C. Das et al.
InorganicaChimicaActa482(2018)292–298
because of the discovery of presence of Cu(II)-His species in human
body which is helpful in the treatment of Menkes disease [53]. A Cu(II)-
containing chemosensor has been developed by Chan and coworkers
which has high selectivity and sensitivity towards histidine and has
applicability towards intracellular fluorescence imaging [54]. Al3+
complex of a pyridoxal derivatized polyether ligand was found to detect
histidine in urine with a detection limit of 0.3 µM in HEPES buffer [55].
However, displacement of Cu(II) by His to turn on excited-state in-
tramolecular proton transfer (ESIPT) is unknown yet. ESIPT mechanism
is a very rapid process which ranges from fraction of picoseconds to
tens of picoseconds, as proton transfer is much easier and speedy than
that of electron transfer in excited state [56,57]. ESIPT compounds
have drawn much attention in recent years due to their potential ap-
plications in optical devices that may take advantage of the salient
properties such as the ultra-fast reaction rate and extremely large
fluorescence Stokes shift [58] which helps to avoid spectral overlap
between absorption and emission spectra.
Vidovic et al. reported copper complexes of pyridoxal-semicarbazide
Schiff base [59]. Herein, we report a pyridoxal-semicarbazone based Cu
(II) complex, [Cu(LH2)Cl2]·2H2O, which is a solvatomorph of the earlier
reported Cu(II) complex of the same lignad, as ESIPT based turn-on
fluorescent probe for selective detection of L-histidine not only in
aqueous medium but also in living cells. Though various metal com-
plexes of the pyridoxal-semicarbazone Schiff base are known [60–62]
the possibility of vast range of applications using these complexes are
yet to be adequately explored. The Cu(II) present in the chemosensor
quenches the fluorescence of the ligand and constructs a non fluor-
escent off state due to its intrinsic paramagnetic property and due to the
deprotonation of the particular eOH proton which participates in
ESIPT. Upon addition of copper binding analyte His, the strong and
effective chelation of Cu(II) by His leads to displacement of the fluor-
escent ligand from the Cu(II) coordination sphere leading to switch on
of the fluorescence. Using this principle, one can also detect histidine in
human urine with our reported Cu(II)-complex.
added drop by drop. The mixture was neutralised by adding small
amount of anhydrous Na2CO3 solution (0.158 g, 1.5 mmol dissolved in
5 ml H2O) and stirred at room temperature for two hours and then
filtered [61,63]. The filtrate was allowed to evaporate slowly at room
temperature. After 2 days white, rod shaped, shiny crystals of ligand
LH2·2H2O, suitable for X-ray diffraction studies, were obtained. Since
the quality of the single crystals were found to be not of a satisfying
quality, therefore, only preliminary structural parameters are being
reported here. The details of X-ray diffraction studies with ORTEP
diagram were given in Electronic Supporting Information (Fig. S1,
Table S1, ESI).
Yield: 225 mg (88%). Anal.calc. for C9H16N4O5 (M.Wt.:260.25): C,
41.54; H, 6.20; N, 21.53. Found: C, 41.93; H, 6.37; N, 21.81%. 1HNMR
(DMSO-d6): δ(ppm): 10.61(1H,s), 8.38(1H,s), 7.92(2H,s), 6.44(1H,s),
5.25(1H,s), 4.55(2H,s), 2.44(3H,s) (Fig. S2). Electronic spectrum in
H2O (pH 7.4, 0.01 M HEPES buffer solution,
−1cm−1): 290 (57292), 372 (42330) (Fig. S3). MS: ESI-MS- m/z:
225.12 [LH2 + H+] (Fig. S4). Selected IR bands (cm−1): 3436(νNasHym2 ),
3358(νNH2
sym ), 3173(νNH), 1709(νC=O), 1596(νC=N) and 1476 (νC-O) (Fig.
λmax/nm (εmax/
M
S5).
2.3. Preparation of [Cu(LH2)Cl2]·2H2O
To a vigorously stirred methanolic solution (10 ml) of the pyridoxal
hydrochloride (0.203 g, 1 mmol) a solution of CuCl2·2H2O (170 mg,
1 mmol) in methanol (10 ml) was added drop wise. Next, semicarbazide
hydrochloride (0.111 g, 1 mmol) dissolved in methanol (10 ml) was
added drop by drop to the suspension. The solution turned deep green.
The mixture was stirred at room temperature for half an hour, refluxed
for another 2 h and then filtered. The filtrate was allowed to evaporate
slowly at room temperature. After 2 days green, square shaped, shiny
crystals of [Cu(LH2)Cl2]·2H2O, suitable for X-ray diffraction studies,
were obtained (Fig. 1). The details of X-ray diffraction studies are given
in Electronic Supporting Information (Table S2 and S3 in ESI). A
comparison of the crystallographic refinement details along with bond
lengths and bond angles of our Cu(II) complex with the previously re-
ported complex are presented in Tables S4 and S5 in ESI.
2. Experimental
2.1. Materials and methods
Yield:
333 mg
(84%).
Anal.calc.
for
C9H16Cl2N4O5Cu
(M.Wt.:394.70): C, 27.36; H, 4.05; N, 14.19. Found: C, 27.32; H, 4.07;
Pyridoxal hydrochloride and CuCl2·2H2O were obtained from
Aldrich and semicarbazide hydrochloride was purchased from BDH Ltd.
All other chemicals and solvents were of reagent grade and used as such
while solvents for spectroscopic and cyclic voltammetry studies were of
HPLC grade obtained from Merck or Aldrich. Elemental analyses were
performed on a Perkin-Elmer 2400C, H, and N analyzer. Infrared
spectra were recorded as KBr pellets on a JASCO FT-IR-460 spectro-
photometer. UV-Vis spectra were recorded using a JASCO V-530 spec-
trophotometer. Cyclic and differential pulse voltammetry experiments
were carried out using a CH1106A potentiostat. A three-electrode
configuration, with glassy-carbon working electrode and Pt-auxiliary
electrode, Ag, AgCl/saturated KCl reference electrode and TEAP as
supporting electrolyte, was used. Under our experimental conditions
the ferrocene/ferrocenium couple was observed at E0 (△Ep) = 0.48 V
(100 mV). 1H NMR spectra were recorded on a Bruker AVANCE DPX
300 MHz or 400 MHz spectrometer using, Si(CH3)4 as internal standard.
ESI-MS spectra of the samples were recorded on JEOL JMS 600 in-
strument. Fluorescence titration experiments were carried out using PTI
made QuantaMaster40 spectrofluoremeter.
N, 14.21%. MS: ESI-postive ion mode- m/z: 358.13 [Cu(LH2)Cl2] (Fig.
2.2. Preparation of ligand LH2·2H2O
Though the ligand was reported earlier [59], we synthesized it in a
modified procedure to grow single crystals suitable for X-ray diffraction
studies. To a vigorously stirred methanolic solution (10 ml) of the
pyridoxal hydrochloride (0.203 g, 1 mmol) a solution of semicarbazide
hydrochloride (0.111 g, 1 mmol) dissolved in methanol (10 ml) was
Fig. 1. Molecular Structure of [Cu(LH2)Cl2]·2H2O. Only non hydrogen atoms
are labeled and their thermal ellipsoids are drawn at 50% probability. The
water molecules of crystallization are omitted for clarity.
293