Anthracene coupled adenine for the selective sensing of copper ions

Article abstract of DOI:10.3762/bjoc.6.44

Anthracene-based adenines 1 and 2 have been designed and synthesized, and their metal ion recognition properties have been established fluorometrically. Both molecules exhibit Cu ²⁺ induced ON-OFF type signaling patterns over the other representative metal ions studied. Compound 1 exhibits 97% quenching of emission in the presence of Cu ²⁺ whilst derivative 2 shows 81% quenching under similar experimental conditions.

Full text of DOI:10.3762/bjoc.6.44

Anthracene coupled adenine for the selective
sensing of copper ions
Kumaresh Ghosh* and Tanushree Sen
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, No. 44.
Department of Chemistry, University of Kalyani, Kalyani-741235, India
doi:10.3762/bjoc.6.44
Email:
Received: 21 January 2010
Accepted: 09 April 2010
Published: 05 May 2010
Kumaresh Ghosh* - ghosh_k2003@yahoo.co.in
* Corresponding author
Editor-in-Chief: J. Clayden
Keywords:
adenine-based anthracene; copper ion selective; fluorescence; metal
ion recognition; PET sensory systems
© 2010 Ghosh and Sen; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Anthracene-based adenines 1 and 2 have been designed and synthesized, and their metal ion recognition properties have been estab-
lished fluorometrically. Both molecules exhibit Cu2+ induced ON-OFF type signaling patterns over the other representative metal
ions studied. Compound 1 exhibits 97% quenching of emission in the presence of Cu2+ whilst derivative 2 shows 81% quenching
under similar experimental conditions.
Introduction
Fluorescent chemosensors for the detection of biologically metal ion with nucleobases plays an important role in the
relevant metal ions have been widely exploited in the field of stability of the nucleic acid structure [25]. Among the nucleo-
supramolecular chemistry [1-4]. Among the various transition bases adenine provides five interactional sites for coordination
metal ions, the copper ion draws significant attention due to its with metal atoms. Although it has already been established that
crucial role in biological systems. This metal ion causes signifi- adenine moiety exhibits preferential coordination of silver ions
cant environmental pollution and also serves as a catalytic [26], the formation of Cu(II) complexes of substituted adenines
cofactor for a variety of metalloenzymes [5-9]. However, is also known in the literature [27]. Depending on the smaller
exposure to high levels of copper, even for a short period of size of the substituent at N9 position of adenine, a preferential
time, can cause gastrointestinal disturbance, while long-term coordination of Cu2+ ion at N7 over N1 or N3 is found to occur
exposure can lead to liver or kidney damage [10-16]. For these for bisadenine ligands [27]. Smaller substituents lead to simul-
reasons, the past few years have witnessed a number of reports taneous binding of silver ion at all the three centers. However,
on the design and synthesis of fluorescent sensors for the detec- in this paper we wish to report that introduction of an anthra-
tion of Cu2+ ions [17-24]. Although there are various reports in cenyl group (a flat hydrophobic surface) at the different posi-
this regard, synthetic receptors with improved binding effi- tions in adenine leads to the preferential binding of Cu2+ ion
ciency are still in demand. In addition, the coordination of the over Ag+ and other transition metal ions studied. In this connec-
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Beilstein J. Org. Chem. 2010, 6, No. 44.
tion, adenine-based receptors 1 and 2 (Scheme 1) have been
synthesized and their metal ion binding properties have been
studied by UV–vis and fluorescence methods. Both 1 and 2 are
found to be selective for Cu2+ in CH3CN containing 0.02%
DMSO. Importantly, 1 is found to be more efficient binder than
2 as reflected in the study.
Figure 1: Design principle for 1 and 2.
In order to find out the possible site for binding of metal ions,
we optimized the geometries of both 1 and 2 at AM1 level in
the gas phase [31]. In 1 the disposition of anthracene at the
Watson–Crick (WC) site is favored over its orientation at the
Hoogsteen (HG) site. This is in accord with the previous report
by Engel et al. [32]. We also noted this phenomenon in our
previous report [30]. The charge densities at the different
nitrogen centers are shown in Figure 2. It is evident from
Figure 2, that the WC sites in 1 and 2 provide relatively greater
charge density than the HG sites. Thus it is presumed that the
binding of metal ion will preferably take place in solution at the
WC site in both cases, although the binding at the HG site
cannot be ruled out. This is in accord with the observation of
Glass et al. [33]. It is worth noting that the introduction of the
alkyl group on the amino group of adenine modulates the
charge densities at the WC site. On going from structure 1 to
Scheme 1: Adenine-based receptors 1 and 2.
Results and Discussion
The syntheses of 1 and 2 are outlined in Scheme 2. First, 9-posi- structure 2, the charge densities at the WC sites are marginally
tion of adenine was alkylated in the presence of NaH in dry increased (Figure 2).
DMF using 9-chloromethylanthracene, obtained from the reac-
tion of 9-anthracenyl alcohol with thionyl chloride, to afford 2 Fluorescence and UV–vis spectroscopic techniques were then
[28] in 50% yield. In a similar manner, 9-butyladenine 3 was employed to ascertain the metal ion recognition properties of
obtained from the reaction of adenine with n-butyl bromide in both 1 and 2 in solution.
the presence of K2CO3 in dry DMF. Further reaction of 3 with
9-chloromethylanthracene led to the compound 1 [29] in 60% Receptor 1 (c = 5.09 × 10−5 M) upon excitation at 348 nm in
yield. All the compounds were characterized by conventional CH3CN containing 0.02% DMSO showed structured emission
methods [30].
centered on 413 nm. Upon the gradual addition of metal ions as
their perchlorate salts to the receptor solution of 1, the emission
The adenine-based molecules 1 and 2 were designed according of 1 decreased by different extents. Figure 3a demonstrates the
to the design principle of a PET sensor, shown in Figure 1. In change in emission of 1 upon the addition of 20 equivalents of
both 1 and 2, adenine is defined as the binding site, which is metal ion and clearly shows that the emission of 1 is quenched
connected to the anthracene probe via –CH2– spacer at the to the greatest extent by the Cu2+ ion. Figure 3b is the
different regions of adenine.
Stern–Volmer plot, which gives a comparative view on the
Scheme 2: Syntheses of receptors 1 and 2.
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Beilstein J. Org. Chem. 2010, 6, No. 44.
Figure 2: AM1 optimized geometries of 1 (E = −162.48 au) of 2 (E = −139.59 au).
quenching of emission of 1. The almost linear nature of the on titration with metal ion solutions as used with compound 1.
curves for 1 in Figure 3b corroborates the quenching dynamic in Like 1, the maximum quenching occurred on the successive
nature and shows a clear distinction between Cu2+ ion and the addition of the Cu2+ salt to a solution of receptor 2. Figure 5a
other metal ions investigated. During interaction of 1 with metal shows the changes in emission intensity of receptor 2 during
ions no additional peak at higher wavelengths either for excimer complexation with Cu2+ ions. Titration experiments were also
or exciplex formation should be observed. Figure 4 shows the performed with the other metal salts, which also caused
spectral change of 1 upon titration with Cu2+ metal ions.
quenching, but not to the same extent as with Cu2+ ion.
Figure 5b shows a comparative view in the change in emission
A similar trend in emission behavior was observed with of 2 during titration with different metal ion solutions and
receptor 2. Receptor 2 (c = 1.48 × 10−5 M) gave an emission Stern–Volmer plot in Figure 6 represents the linear nature of the
spectra of triplet nature with the highest peak at 413 nm on curves indicating dynamic quenching during the interaction
excitation at 348 nm. The emission intensity of 2 was quenched process.
Figure 3: a) Fluorescence ratio (I-I0/I0) of receptor 1 (c = 5.09 × 10−5 M) at 413 nm upon addition of 20 equiv of a particular guest in CH3CN
containing 0.02% DMSO (λex = 348 nm); b) Stern–Volmer plot of receptor 1 (c = 5.09 × 10−5 M) at 413 nm upon addition of 20 equiv of a particular
guest in CH3CN containing 0.02% DMSO (λex = 348 nm).
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Beilstein J. Org. Chem. 2010, 6, No. 44.
Figure 4: Emission spectra of receptor 1 (c = 5.09 × 10−5 M) during
titration with Cu2+ ion (c = 1.4 × 10−3 M) in CH3CN containing 0.02%
DMSO (λex = 348 nm).
Figure 6: Stern–Volmer plot of receptor 2 (c = 1.48 × 10−5 M) at 413
nm upon addition of 20 equiv of a particular guest in CH3CN containing
0.02% DMSO (λex = 348 nm).
From both Figure 5b and Figure 6 it is evident that receptor 2
also shows a similar fluorometric response as observed for 1
and selectivity for Cu2+ ions. The only difference between 1 other metal ions are absent and present in the receptor solutions.
and 2 is the extent of quenching of emission. The greater degree The greater quenching of emission upon addition of Cu2+ to the
of quenching of emission in both 1 and 2 in presence of Cu2+ solutions of both 1 and 2 containing other metal ions (Figure 7)
ions may presumably occur by the chelation enhanced anthra- corroborates the selectivity in the binding process.
cene → Cu2+ π-cation interactions, the paramagnetic effect of
Cu2+ [34-36].
The relative change in emission intensity of 1 at 413 nm was
used to determine the binding constants [37]. The measured
To understand the selective sensing of Cu2+ by both 1 and 2, we emission (I0/ΔI) at 413 nm when plotted against the inverse of
recorded the emission spectra of the receptors with the addition the concentration of guest solution fits almost a linear relation-
of 10 equivalents of Cu2+ to the receptor solutions containing ship. The ratio of the intercept versus slope gives the associ-
10 equivalents of a mixture of other metal ions examined in the ation constants (Table 1). A similar method for determining the
present study. Figure 7 displays the comparative view of the binding constants with 2 was employed. As can be seen from
change in emission of 1 and 2 in the presence of Cu2+ when the Table 1, the receptor 1 has a marked selectivity for Cu2+ over
Figure 5: a) Emission spectra of receptor 2 (c = 1.48 × 10−5 M) during titration with Cu2+ ion (c = 4.4 × 10−4 M) in CH3CN containing 0.02% DMSO
(λex = 348 nm); b) fluorescence ratio (I-I0/I0) of receptor 2 (c = 1.48 × 10−5 M) at 413 nm upon addition of 20 equiv of a particular guest in CH3CN
containing 0.02% DMSO (λex = 348 nm).
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Beilstein J. Org. Chem. 2010, 6, No. 44.
Table 1: Association constants (Ka in M−1) for 1 and 2 with the metal
ions by fluorescence method.
Guestsa
Kab,d for 1
Kac,d for 2
Cu2+
Ag+
5.33 × 104
2.17 × 102
8.80 × 102
2.21 × 103
1.89 × 103
9.10 × 102
2.01 × 103
3.37 × 103
8.88 × 103
1.40 × 103
5.16 × 103
5.72 × 103
5.78 × 103
5.09 × 103
3.41 × 103
1.19 × 103
Co2+
Cd2+
Pb2+
Mn2+
Ni2+
Zn2+
aPerchlorate salts were used;
bDetermined at 413 nm;
cDetermined at 413 nm;
dErrors in Ka were ≤ 5%.
well as the ring nitrogen at WC site in 1 in the presence of the
anthracenyl group on the amine functionality. The binding
constant curves are given in Figure 8 and the linear nature of the
curves establishes 1:1 binding model during the interaction
process.
The ground state interaction properties of 1 and 2 were also
investigated with the same metal ions in CH3CN containing
0.1% DMSO. The concomitant changes in absorption spectra of
1 and 2 were, however, only minor for all metal ions examined
with the exception of Cu2+. Figure 9a shows the changes in the
absorption spectra of 1 upon titration with Cu2+ ions. The
greater change in absorption of 1 at 268 and 325 nm produced
Figure 7: Change in emission of 1 (c = 8.66 × 10−5 M) (a) and 2 (c =
1.29 × 10−4 M) (b) upon addition of 10 equiv amounts of Cu2+ in the
presence and absence of other metal ions in CH3CN containing 0.2%
DMSO.
other metal ions. A similar feature is noted for 2 but the effi- an isosbestic point, indicating the presence of a unique complex
ciency in selectivity is found to be less when compared to 1. in equilibrium with the neutral receptor. The progressive
This is due to the more basic character of the amino group as decrease in absorbance at 366 nm for anthracene moiety further
Figure 8: a) Binding constant curve for 1 with Cu2+ ion; b) binding constant curve for 2 with Cu2+ ion.
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Beilstein J. Org. Chem. 2010, 6, No. 44.
Figure 9: a) Absorption spectra of receptor 1 (c = 5.09 × 10−5 M) upon gradual addition of Cu2+ ion (c = 1.4 × 10−3 M) in CH3CN containing 0.02%
DMSO; b) absorption spectra of receptor 2 (c = 1.48 × 10−5 M) upon gradual addition of Cu2+ ion (c = 4.4 × 10−4 M) in CH3CN containing 0.1%
DMSO.
suggests the strong participation of anthracene in a π-cation weak cation-π interaction with the anthracenyl function, situ-
complexation event. Similarly, receptor 2 (c = 1.48 × 10−5 M) ated at the distal position from the interacting region. Such find-
showed intense absorption bands, characteristic of the anthra- ings in the ground state were not observed in case of other
cene moiety at 348, 365 and 385 nm in CH3CN containing cations even with Ag+ (Figure 10a for receptor 2). This was also
0.1% DMSO. Upon adding metal ions as their perchlorate salts found for 1 with Ag+ ions (Figure 10b). The resulting isotherm
the absorption intensity decreased to the different extents. In fits nicely a 1:1 binding model (Supporting Information File 1).
case of Cu2+, the absorption changed significantly in the region
at 314 nm and was accompanied by an isosbestic point at 372 However, the cation-π interaction in both 1 and 2 is due solely
nm, which indicates the formation of a supramolecular complex to the presence of adenine moiety which is involved in the
between 2 with Cu2+ ion. Figure 9b displays the change in binding of metal ions. This was proved by carrying out the
absorption of 2 with addition of Cu2+ ions in CH3CN UV–vis titration experiment using anthracene only. Figure 11
containing 0.1% DMSO. It is of note that the change in absorb- shows the decrease in absorbance of anthracene upon gradual
ance of the peaks for anthracene moiety is small compared to addition of Cu2+ ions. It is of note that the decrease in absorb-
that in the case of 1 with Cu2+ thereby suggesting a relatively ance of anthracene is much less compared to the cases of 1 and
Figure 10: a) Absorption spectra of receptor 2 (c = 1.48 × 10−5 M) upon gradual addition of Ag+ ion (c = 4.4 × 10−4 M) in CH3CN containing 0.1%
DMSO; b) absorption spectra of receptor 1 (c = 5.09 × 10−5 M) upon gradual addition of Ag+ ion (c = 1.4 × 10−3 M) in CH3CN containing 0.02%
DMSO.
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Beilstein J. Org. Chem. 2010, 6, No. 44.
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Supporting Information File 1
Stoichiometry curves for 1 and 2 with Cu2+ ions.
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