1-(d-Glucopyranosyl-2′-deoxy-2′-iminomethyl)-2-hydroxynaphth alene as chemo-sensor for Fe3+ in aqueous HEPES buffer based on colour changes observable with the naked eye

Article abstract of DOI:10.1016/j.tetlet.2008.11.116

A new glucose-based C2-derivatized colorimetric chemo-sensor (L₁) has been synthesized by a one-step condensation of glucosamine and 2-hydroxy-1-naphthaldehyde for the recognition of transition metal ions. Among the eleven metal ions studied, v

Full text of DOI:10.1016/j.tetlet.2008.11.116

Tetrahedron Letters 50 (2009) 776–780
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Tetrahedron Letters
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1-(D-Glucopyranosyl-20-deoxy-20-iminomethyl)-2-hydroxynaphthalene as
chemo-sensor for Fe3+ in aqueous HEPES buffer based on colour changes
observable with the naked eye
*
Atanu Mitra, Balaji Ramanujam, Chebrolu P. Rao
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new glucose-based C2-derivatized colorimetric chemo-sensor (L1) has been synthesized by a one-step
condensation of glucosamine and 2-hydroxy-1-naphthaldehyde for the recognition of transition metal
ions. Among the eleven metal ions studied, viz., Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+
and Hg2+, L1 results in visual colour change only in the presence of Fe2+, Fe3+and Cu2+ in methanol. How-
ever, in an aqueous HEPES buffer (pH 7.2) it is only the Fe3+ that gives a distinct visual colour change even
in the presence of other metal ions, up to a concentration of 280 ppb. The changes have been explained
based on the complex formed, and the composition has been determined to be 2:1 between L1 and Fe3+
based on Job’s plot as well as ESI MS. The structure of the proposed complex has been derived based on
HF/6-31G calculations.
Received 13 October 2008
Revised 19 November 2008
Accepted 28 November 2008
Available online 3 December 2008
Keywords:
1-(D
-Glucopyranosyl-20-deoxy-20-
iminomethyl)-2-hydroxynaphthalene
Colorimetric detection
Naked-eye sensor for Fe3+
Aqueous buffer medium
HF/6-31G calculations
Ó 2008 Elsevier Ltd. All rights reserved.
Iron is the most abundant transition metal ion present in the
earth’s crust. It is also an important biological element owing to
its diverse functions. Some of these functions are driven based on
the concentration of the ions.1 Though iron can exhibit a variety
of oxidation states, the most prominent ones are +3 and +2. To
our knowledge, the literature reports on the detection of Fe(III)
have been based mainly on techniques such as fluorescence, poten-
tiometry and redox titrations.2 The early studies of photometric
determination of Fe(III) carried out using a variety of reagents were
mainly effective at millimolar concentrations though there were
few suitable for low concentrations.3 Among these, the reagents
that may have some chemical relevance to the present study in-
clude salicylic-3f,g,k/sulfosalicylic3f,h,j acid derivatives and also
those based on phenolic-OH, viz., tiron indicator3a The chemo-sen-
sor receptors that work through colour changes at micro- or sub-
micromolar levels were most in demand owing to the easy visual
detection of the colour change that occurs in the presence of guest
species with the naked eye.4 Therefore, the design of synthetic
receptors for the selective recognition of biologically and environ-
mentally relevant ions, especially those of transition metal ones
including iron, is currently of great importance. The applicability
of such synthetic receptors will be broadened if these were built
with chemical species having water-solubility and biocompatibil-
ity. Carbohydrates possess both these properties, and hence can
act as useful platforms for making such molecular receptors.
Therefore, this Letter deals with the synthesis and characterization
of a glucose-based 2-hydroxy-1-naphthylidene derivative linked
through imine, viz., 1-(D
-glucopyranosyl-20-deoxy-20-iminometh-
yl)-2-hydroxynaphthalene (L1), and its selective visual recognition
towards Fe3+ against a number of other metal ions in an aqueous
HEPES buffer (pH 7.2) at biological pH. To our knowledge, this is
the first Letter on the visual recognition of Fe3+ in a buffer, based
on a biologically benign molecular receptor. The derivative L2, pos-
sessing a salicylidene moiety, viz., 1-(D
-glucopyranosyl-20-deoxy-
20-iminomethyl)-2-hydroxybenzene, is less effective and hence
acts as a control system in the present study.
Both the chemo-sensor (L1) and the control (L2) molecules have
been synthesized by a one-step simple condensation5 of C2-deoxy-
C2-amino-glucose (glucosamine) with 2-hydroxy-1-naphthalde-
hyde/salicylaldehyde as shown in Scheme 1. The products were
characterized by analytical and spectral techniques6 (SI 01). Simi-
lar to the binding core observed with the 4,6-di-O-protected7 glu-
cose-based derivatives, even L1 and L2 are capable of exhibiting an
ONO binding core.
In order to explore the chemo-sensor properties of L1 and L2,
these molecules were titrated against a number of metal ions,
viz., Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and
Hg2+, in methanol as well as in an aqueous HEPES buffer (pH
7.2). The titrations were monitored qualitatively by observing the
colour change with the naked eye. In order to observe visual colour
changes, if any, studies were carried out8 by mixing L1 or L2 with
* Corresponding author. Tel.: +91 22 2576 7162; fax: +91 22 2572 3480.
E-mail addresses: cprao@iitb.ac.in, cprao@chem.iitb.ac.in (C.P. Rao).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.116

A. Mitra et al. / Tetrahedron Letters 50 (2009) 776–780
777
metal ions in a 1:1 mole ratio in methanol, and the corresponding
results are shown in Figure 1. Distinct visual colour changes were
noticed only in the presence of Fe2+ and Fe3+ with both the ligands,
wherein these ions are light and dark purple, respectively, to differ-
entiate the presence of iron in the +2 and +3 oxidation states. The
other ion that turns the solution colour of L1 and L2 to light green
was Cu2+. The presence of all other ions exhibited practically no
change in the colour of the ligand solution.
as competitive ones, and the results are shown in Figure 2. The
titration of {L1 + Fe2+} with other M2+ results in the fading away
of the original colour, and hence the resulting solutions cannot
be distinguished. However, the original colour of {L1 + Fe3+} per-
sists even after the addition of other M2+ ions including that of
Cu2+. Thus, the results shown in Figure 2 are clearly suggestive of
the selective recognition of Fe3+ by L1 even in the presence of other
M
2+ ions with the naked eye and not so in the case of Fe2+. Similar
Though L1 and L2 seem to recognize Fe2+ and Fe3+ with high vi-
sual clarity, Cu2+ can also be recognized by these only by a mar-
ginal colour variation that is barely noticed. The results shown in
Figure 1 are suggestive of L1 being more sensitive towards metal
ions than L2. Since L1 and L2 are sensitive towards both Fe2+ and
Fe3+, the selectivity of these ions has been addressed by challeng-
ing the persistence of the colour in the presence of other metal ions
competitive metal ion titrations carried out in the case of the sali-
cylidene derivative, L2, did not result in colour changes that are dis-
tinguishable either with Fe2+ or with Fe3+ (SI 02), and hence L2 is
not selective towards either of these ions in methanol.
Titrations carried out between {L1 + Cu2+} or {L2 + Cu2+} and
other ions (Fig. 3) resulted in the fact that Cu2+ can still be recog-
nized in the presence of other ions except Fe3+; however, the over-
OH
OH
O
HO
O
HO
OH
OH
HO
OH
(i)
HO
(i)
O
HO
HO
N
N
OH
(ii)
(iii)
NH2.HCl
HO
HO
L1
L2
Scheme 1. Synthesis of glucose-based derivatives, viz., L1 and L2.6 (i) Triethylamine [N(CH2CH3)3], (ii) 2-hydroxy-1-naphthaldehyde and (iii) salicylaldehyde. The enclosed
portions exhibit an ONO core.
L
1
2
3
4
5
6
7
8
9
10
11
Figure 1. Colour of the methanolic solutions of L1 (top row) and L2 (bottom row) in the presence of different metal ions at a 1:1 mole ratio: Vial L is a simple ligand; Mg2+
,
Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ are present in vials 1–11, respectively.
L1+Fe3+/2+
1
2
3
4
5
6
7
8
9
Figure 2. Competitive ion titration of {L1 + Fe3+} (bottom row) or {L1 + Fe2+} (top row) with different metal ions in methanol: Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and
Hg2+ are present in vials 1–9, respectively.
L+Cu2+
1
2
3
4
5
6
7
8
9
10
Figure 3. Competitive ion titration of {L1 + Cu2+} (top row) or {L2 + Cu2+} (bottom row) with different metal ions in methanol: Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+
and Hg2+ are present in vials 1–10, respectively.

778
A. Mitra et al. / Tetrahedron Letters 50 (2009) 776–780
all sensitivity is low. Thus, in a methanolic solution, L1 can clearly
sense Fe2+ and Fe3+ largely and Cu2+ to a reasonable extent by sim-
ple visual colour changes that can be noticed with the naked eye.
However, in the presence of other ions, it is only Fe3+ that can be
sensed by L1 and not by L2.
sual colour changes which are noticed with the naked eye, and
hence L1 can act as a selective and biologically benign molecular
sensor for Fe3+. The selectivity of L1 is increased towards Fe3+ in
the HEPES buffer (pH 7.2) as compared to that in the methanol
medium.
Similar experiments have been carried out in an aqueous HEPES
buffer at pH 7.2. Even in a buffer medium, it is only Fe2+ and Fe3+
which show distinguishable colour changes mainly with L1 and
to a little extent with L2 that have already been noticed even in
the methanolic solution (Fig. 4). The colour changes observed with
Cu2+ were only marginal.
In order to quantify the visual colour changes that occurred
upon mixing L1 with metal ions and also to establish the complex
formation between these, titrations were carried out by measuring
the absorption spectra.9 Several metal ions, viz., Mg2+, Ca2+, Mn2+
,
Co2+, Ni2+, Zn2+, Cd2+ and Hg2+ ,exhibited either no change in the
absorption spectra and/or no new band formation during their
titrations (SI 04). The ions, viz., Fe2+, Fe3+ and Cu2+, showed consid-
erable changes in the absorption spectra (Fig. 6) in methanol.
Among Fe2+ and Fe3+, it is the latter that exhibits great changes
in the absorption spectra. The changes observed with 300, 420 and
550 nm bands are suggestive of the metal ion complex formation
(Fig. 6, SI 04). Even the d?d transitions (Fig. 6, inset) have sup-
ported the formation of the complex between L1 and these ions.
Greater changes were noticed in the presence of Fe3+ as compared
to those in the presence of Fe2+. These results are in accordance
with those observed by the visual colour changes.
Competitive metal-ion titrations carried out between {L1 + Fe2+
}
or {L2 + Fe2+} and other metal ions in the HEPES buffer (pH 7.2)
exhibited no distinguishable colour change, as the original colour
of the solution does not persist in the presence of other ions (SI
03). However, the experiments carried out between {L1 + Fe3+} or
{L2 + Fe3+} and other metal ions could not bleach out the original
colour developed by Fe3+ at least in the case of L1 as evident from
Figure 5.
Thus, L1 can distinctly recognize only Fe3+ even in the presence
of other metal ions in the HEPES buffer (pH 7.2) by producing vi-
L
1
2
3
4
5
6
7
8
9
10
11
Figure 4. Colour of the HEPES buffer solutions of L1 (top row) and L2 (bottom row) in the presence of different metal ions at a 1:1 mole ratio: vial L is a simple ligand; Mg2+
,
Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ are present in vials 1–11, respectively.
L1+Fe3+/2+
1
2
3
4
5
6
7
8
9
Figure 5. Competitive ion titration of {L1 + Fe3+} (top row) or {L1 + Fe2+} (bottom row) with different metal ions in an aqueous HEPES buffer (pH 7.2): Mg2+, Ca2+, Mn2+, Co2+
,
Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ are present in vials 1–9, respectively.
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
1.00
0.75
0.50
0.25
0.00
1.5
1.0
0.5
0.0
3.0
2.5
2.0
1.5
1.0
450 500 550 600 650 700 750 800
500
600
700
800
0
2
4
6
8
10
200
300
400
500
200
300
400
500
Wavelength (nm)
Mole Ratio; [Metal ion]/[L1]
Figure 6. Absorption spectral data during the titration of L1 with metal ions in methanol: Spectral traces in the case of (a) Fe2+ and (b) Fe3+. (c) Relative absorbance (A/A0)
plots of 303 nm band as a function of the mole ratio of [Metal ion]/[L1]. The symbols, J, j and w are for Fe2+, Fe3+ and Cu2+, respectively. Insets given in (a) and (b) are the
spectra in visible region at a higher concentration.

A. Mitra et al. / Tetrahedron Letters 50 (2009) 776–780
779
Titrations carried out between the receptor molecules L1 and
Fe2+, Fe3+ or Cu2+ in the aqueous HEPES buffer (pH 7.2) exhibited
changes in the absorption spectra, and these changes were found
to be greater in the case of Fe3+ titration (Fig. 7, SI 04), supporting
the formation of a complex between L1 and Fe3+ in methanol as
well as in the HEPES buffer (pH 7.2). The stoichiometry of the com-
plex formed between L1 and Fe3+ has been found to be 2:1 based on
Job’s plot10 (SI 05). This has been further confirmed by the mass
spectral peak observed at m/z = 720 in ESI MS, and the presence
of iron in this has been augmented by the isotope peak pattern
(SI 06).
ety and the average plane of the carbohydrate moiety (Fig. 8b).
Thus, one of the ligands exhibited an ONO binding core and the
other exhibited an OO binding core. Indeed, the di-O-protected
glucosyl derivatives mainly exhibited ONO binding cores in their
metal ion complexes of cis-VO2þ, cis-MoO22þ, trans-UO22þ, Ni2+
and Cu2+ expect in the case of Zn2+ where the ligand exhibited only
an ON binding core. 12 Even the un-protected galactosyl derivative
also exhibited a ONO binding core with Cu2+ 5a
Thus, the glucose-based 2-hydroxy-1-naphthalidene derivative
linked through imine, viz., 1-(
-glucopyranosyl-20-deoxy-20-imi-
.
D
nomethyl)-2-hydroxynaphthalene (L1), has been found to be selec-
tive towards Fe3+ in an aqueous HEPES buffer (pH 7.2) based on
visual colour changes, absorption spectroscopy and ESI MS. The
In order to establish the lowest concentrations of Fe3+ that can
be detected visually by L1 in methanol as well as in the HEPES buf-
fer (pH 7.2), titrations were carried out by keeping the mole ratio
between L1 and Fe3+ as 2:1 and it was found that L1 and L2 can de-
tect up to a concentration of 5 ꢀ 10ꢁ6 M (280 ppb) (SI 07). At this
concentration, the HEPES buffer (pH 7.2) solution is darker as com-
pared to the methanol one.
Having determined the composition of the complex formed as
1:2 between Fe3+ and L1 based on ESI MS and absorption data,
the species has been modelled based on the HF/6-31G level of cal-
culations using the GAUSSIAN 03 package.11 The optimized structure
(SI 08) was found to be a neutral complex, viz., Fe(L1)2, with a
square pyramidal geometry at Fe3+, wherein one of the ligands acts
as a di-anionic tridentate and the other as a mono-anionic bi-den-
tate (Fig. 8a). Both the ligands use phenolate-Oꢁ and C3-Oꢁ of the
carbohydrate. The fifth coordination is extended from the imine
nitrogen of the tri-dentate ligand. The tri-dentate ligand exhibited
an equi-planar conformation between the plane of the phenyl moi-
same molecular receptor (L1) exhibited recognition towards Fe2+
,
Fe3+ and Cu2+ in methanol owing to their coordination preferences
as judged from the charge transfer transition noticed in the corre-
sponding absorption spectra. The 2:1 complex formed between L1
and Fe3+ has been further supported by computational calculations
to result in a square pyramidal Fe3+ bound through both the phe-
nolate-Oꢁ, C3-Oꢁ and imine nitrogen only from one of the ligands.
Such tri-dentate binding through the ONO core has already been
shown by us in the past in the case of transition metal ion com-
plexes of the 4,6-di-O-blocked-glucosyl- as well as the free-galac-
tosyl-based derivatives. To our knowledge, this is the first letter
on the selective recognition of Fe3+ by a biologically benign carbo-
hydrate receptor in a buffer medium where the recognition can be
noticed through a visual colour change. The literature reports on
Fe3+ recognition were mostly in alcohol, though a few were in
water, but the receptors were non-carbohydrate derivatives.
1.5
1.5
5
4
3
2
1
1.0
1.0
0.5
0.0
0.5
0.0
300
400
500
600
300
400
500
600
0
2
4
6
8
10
Mole Ratio; [Mx+]/[L1]
Wavelength (nm)
Figure 7. Absorption spectral data during the titration of L1 with metal ions in an aqueous HEPES buffer: Spectral traces in the case of (a) Fe2+ and (b) Fe3+. (c) Relative
absorbance (A/A0) plots of a 303 nm band as a function of the mole ratio of [metal ion]/[L1]. The symbols, j, d and N are for Fe2+, Fe3+ and Cu2+, respectively.
Figure 8. (a) Primary coordination sphere about iron in the complex, Fe(L1)2 optimized at HF/6-31G level. (b) A stereo view of the optimized Fe(L1)2 complex. Bond distances
0
are given in the figure in ÅA. The bond angles (°) are O1–Fe–N = 88.4; O1–Fe–O2 = 151.3; O1–Fe–O3 = 87.2; O1–Fe–O4 = 103.4; N–Fe–O2 = 84.5; N–Fe–O3 = 154.2; N–Fe–
O4 = 103.3; O2–Fe–O3 = 87.2; O2–Fe–O4 = 105.3 and O3–Fe–O4 = 102.5.

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A. Mitra et al. / Tetrahedron Letters 50 (2009) 776–780
3
H, C1–H, JC1–H–C2–H 5.2 Hz), 6.60–8.12 (6H, Ar-H), 8.90 (d, H, CH@N), 13.60 (t,
Acknowledgements
H, Aromatic-OH). ESI MS m/z = 334 ([M+H]+, 100%).(b) L2 has been
synthesized similarly by using salicylaldehyde (0.15 ml; 1 mmol) in place of
2-hydroxy-naphthaldehyde. Yield = 0.23 g (81%). 1H NMR (DMSO-d6, ppm):
3.25–3.80 (m, 5H, C2–H, C3–H, C4–H, C5–H), 4.54–4.95 (4d, 4H, C1–OH, C3–
C.P.R. acknowledges the financial support received from DST,
CSIR and DAE-BRNS. A.M. acknowledges CSIR for SRF.
3
OH, C4–OH and C6–OH), 5.16–5.18 (d, H, C1–H, JC1–H–C2–H, 5.5 Hz), 6.19–7.59
(2d, 2t, 4H, Ar-OH), 8.6 (S, H, CH@N), 13.2 (S, H, Ar-OH). ESI MS m/z = 284
([M+H]+, 100%).
Supplementary data
7. (a) Rajsekhar, G.; Gangadharmath, U. B.; Rao, C. P.; Guionneau, P.; Saarenketo,
P. K.; Rissanen, K. Carbohydr. Res. 2002, 337, 1477; (b) Sah, A. K.; Rao, C. P.;
Saarenketo, P. K.; Rissanen, K. Carbohydr. Res. 2002, 337, 79; (c) Sah, A. K.; Rao,
C. P.; Saarenketo, P. K.; Kolehmainen, E.; Rissanen, K. Carbohydr. Res. 2001, 335,
33.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.11.116.
8. The visual colour detection experiments were carried out by using 10ꢁ2 M.
Methanolic solutions of the ligand and metal ions (as hydrated perchlorate
salts) added together to give a mixture with a 1:1 mole ratio wherein the final
concentration is 5 ꢀ 10ꢁ3 M. The ligand was initially dissolved in a minimum
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C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi,
R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
GAUSSIAN 03, Revision C.02; Gaussian: Wallingford, CT, 2004.
6. (a) Glucosamine hydrochloride (0.215 g, 1 mmol, 1) salt has been neutralized
with triethylamine in ethanol before it is used in the synthesis. To this, an
ethanolic solution of 2-hydroxy-1-naphthaldehyde (0.172 g, 1 mmol) was
added. The reaction mixture was refluxed for 6 h at 60 °C. The solid product
formed, L1, (0.896 g) was filtered and washed with ethanol several times
12. (a) Rajsekhar, G.; Sah, A. K.; Rao, C. P.; Guionneau, P.; Bharathy, M.; GuruRow, T.
N. Dalton Trans. 2003, 3126; (b) Rajsekhar, G.; Rao, C. P.; Nättinen, K.; Rissanen,
K. Inorg. Chem. Commun. 2003, 6, 1156; (c) Sah, A. K.; Rao, C. P.; Saarenketo, P.
K.; Rissanen, K.; van Albada, G. A.; Reedijk, J. Chem. Lett. 2002, 348; (d) Sah, A.
K.; Rao, C. P.; Saarenketo, P. K.; Rissanen, K. Chem. Lett. 2001, 1296; (e) Sah, A.
K.; Rao, C. P.; Wegelius, E. K.; Kolehmainen, E.; Rissanen, K. Carbohydr. Res.
2001, 336, 249; (f) Sah, A. K.; Rao, C. P.; Saarenketo, P. K.; Wegelius, E. K.;
Kolehmainen, E.; Rissanen, K. Eur. J. Inorg. Chem. 2001, 2773.
followed by diethyl ether at the end which was dried under
a vaccum.
Yield = 89%. 1H NMR (DMSO-d6, ppm): d 3.12–3.75 (m, 5H, C2–H, C3–H, C–4H,
C5–H and C6–H), 4.46–5.38 (m, 4H, C1–OH, C3–OH, C–4OH, C6–OH), 5.62 (d,