Protein assisted fluorescence enhancement of a dansyl containing fluorescent reagent: Detection of Hg+ ion in aqueous medium

Article abstract of DOI:10.1039/c1ob05540b

Intramolecular charge transfer (ICT) based fluorescent reagents containing a dansyl fluorophore have been synthesized and characterized. The reagent 1 and its complex, 1+Hg²⁺ in sodium acetate buffer (pH 6.7) revealed considerable fluorescence

Full text of DOI:10.1039/c1ob05540b

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Organic &
Biomolecular
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Protein assisted fluorescence enhancement of a dansyl containing fluorescent
+
reagent: Detection of Hg ion in aqueous medium†
Priyanka Srivastava, Mohammad Shahid and Arvind Misra*
Received 6th April 2011, Accepted 20th May 2011
DOI: 10.1039/c1ob05540b
Intramolecular charge transfer (ICT) based fluorescent
reagents containing a dansyl fluorophore have been syn-
thesized and characterized. The reagent 1 and its complex,
2+
1
+Hg in sodium acetate buffer (pH 6.7) revealed consider-
able fluorescence enhancement (switched-on) in the presence
of bovine serum albumin (BSA) with 10 ppb detection
1
sensitivity. H NMR spectral analysis suggests complexation
2+
between 1 and Hg ion involving the N,N-dimethylamino
and carboxylic functions.
Fig. 1 Synthesized fluorescent reagents.
The development of a selective and sensitive fluorescent tool for
detection of heavy and transition metal (HTM) ions is currently
receiving great interest among the scientific community because
7
4
-aminomethylcyclohexanecarboxylic acid (TAC). As a model,
reagent 3 (Fig. 1) with an extended flexible linker arm was
prepared by reacting DNS first with caproic acid to give 2. The
activated NHS ester of 2 subsequently coupled with TAC to
obtain 3 by a carboxamide bond formation (Scheme 1, ESI†).
The use of an amino acid not only enhances the aqueous medium
compatibility of the probe but also the typical trans arrangement of
the dansyl and carboxyl functions at the 1,4-termini will segregate
the amino and carboxyl functions from self-interaction, and would
also enhance the electrostatic interaction with protein molecules.
Moreover, a cis-1,3 isomer based probe could also be employed,
however, it has not been used because it is expected that in the cis-
1
2+
of their related environmental and health problems. The Hg
ion is commonly spread in the environment by anthropogenic and
industrial releases and is converted to toxic methylmercury by
bacterial and chemical actions. The bioaccumulation of such toxic
stuff in living tissues of human and animal bodies, through the
2
food chain, causes mercury poisoning and lethal diseases. The
maximum allowable concentration of mercury in food and the
environment, as recommended by the FDA and EPA, is in the
1b
range of 1–2 ppb and beyond that it becomes toxic.
Although several methods have been developed for the detection
of HTMs at low concentrations, fluorescence based detection
methods are more reliable, inexpensive, sensitive, easy to perform,
1
,3 conformation the probability of self interaction between two
functional groups will be high, and also complexation would be
more favored in the close vicinity of sulfonamide and carboxylic
functions rather than between the amino and carboxyl functions,
and under these conditions the desired optical behavior might
be hampered. Secondly, the synthesis of the 1,3 derivative is
tedious and favors lactamization, and opening of the lactam most
of the time leads to polymerization. Dansyl as a fluorophore
is implicated because it is sensitive to the variation in external
environment and a slight variation in the medium would result
in a dramatic change in its typical and strong fluorescence
with high emission quantum yield, attributed to intramolecular
charge transfer (ICT), involving the N,N¢-dimethylamino and
3
and are suitable for real time monitoring of anions and metal ions.
4
The poor solubility of sensing systems in aqueous medium and
nonspecific fluorescence quenching induced by HTMs due to en-
5a
5b,c
hanced spin–orbit coupling and electron transfer mechanism
make them impractical as good analytical tools. The conjugation
of a fluorophore with a protein will be a promising way to
7d,e
5d
overcome this limitation. Therefore, a sensitive fluoroionophore
having good optical properties in aqueous media and upon interac-
2
+
tion with metal ions like Hg showing fluorescence enhancement
6
(
FE) is essential for naked-eye visible recognition.
In view of this new reagents have been synthesized by reacting
dansyl chloride (DNS) with an analogous amino acid, trans-
8–9
sulfonamide groups. Such typical optical behavior has led dansyl
fluorophore to be a core structure in most fluorescent sensors for
detection of cations, anions and in supramolecular structures
10
11a
11b–c
11d
Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, 221005, India. E-mail: arvindmisra2003@yahoo.com; Fax: +91-
such as calixarenes
and dendrimers.
-4
The absorption spectra of reagents (1 ¥ 10 M) in sodium
acetate buffer (50 mM) illustrated the high and low energy bands
in the range of ~247–244 (e = 103 200, 83 100, 56 500 M cm )
and 332–336 nm (e = 29 200, 23 500, 13 900) due to p–p* and
0
542-2368127; Tel: +91-542-6702503
†
Electronic supplementary information (ESI) available: Experimental
-
1
-1
1
13
details, UV-Vis, fluorescence, H and C NMR spectroscopic data. See
DOI: 10.1039/c1ob05540b
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n–p* electronic transitions, respectively. At 335 nm excitation
wavelength reagents exhibited an emission band at ~555 nm
In order to understand the possible reversibility and binding of
2
+
1 with Hg ion, a strong chelating reagent, EDTA, was added
2
+
(
Fig. 2) in which the fluorescence intensity of 1 was relatively higher
to the solution of the probable complex, 1+Hg . The revival
of fluorescence of almost similar emission intensity suggested
a reversible mode of complexation. Conversely, when excess of
compared to 3 (U = 0.65 and U = 0.54 with respect to qunine
1
3
sulfate in ACN). The low fluorescence intensity of 3 may obviously
be attributed to restricted ICT, because of the extended flexible
linker arm of TCA that favors more self interaction between the
N,N-dimethylamino and carboxylic functions, and hence leads to
diminished fluorescence due to hindrance in charge transfer from
the amino to the sulfonamide unit.
2
+
Hg was added to the solution of 1 containing EDTA (50 equiv)
the relative fluorescence intensity remained unaffected (data not
shown). Thus, the preliminary observations suggested a high
2
+
affinity of 1 for Hg ion.
Further, bovine serum albumin (BSA) is known to generate well
12
defined hydrophobic–hydrophilic cavities in aqueous media and
the microenvironment sensitive dansyl fluorophore exhibits en-
hanced fluorescence in hydrophobic media while the fluorescence
quenching often occurs in hydrophilic media, and also in acidic
conditions possibly due to the protonation of dimethylamino
group, and consequently, hindrance in charge transfer to the
8–9
naphthyl ring from the dimethylamino unit. Thus, keeping in
2
+
mind the challenges of turn-on response to Hg ions in aqueous
medium, chemical sensors based on amino acids would be a
13
possible route for development, in conjunction with proteins.
Therefore, we observed the sensing behavior of reagents in the
presence of proteins like BSA and fetal bovine serum (FBS).
The emission spectrum of BSA at excitation wavelength lex
=
2
78 nm shows a strong emission at ~345 nm due to tryptophan
Fig. 2 UV-Vis absorption spectra. (b) Emission spectra of 1 and 3 with
and without BSA (10 mM) in AcONa buffer (50 mM; pH = 6.7).
14
2+
domains (Fig. S4†). Upon gradual addition of Hg ions (0–
4 equiv) to BSA solution significant fluorescence quenching was
4
observed, in which the emission band centered at 345 nm was
blue-shifted (~8 nm) to appear at 337 nm (Fig. S4). On the other
hand, on a gradual increase in BSA concentration (0–30 mM) in
the solution of 1 (40 mM), fluorescence enhancement (~5 fold)
occurred with a blue shift of ~56 nm, and emission spectra were
almost saturated after the addition of 20 mM of BSA (Fig. 4a).
+
+
2+
+
2+
Upon addition of different metal ions (Na , K , Ca , Ag , Co ,
2
+
2+
2+
2+
2+
2+
Pb , Zn , Cu , Hg , Ni Cd ; 15 equiv, 12 ml) to individual
solutions of reagents (40 mM) the emission spectra of 1 only
revealed significant fluorescence quenching with Hg ion (Fig. 3)
while no significant change was observed with reagents 2 and 3
Fig. S1†). A competitive metal ion study exhibited insignificant
2
+
(
2
+
change in the fluorescence spectrum of complex 1+Hg , either by
the addition of Hg (30 equiv) to the solution of 1 and then an
2
+
2
+
excess of other metal ions, or by the addition of Hg to a solution
of 1 containing an excess of other tested metal ions (Fig. S2). Job’s
plot analysis from the fluorescence titration data have revealed
2
+
a 1 : 1 stoichiometry between 1 and Hg ion with a binding
4
-1
constant, Kass. = 2.38 ¥ 10 M calculated by Benesi–Hildebrand
B–H) method. A good linear (R 0.992) Stern–Volmer plot with
quenching constant, KS–V = 9000 M suggested a dynamic mode
of fluorescence quenching with Hg (Fig. S3b, c).
2
(
-
1
2
+
Fig. 4 Fluorescence titration spectra of 1 with BSA in AcONa buffer
(
50 mM; pH = 6.7). Inset shows the change in fluorescence intensity with
respect to concentration of BSA. (b) Job’s plot indicating 1 : 1 and 1 : 2
stoichiometry between 1 and BSA.
Similarly, 3 also shows a blue shift but the increase in intensity
was relatively less (Fig. 2b). This is attributable to a structurally
more favored interaction of 1 compared to 3 in the hydrophobic
region of BSA. The observed 1 : 1 and 1 : 2 stoichiometries between
1
and BSA from the Job’s plot have suggested concentration
dependence. On increasing the concentration of either 1 or BSA,
keeping the other one constant, the stoichiometry varies from
1 : 1 to 2 : 1 and vice versa with relative increase and decrease in
fluorescence intensities. Since the titration curves could not be well
fitted mixed interaction between 1 and BSA is expected.
Fig. 3 Emission spectra of 1 upon interaction with various metal ions
+
+
2+
+
2+
2+
2+
2+
2+
2+
2+
(
M = Na , K , Ca , Ag , Co , Pb , Zn , Cu , Hg , Ni Cd ; 15
2+
equiv). Job’s plot shows 1 : 1 equilibrium between 1 and Hg ion in AcONa
buffer.
5
052 | Org. Biomol. Chem., 2011, 9, 5051–5055
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At low concentration of 1 a good linear fit of the fluorescence
titration curves favored a 1 : 1 equilibrium with an association
4
-1
constant Kass = 6.9 ¥ 10 M (Fig. 4b, 5a) while a low binding
3
-1
constant, Kass = 4 ¥ 10 M was obtained for 3. Similarly, the
interaction of 1 with FBS shows an increase in fluorescence
intensity with a blue-shift (Fig. S5†). Further, the unfolding
process of serum albumin on increasing the concentration of urea
12
is very well established. To see the denaturing effect of the protein
on its binding affinity with 1 and related optical properties, a
urea solution (0–3 M) was added gradually to the solution of
protein-bound probe, 1+BSA. The emission profile (Fig. 6) shows
conversely a decrease in fluorescence intensity along with a red-
shift. This clearly suggested about the weakening of probe–protein
binding upon addition of urea, and hence, the release of the probe
in a hydrophilic environment.
Fig. 7 Bar diagram showing interaction of 1 and 3 with tested metal ions
in the presence of BSA in AcONa buffer. Inset shows change in color of 1.
The competitive metal ion interaction studies of 1 in BSA
2
+
revealed a high affinity for Hg ion, and the emission generated
2
+
in 1+BSA+Hg solution remained unaffected when other metal
2
+
ions tested were added, except for Cu ion which shows a marginal
decrease in the relative fluorescence intensity (Fig. 8b, Fig. S7 bar
diagram). To observe the binding affinity of the protein-bound
2
+
probe, 1+BSA for Hg ion, a fluorescence titration experiment
was performed. The spectral data revealed fluorescence enhance-
ment upon the addition of 1 equiv of Hg ion. The Job’s plot
analysis revealed a 1 : 1 stoichiometry and the association constant
2
+
2
Fig. 5 Benesi–Hildebrand plots (a) 1+BSA and (b) 1+BSA+Hg (c) Job’s
plot indicates 1 : 1 equilibrium between 1+BSA and Hg ion.
2+
estimated from fluorescence titration data was found as Kass
=
=
5
-1
5d
2.2 ¥ 10 M (Fig. 5c, 5b) with 10 ppb level sensitivity (LOD
.05 mM).
0
Fig. 6 Change in emission spectra of 1+BSA upon increasing concentra-
tion of urea (0–3 M) in AcONa buffer (50 mM; pH = 6.7).
2
+
Fig. 8 Fluorescence titration spectra upon addition of Hg ion (0–1.0
equiv) to 1 + BSA (1 : 1/4; v/v) in AcONa buffer (50 mM; pH = 6.7). (b)
2
+
Thus, the turn-on fluorescence to detect Hg ion using BSA
as a support protein is expected. The optimum concentration of
2+
Interference study of 1 + BSA + Hg with tested metal ions.
2
+
2+
BSA for Hg ion has been estimated by the addition of different
To arrive at the actual mechanism involved in Hg ion
interaction, H NMR spectra were recorded in DMSO-d
1
concentrations (0–25 mM) of BSA to a solution of 1 (40 mM).
6
(Fig. 9).
2
+
1
After addition of Hg (in excess) in each vial, emission spectra
The H NMR spectrum of 1 shows a doublet at d 8.48–7.29
ppm due to the H2, H8, H4, and H6 protons. The carboxylic
group appears as a broad resonance at d 12.0 ppm (Fig. S9). A
triplet attributable to the -NH proton of sulfonamide appeared
at d 7.91 ppm and H3, H7 as a multiplet at d 7.64 ppm. The
aliphatic protons of the cyclohexane ring resonated in the range of
and methylene (-CH NH) protons
appeared at d 2.84and2.61ppmas asinglet andtripletrespectively.
Upon addition of 2 equiv of Hg ion, the H2 and H8 protons
were recorded and found a maximum fluorescence enhancement
(
1
(
~4.5 fold) with a blue shift of ~56 nm for the solution containing
0–12.5 mM (0.25–0.3 equiv) of BSA, and a further increase of BSA
15–30 mM) exhibited only ~1.2 fold rise in emission intensity (Fig.
S6†). Therefore, keeping the favored 1 : 1 complexation, a metal ion
10 equiv) interaction study was performed by reducing the BSA
concentration to 1 : 1/4 (v/v) in NaOAc buffer. Upon addition
(
d 2.61–0.75 ppm. The -N(CH
3
)
2
2
2
+
2+
of Hg (0–1 equiv) to 1+BSA solution, ~10 fold fluorescent
enhancement is observed in comparison to the probe without BSA,
merged together to appear at d 8.56 ppm (Dd = 0.086), whereas
the resonance of the carboxylic function disappeared (Fig. S11).
The resonances of the cyclohexane ring and methylene protons
more or less shifted upfield, and the H2¢ and H3¢ protons exhibited
maximum upfield shifts of 0.028 and 0.031 ppm respectively. The
2
+
2+
while the other tested metal ions including Cu and Cd have
not induced any significant change (~1–1.2 fold) Also, the model
reagent 3 does not show any significant change in the emission
signal under the same conditions (Fig. 7).
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1
2+
2+
Fig. 9 H NMR spectra of 1 in DMSO-d
6
and after addition of Hg ion (1.5 equiv). Plausible mode of complexation between 1 and Hg ion.
H3, H7 (Dd = 0.12), H6 (Dd = 0.35), -NH (Dd = 0.08), and -N(CH
3
)
2
free thiol of from cysteine amino acid residues present in BSA.
Consequently, the increase in electron withdrawing effect on the
sulfonamide group of naphthalene, and hence the disruption in
(
Dd = 0.24) protons exhibited downfield shifts respectively. This
2
+
clearly suggested the interaction of Hg ion was most favorable
through the N,N¢-dimethylamino and carboxyl functions of the
cyclohexane ring, without deprotonation of sulfonamide, hence
the fluorescence quenching, due to hindrance in ICT. An almost
similar behavior was observed when we measured the H NMR
spectra of reagent 2 in the presence of Hg ion (Fig. S14).
14c
internal charge transfer, causes fluorescence enhancement. The
least possibility of deprotonation was further evidenced when we
carried out the pH titration study (Fig S8). In acidic medium 1
shows fluorescence quenching, possibly due to the protonation of
1
2
+
8
the amino function of the probe, whereas in alkaline medium no
1
9e
Since, H NMR titration spectral data revealed no dramatic
significant fluorescence enhancement was observed.
variations in the resonances of the coupled cyclohexane ring, the
retention of the typical conformation of probe and the possibility
of the formation of any strained geometry after complexation
In conclusion, we have synthesised dansyl containing fluo-
rescent reagents and demonstrated the turn-on fluorescence for
detection of Hg ion in aqueous media, using BSA as a support
protein. Further, studies to deepen our understanding of the
sensing mechanism are in progress.
The authors are thankful to The Department of Science
and Technology (DST) and Council of Scientific and Industrial
Research (CSIR), New Delhi for financial support and fellowships
(to PS and MS).
2
+
2
+
with Hg ion is therefore ruled out. Thus, taking into account the
planar structure of the naphthalene ring we proposed a plausible
2
+
mode of complexation with Hg ion laterally, between the two
functionalities as depicted in Fig. 9. The projected geometrical
arrangement in that sense would generate no significant strained
structure in which interaction between the two moieties is
1
expected, as observed in the H NMR spectra. Further, it is
noteworthy to add that the typical conformation of the probe
remained unchanged even before and after the interaction of Hg
ion in protein medium (BSA, FBS).
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