new band at 316 nm (phenolate)
together with the strengthening of the band assigned to
picrate at 374 nm was detected. In several cases, the
appearance of a new absorption band in the region of
618 nm, usually very weak and broad was observed,
which in the case of receptor 6c disappears very quickly
and is ascribed to the charge transfer complexes,13 TBA
fluoride (615 nm), TBA acetate (625 nm) and TBA cit-
rate (618 nm) (Fig. 3). All the anions presenting a posi-
tive change show similar behaviour. However, anions
which do not alter the colour of the system do not
usually show changes in the UV–vis spectrum of the
chemosensor system.
In the same way, we have proved that our chemosensor
systems formed independently with receptors 6a or 6c,
with either ammonium picrate, sodium picrate or
potassium picrate showing the same colouration change
when adding a TBA acetate solution. However, in the
absence of the picrate ion there was no colour variation
detected. The formation of the 6a/picrate-anion com-
plex, implies the change of colour from yellow to green,
which usually takes a few seconds to occur, the final
green colour being very stable (more than 4 h). Never-
theless, in the sensor developed from 6c/picrate, colour
changes were always slower than those in the 6a/picrate
system, recovering the initial yellow colour in a few
minutes. Unfortunately, calculation of the stability
constants of the complexes from the linear titration
curves happened to be unsuccessful, presumably, due to
the coexistence of different species at equilibrium with
either the formation of weak complexes or due to the
existence of slow reactions, as suggested by the colour
variation delay present in several anions. The study of
Most probably, colour variation, is due to a charge
transfer process between the receptor–phenolate, and
the picrate, electron donor–acceptor (EDA) complex.
After the attack of an anion on the phenolic OH of the
receptor, the formation of phenolate takes place, usually
occurring at an apparent pH of 8. Due to the aromatic
nature of receptors 6a and 6c, this negative charge is
delocalised in the p system of the receptor, conferring
electron donating properties of negative charge to the
receptors. The spatial proximity of the electron deficient
picrate molecule, coordinated to the crown ether ring,
allows the charge transfer, within the EDA complex,
which is responsible for the visual colour change from
Figure 2. (a) Absorption spectra of 6a + potassium picrate. (b)
Absorption spectra of potassium picrate. (c) Absorption spectra
changes of 6a (4.5 · 10ꢀ5 M) + potassium picrate (4.5 · 10ꢀ5 M) with
trimesoate n-Bu4Nþ, Fꢀn-Bu4Nþ, AcOꢀn-Bu4Nþ, citrate n-Bu4Nþ,
isophthalate n-Bu4Nþ, H2PO4ꢀn-Bu4Nþ, respectively, in DMSO. [Xꢀn-
Bu4Nþ] ¼ 1.70 · 10ꢀ4 M.
Figure 3. Absorption spectral changes of 6c (4.7 · 10ꢀ4 M) + potassium picrate (4.7 · 10ꢀ5 M) with citrate n-Bu4Nþ (4.7 · 10ꢀ3 M) in DMSO. 1 at
0 min, 2 at 1 min, 3 at 2 min, 4 at 3 min and 5 at 5 min.

1264
J. Morey et al. / Tetrahedron Letters 45 (2004) 1261–1265
James, T. D. Chem. Lett. 2001, 406–407; (c) Lee, D. H.;
Lee, H. Y.; Hong, J.-I. Tetrahedron Lett. 2002, 43, 7273–
7276; (d) Kim, Y.-H.; Hong, J.-I. Chem. Commun. 2002,
yellow to dark green. In this way, the ability to intercept
a phenolic proton of the receptor by a strong basic anion
(derived from a weak acid) is the first cause of the charge
transfer.
ꢁ
512–513; (e) Ros-Lis, J. V.; Martınez-Manez, R.; Soto, J.
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Several complementary tests support, to a certain extent,
this suggested process. For example, if the formation of
phenolate was hindered, by protecting the phenolic OH
group with allyl ethers, which is the case in the system
formed by receptor 5b or 5d/potassium picrate, no
change in colour is observed. Accordingly, all the assays
carried out with receptor 7,13c (Fig. 3), which lacks the
crown ether units but possesses the ability to form the
corresponding phenolate, did not show a colour change
in the presence of an anion giving a positive result, for
example, phytate, even when an adequate ratio of benzo-
15-crown-5 was added to this, 7/picrate system. This,
reveals the importance of the crown ether subunits,
which allow the receptor–chromophore to be found, not
covalently coordinated, but at an adequate distance
so as to produce the intermolecular charge transfer.
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Summing up, we have synthesised two new ditopic
receptors (6a and 6c) capable of complexing a cation
(picrate salt) and reacting with a strong basic anion or
polyanion, which caused a colour change in an aqueous
solution of DMSO. This colour change can be utilised
for the easy detection of sodium phytate.
6. Harland, B. F.; Oberleas, D. World Rev. Nutr. Diet. 1987,
52, 235–239.
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Acknowledgements
ꢀ
ꢁ
The authors thank Dr. Felix Grases and Dr. Bartolome
M. Simonet for their support and encouragement. This
work was supported by a Grant from DGICYT (ref.
BQU-2002-0461).
8. (a) Vucenik, I.; Kalebic, T.; Tantivejkul, K.; Shamsuddin,
A. M. Anticancer Res. 1998, 18, 1377–1384; (b) Vucenik,
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References and notes
1. (a) Desvergne, J.-P.; Czarnik, A. In Chemosensors of Ion
and Molecular Recognition; Kluwer: Dordrecht, 1997; Vol.
ꢁ
492; (b) Brozka, Z. In Comprehensive Supramolecular
Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D.
D., Vogtle, F., Suslick, K. S., Eds.; Pergamon: Oxford,
€
ꢁ
~
1996; (c) Bianchi, A.; Bowman-James, K.; Garcıa-Espana,
E. Supramolecular Chemistry of Anions; Wiley-VCH: New
York, 1997; (d) Schimdtchen, F. P.; Berger, M. Chem.
Rev. 1997, 97, 1609–1646; (e) Beer, D. B.; Gale, P. A.
Angew. Chem., Int. Ed. 2001, 40, 486–516; (f) McCleskey,
S. C.; Griffin, M. J.; Schneider, S. E.; McDevitt, J. T.;
Anslyn, E. V. J. Am. Chem. Soc. 2003, 125, 1114–1115; (g)
Gale, P. A. Coord. Chem. Rev. 2003, 240, 191–221.
ꢀ
11. Ballester, P.; Costa, A.; Deya, P. M.; Vega, M.; Morey, J.
Tetrahedron Lett. 1999, 40, 171.
12. All new compounds gave spectroscopic data in agreement
with the structures indicated. Selected data for compound
1
6a: H NMR d (DMSO-d6, 300 MHz, ppm): 12.30 (s, 3H,
OH), 10.52 (s, 3H, NH), 8.52 (d, 3H, J ¼ 1:1 Hz), 8.10 (dd,
3H, J ¼ 8:4, 1.1 Hz), 8.04 (s, 3H), 7.48 (s, 3H), 7.28 (d, 3H,
J ¼ 6:6 Hz), 7.22 (d, 3H, J ¼ 6:6 Hz), 7.04 (d, 3H,
J ¼ 8:4 Hz), 4.13–3.51 (m, 48H); 13C NMR d (DMSO-d6
75.4 MHz, ppm): 170.96, 163.10, 152.39, 149.62, 144.83,
135.41, 135.18, 131.04, 122.02, 121.03, 118.31, 117.96,
112.49, 74.40, 73.81, 72.90, 72.78, 72.51; IR (KBr): m 3330,
2923, 2871, 1662, 830 cmꢀ1. MS (m=z) MALDI TOF 1304
(M+Na) C69H75N3O21 requires Mþ ¼ 1281. Mp ¼ 93 ꢁC.
Compound 6c: 1H NMR d (DMSO-d6, 300 MHz, ppm):
2. (a) Metzger, A.; Anslyn, E. V. Angew. Chem., Int. Ed.
1998, 37, 649–652; (b) Gale, P. A.; Twyman, L. J.;
Handlin, C. I.; Sessler, J. L. Chem. Commun. 1999, 1851–
1852; (c) Lavigne, J. J.; Anslyn, E. V. Angew. Chem., Int.
Ed. 1999, 38, 3666–3669; (d) Yamaguchi, S.; Akiyama, S.;
Tamao, K. Tetrahedron Lett. 2002, 43, 7273–7276.
3. (a) Niikura, K.; Bisson, A. P.; Anslyn, E. V. J. Chem. Soc.,
Perkin Trans. 2 1999, 1111–1114; (b) Ward, C. J.; Patel, P.;

J. Morey et al. / Tetrahedron Letters 45 (2004) 1261–1265
1265
12.78 (s, 3H, NH), 9.13 (s, 3H, OH), 8.39 (d, 3H,
J ¼ 1:2 Hz), 8.00 (dd, 3H, J ¼ 8:6, 1.2 Hz), 7.92 (s, 3H),
7.17 (d, 3H, J ¼ 8:6 Hz), 3.79–3.57 (m, 75H); 13C NMR
d (DMSO-d6 75.4 MHz, ppm): 172.82, 163.70, 144.83,
135.03, 130.35, 127.30, 121.97, 119.56, 81.11, 75.39, 74.19,
74.10, 73.89, 73.83, 73.72, 72.91, 44.30; IR (KBr): m 2902,
2872, 1642, 1246, 1098, 830 cmꢀ1. MS (m=z) MALDI TOF
13. (a) March, J. Advanced Organic Chemistry. Reactions,
Mechanisms, and Structure, 4th ed.; John Wiley & Sons:
New York, 1992; pp 79–81; (b) Furniss, B. S.; Hannaford,
A. J.; Smith, P. W. G.; Tatchell, A. R. VogelÕs Textbook of
Practical Organic Chemistry, 5th ed.; 1989; p 1240. 14
Triamide 7 (mp ¼ 247–248 ꢁC) was prepared in 37% yield.
ꢀ
ꢁ
See: Ballester, P.; Costa, A.; Deya, P. M.; Gonzalez, J. G.;
Rotger, M. C.; Deslongchamps, G. Tetrahedron Lett.
1994, 35, 3813–3816.
1312
(Mþ)
C66H93N3O24
requires
Mþ ¼ 1312.
Mp ¼ 100 ꢁC.