1,8-Naphthyridine modified rhodamine B derivative and Cu2+ complex: Colorimetric sensing of thiols in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2012.09.089

A 1,8-naphthyridine modified rhodamine B derivative (1) has been designed and synthesized. Compound 1 is the first 1,8-naphthyridine-modified rhodamine B sensor that can detect Cu²⁺ selectively with a dramatic color change from colorless to pin

Full text of DOI:10.1016/j.tetlet.2012.09.089

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Tetrahedron Letters 53 (2012) 6544–6547
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
1,8-Naphthyridine modified rhodamine B derivative and Cu2+ complex:
colorimetric sensing of thiols in aqueous media
Yu Lian Duan a, Yong Gang Shi a, Jian Hua Chen c, Xiang Hua Wu a, Guang Ke Wang b, Ying Zhou b,
,
⇑
Jun Feng Zhang a,
⇑
a College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, PR China
b College of Chemical Science and Technology, Yunnan University, Kunming 650091, PR China
c Yunnan Academy of Tobacco Science, Kunming 650106, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2012
Revised 11 September 2012
Accepted 21 September 2012
Available online 28 September 2012
A 1,8-naphthyridine modified rhodamine B derivative (1) has been designed and synthesized. Compound
1 is the first 1,8-naphthyridine-modified rhodamine B sensor that can detect Cu2+ selectively with a dra-
matic color change from colorless to pink. The complex of 1 and Cu2+ was utilized as a chemosensing
ensemble for cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) detection, which showed highly
sensitive and selective colorimetric response to Cys, Hcy, and GSH among the tested naturally occurring
a
-amino acids in EtOH-HEPES (0.02 M, pH 7.4) (3:7, v/v) buffer solution.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Rhodamine B
1,8-Naphthyridine
Copper
Thiols
Colorimetric
Introduction
can be easily observed by naked eyes.8 Therefore, it is still a chal-
lenge for synthesizing sensors that selectively and sensitively de-
tect Cys, Hcy, and GSH with obvious colorimetric changes.
Rhodamine derivatives and its ring-opening reaction received a
great deal of attention and have been applied as fluorescent and
colorimetric metal ion sensors in organic and biological re-
searches.9 Recently, Li and Zhang et al. reported a sensitive colori-
The recognition and sensing of biothiols are of intense interests
because the intracellular thiols play an important role in maintain-
ing biological systems. Cysteine (Cys), homocysteine (Hcy), and
glutathione (GSH) as the well-known biothiols, are essential bio-
logical materials required for the growth of cells and tissues in liv-
ing systems. For example, Cys deficiency is associated in many
syndromes, including retarded growth, hair depigmentation, lethargy,
liver damage, muscle and fat loss, and weakness.1 Similarly, an
elevated level of Hcy in human plasma is a risk factor for Alzheimer’s,
cardiovascular diseases, neural tube defect, inflammatory bowel
disease, and osteoporosis.2 Therefore, the optical method for the
detection and sensing of Cys, Hcy, and GSH with high sensitivity
is of current interest in the chemosensor research field.
Up to now, various biothiol chemosensors based on Michael
addition,3 cyclization reaction with aldehyde,4 cleavage reaction
by thiols,5 or metal complexes-displace coordination mechanisms6
have been synthesized.7 In the Cys, Hcy, and GSH binding, these
chemosensors show multifarious absorbance or fluorescence
changes. However, relatively few examples of chemosensors for
Cys/Hcy/GSH sensing showed obvious colorimetric change, which
metric sensor toward
a-amino acids based on a rhodamine B
derivative and copper ion ensemble system.10 However, the pink
solution of this system changed to colorless immediately upon
the addition of all kinds of the tested
a-amino acids, which is
showing no selectivity to any individual
a-amino acid. Hence
choosing an appropriate binding site in a sensor is crucial and a
decisive factor for the sensor’s selectivity. With the intriguing
structures, good bonding properties, as well as the multiple bio-
compatibility and spectroscopic properties, 1,8-naphthyridine
and its derivatives have been applied in the coordination chemistry
and molecular recognition fields.11 It exhibits various coordination
modes with Cu+, Cu2+, Zn2+, and many other metal ions12 with
interesting spectroscopic properties. Therefore, 1,8-naphthyridine,
as a typical N-containing receptor, was introduced to a rhodamine
B chromophore in company with Cu2+ ion to form the high selec-
tivity sensing ensemble system for Cys, Hcy, and GSH in the pres-
ent work.
⇑
Corresponding authors. Tel./fax: +86 871 5033679 (Y.Z.); tel.: +86 871 5941087;
fax: +86 871 5941088 (J.F.Z.).
Herein, we report the design and synthesis of
a new1,
8-naphthyridine modified rhodamine B derivative (1), which displays
E-mail addresses: yingzhou@ynu.edu.cn (Y. Zhou), junfengzhang78@yahoo.cn
(J.F. Zhang).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.089

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Y. L. Duan et al. / Tetrahedron Letters 53 (2012) 6544–6547
6545
a dramatic color change from colorless to pink only toward Cu2+
among all the examined metal ions in EtOH-HEPES (0.02 M, pH
7.4) (3:7, v/v) buffer solution. Moreover, the complex of 1 and
Cu2+ was utilized as a chemosensing ensemble for Cys, Hcy, and
GSH detection, which showed a highly sensitive and selective
colorimetric response to thiols among the tested
a-amino acids,
including histidine (His), leucine (Leu), isoleucine (Ile), methionine
(Met), lysine (Lys), threonine (Thr), tyrosine (Tyr), valine (Val),
aspartic acid (Asp), arginine (Arg), alanine (Ala), serine (Ser), gluta-
mine (Gln), proline (Pro), glutamic acid (Glu), glycine (Gly), phen-
ylalanine (Phe), Cys, Hcy, and GSH, in the aqueous solution.
Results and discussion
N-(7-formyl-1,8-naphthyridine-2-yl) acetamide (2)13 and Rho-
damine B-hydrazide (3)14 were synthesized by modifying the re-
ported procedure. The synthesis of 1 is outlined in Scheme 1.
Reaction of 2 and 3 in ethanol at reflux for 12 h afforded 1 in
41% yield. The detailed experimental procedures and 1H and 13C
NMR spectra are summarized in Supporting Information.
Figure 1. Absorbance spectra of 1 (2 Â 10À5 M) upon addition of 2 equiv of Cu2+and
other cations in EtOH-HEPES (0.02 M, pH 7.4) (3:7, v/v) buffer solutions. Inset: color
changes of 1 in the presence of different cation perchlorates (10 equiv) in EtOH-
HEPES (0.02 M, pH 7.4) (3:7, v/v).
The photophysical properties of 1 was evaluated in EtOH-HEPES
buffer (0.02 M, pH 7.4) (3:7, v/v) solution. As shown in Figure 1,
compound 1 (2 Â 10À5 M) showed a strong absorption at around
370 nm, which is attributed to the intraligand (IL)
P ? P* transi-
tion of 1,8-naphthyridine. Additions of Li+, Na+, K+, Ag+, Fe2+, Co2+
,
effect on the emission of 1. Commonly, the fluorescence intensity
at around 550 nm would be enhanced because of the rhodamine
B ring-opening process. However, no new emission peak was ob-
served because of the strong fluorescence quench effect of Cu2+ ion.
Beside the high sensitivity, another important feature of 1 is its
high selectivity toward Cu2+ over other competitive species. As
shown in Figure S2 in Supplementary data, the absorption changes
upon the addition of excess amount of competitive metal ions were
measured. The unique absorption change with appearance of pink
of 1 was observed only by the addition of Cu2+, which can be as-
cribed to the spirolactam bond cleavage of the rhodamine B group.
Ni2+, Zn2+, Cd2+, Hg2+, Pb2+, Fe3+, and Cr3+ showed little or no effect
on the absorption of 1 besides Cu2+. Upon addition of 2 equiv Cu2+
ions, a new absorption peak centered at 562 nm appeared because
of the rhodamine B ring-opening process. The above result induced
a clear colorimetric change from colorless to pink that enabled
detection simply by naked eyes.
The changes in the fluorescence emission of 1 were next inves-
tigated. As shown in Figure S1, when excited with 360 nm, there
was a wide band centered at around 415 nm corresponding to
the emission of 1,8-naphthyridine intraligand fluorescence. Addi-
tions of Li+, Na+, K+, Ag+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Cu2+
,
The other competitive metal ions such as Li+, Na+, K+, Ag+, Mg2+
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Fe3+, Cr3+, and Cu+ (Fig. S19) gave
,
Pb2+, Fe3+, and Cr3+ exerted a slight enhancement or quenching
O
O
O
O
O
O
H2N
N
N
O
reflux
H2N
N
NH2
H3PO4
reflux
,
SeO
NH2
2 / 1,4-dioxane
O
N
N
H
N
O
N
O
N
H
N
N
reflux
O
2
O
N
NH
N
HC
N
N
N
O
N
3
N
N
O
2
ethanol, reflux
1
Scheme 1. Synthesis route of compound 1.

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6546
Y. L. Duan et al. / Tetrahedron Letters 53 (2012) 6544–6547
negligible changes in the UV spectra, indicating the selective bind-
ing of 1 to Cu2+
.
To get a further insight into the binding of Cu2+ with 1, the
absorbance spectra of 1 upon titration with Cu2+ were recorded
(Fig. 2). Upon addition of increasing amounts of Cu2+ ions, a new
absorption band centered at 562 nm appeared with increasing
intensity. Meanwhile, the absorption band centered at 366 nm de-
creased gradually with an isosbestic point observed at 385 nm. A
C-curve dependence of the absorbance intensity at 562 nm as a
function of Cu2+ concentration was observed, which indicated that
1 forms a 2:1 complex with Cu2+, whose association constant (Ka)
was determined to be about 4.44 Â 109 MÀ2 from the titration
experiments.15 Moreover, Job’s plot ( Fig. 3) and FAB mass
(Fig. S10) confirmed the 2:1 stoichiometry for the 1-Cu2+ complex,
which also strongly supports the above conclusions.
The sensitivity of 1-Cu2+ complex for 20 naturally occurring
amino acids was determined. As shown in Figure 4, upon additions
of -amino acids, including His, Leu, Ile, Met, Lys, Thr, Tyr, Val, Asp,
a-
Figure 3. Job’s plot showing the 2:1 binding of 1 to Cu2+ ions.
a
Arg, Ala, Ser, Gln, Pro, Glu, Gly, Phe, Cys, Hcy, and GSH to the solu-
tions of 1-Cu2+, respectively, only Cys, Hcy, and GSH (Fig. S14)
caused a significant decrease of the absorption at around 562 nm
along with the color change from pink to colorless instantaneously
(Fig. 4 inset).
A relative stronger binding ability of Cys/Hcy/GSH and Cu2+ over
that of 1 toward Cu2+ in the ensemble of 1-Cu2+ clearly contributes
to high sensitivity in the detection of Cys/Hcy/GSH, and is also
responsible for a good selectivity over other amino acids. Titration
of 1-Cu2+ complex with Cys/Hcy/GSH was then conducted by UV–
vis spectroscopy. As shown in Figure 5, Figure 6 and Figure S14,
upon addition of increasing amounts of Cys/Hcy/GSH to a solution
of 1-Cu2+ (2 Â 10À5 M) in spectroscopic grade EtOH and HEPES buf-
fer (0.02 M, pH 7.4) (3:7, v/v), the absorption band at 562 nm dis-
appeared with decreasing intensity and reached its minimum upon
addition of 4.0 equiv Cys/Hcy/GSH. The clear color change from
pink to colorless was easily observed by naked eyes. The binding
constants for Cys/Hcy/GSH and 1-Cu2+ were then calculated and
found to be in accordance with the observed selectivity trend.
The binding constants of Cys/Hcy/GSH and 1-Cu2+ (6.30 Â 1010
M
À2, 1.39 Â 1011 MÀ2 and 3.74 Â 1012 MÀ2) (Fig. S23–S25) are larger
than that of 1 with Cu2+ (4.44 Â 109 MÀ2).15 Therefore, the ligand 1
in 1-Cu2+ complex can be replaced by Cys/Hcy/GSH, and 1-Cu2+ is
able to selectively discriminate Cys, Hcy, or GSH among other ana-
logs (Scheme 2).
Figure 4. Absorbance spectra of 1-Cu2+(2 Â 10À5 M of 1 with addition of 2.0 equiv
of Cu2+) in EtOH-HEPES (0.02 M, pH 7.4) (3:7, v/v) upon addition of different amino
acids (6.0 equiv). Inset: color changes of 1-Cu2+ upon addition of different amino
acids (6 equiv) (Free:1-Cu2+; 1:Phe; 2:Ala; 3:Met; 4:Gly; 5:Glu; 6:Gln; 7:Arg; 8:Lys;
9:Leu; 10:Pro; 11:Ser; 12:Thr; 13:Asp; 14:Tyr; 15:Val; 16:Ile; and 17:His).
Figure 2. UV titrations of 1 (2 Â 10À5 M) in EtOH -HEPES (0.02 M, pH 7.4) (3:7, v/v)
upon addition of amounts of Cu2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1,
1.2, 1.3, 1.4, and 1.5 equiv). Inset: absorption intensity of 1 at 562 nm as a function
of Cu2+ concentration.
Figure 5. UV titrations of 1-Cu2+ (2 Â 10À5 M) in EtOH-HEPES buffer (0.02 M, pH
7.4) (3:7, v/v) in the presence of different amounts of Cys (0, 0.1, 0.2, 0.3, 0.4, 0.5,
0.7, 1.0, 1.5, 2.0, and 4.0 equiv) Inset: absorption intensity of 1-Cu2+ at 562 nm as a
function of Cys concentration.

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Scheme 2. Proposed mode of 1-Cu2+ and Cys/Hcy/GSH.
In conclusion, a 1,8-naphthyridine modified rhodamine B-based
receptor ensemble bearing a Cu2+ center was investigated as a
selective chemosensor for Cys/Hcy/GSH over other naturally occur-
ring a-amino acids in a 70% aqueous solution. It was demonstrated
that a relative stronger binding ability of Cys/Hcy/GSH and Cu2+
over that of 1 toward Cu2+ in the ensemble of 1-Cu2+ contributed
to the high sensitivity and selectivity in the detection of Cys/Hcy/
GSH. The present system provided an easy, rapid, and selective
detection of Cys/Hcy/GSH with dramatic color changes, from color-
less to pink with Cu2+ and finally to colorless in the presence of
Cys/Hcy/GSH, which was easily observed by naked eyes.
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This work was supported by the Foundation of the Department
of Science and Technology of Yunnan Province of China
(2011FB013, 2011FB047) and the Natural Science Foundation of
China (NSFC Grant Nos. 21002086, 21102127, 21262045,
21262050).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
09.089.