1
1
A
SPECTROSCOPIC INVESTIGATION OF THE
2
COMPLEX
3
COPPER(II) IN AQUEOUS SOLUTION
OF
TURMERIC
DYE
WITH
4
Kanhathaisong*,
5
Sriwai
Saowanee
6
Rattanaphani, and Thanaporn Manyum
Rattanaphani,
Vichitr
7
8
School of Chemistry, Suranaree University of Technology, Nakhon Ratchasima,
9
Thailand, 30000
10
*
Corresponding author. E-mail: sriwaih@hotmail.com
11
12
Abstract
13
The structure of the complex formed between Cu(II) and curcumin in
14
aqueous solution was investigated using UV-visible spectroscopy. The
15
molar ratio method was applied to ascertain the stoichiometric
16
composition of the complex in aqueous solution. The resulting
17
stoichiometry for Cu(II) to curcumin was 1:2. A structure for Cu(Cur)2
18
was proposed. Curcumin uses the β-diketone moiety as the binding site
19
with copper(II) ion. In addition, IR spectra and diffuse reflectance UV-
20
Vis spectra have been performed to support the experimental
21
observations.
22
23
24
Keywords: Curcumin, spectrophotometry, complexation, copper
2
1
Introduction
2
The rhizome of turmeric, Curcuma longa Linn., is an important source of a
3
natural yellow pigment. The compounds responsible for the yellow pigment
4
are curcumin[1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadien-3,5-dione]
5
(Cur)
6
bisdemethoxycurcumin (BDC) (Jayaprakasha et al., 2002; Jayaprakasha et al.,
7
2005; Jiang et al., 2006). Some studies on curcumin are based on the ionic
8
structure where the keto-enol equilibrium (Figure 1) is present or when it is
9
fully in keto form (Tønnesen et al., 1982; Tønnesen, 1989; Wang et al., 1997)
10
and
2
curcuminoids,
demethoxycurcumin
(DC)
and
with the resulting properties depending on the latter.
11
The water solubility of curcumin is relatively low, although it is
12
soluble in alcoholic media. The β-diketo moiety of curcumin undergoes keto–
13
enol tautomerism. Crystal studies (Tønnesen, et al., 1982; Tønnesen, et al.,
14
1983; Karlsen, et al., 1988) have shown that the symmetric structure of
15
curcumin leads to a statistically even distribution of the enol proton between
16
the 2 oxygen atoms. The strong chelating ability of diketones has been widely
17
investigated towards a great number of metal ions. In such conditions the
18
curcumin molecule should bind with species which have a positive or partially
19
positive charge, for example, a transition metal ion (Bates, 2000).
20
Turmeric dyes have been used as a natural dye for centuries (Moeyes,
21
1993). Unfortunately, the use of natural yellow dyes is related to “poor
22
fastness” due to their problematic issues in light fastness and wash fastness
23
properties. Curcuminoids have particular disadvantages when dyeing onto silk
24
fibroin, cotton and wool (Tsatsarosi et al., 1998; Popoola, 2000; Yoshizumi,
3
1
and Crews, 2003). The solution is usually to use metal ions as mordants
2
(Tsatsaroni et al., 1998).
3
Metal complex formation has been a well-known feature of textile
4
dyeing from very early times, since it was recognized that the technical
5
performance, including fastness to washing and light, of many natural dyes
6
could be enhanced by treatment with certain metal ions, a process known as
7
mordanting (Christie, 2001). In the northeast of Thailand, villagers usually use
8
trial and error methods to find some mordants for silk dyeing (Moeyes, 1993).
9
Alum and salt, iron oxide and copper sulphate are common examples.
10
Copper(II) sulphate pentahydrate (CuSO4∙5H2O) is widely used as a
11
mordant for dyeing silk fibre with polyphenolic dyes to obtain bright colours
12
and improve fastness to light and wash (Christie, 2001).
13
The complexes formation of curcumin with various metal ions have
14
been investigated (Vajragupta, et.al., 2003; Borsari et.al., 2002; Barik, et.al.,
15
2007). The stoichiometries of curcumin with some metal ions were also
16
reported (Borsari et.al., 2002; Barik, et.al., 2007). However, the complex
17
formation of copper(II) with curcumin which is a major composition of
18
curcuminoid dyes in aqueous solution has not yet been reported. The aim of
19
this present work is to determine the stoichiometric composition of the
20
curcumin – Cu(II) complex formed between curcumin and the Cu(II) ion in an
21
aqueous solution. The complex formation between curcumin dyes and the
22
Cu(II) ion were investigated by using the UV-Vis spectrophotometry method.
23
These complexes can be directly applied to dyeing silk.
24
25
4
1
Materials and Method
2
All chemicals in this study, copper(II) nitrate trihydrate (Cu(NO3)2·3H2O),
3
copper(II) sulphate pentahydrate (CuSO4·5H2O), sodium hydroxide (NaOH),
4
ammonium acetate, acetic acid and ethanol were A.R. grade obtained from
5
Merck, while 98% curcumin [458-37-7] in amber widemouthed reagent bottle
6
was purchased from Across Organics.
7
Determination of the Mole Ratio for the Cu(II)-Curcumin Complex
8
The molar ratio method was used to determine the composition of this
9
complex in aqueous solution from spectrophotometric spectra. For this
10
method, fresh 5.0×10-3 M curcumin stock solution was prepared by dissolving
11
0.1842 g of curcumin in 100 mL 95%ethanol directly in the volumetric flask
12
and kept it in an amber reagents bottle. A 5.0×10-3 M copper(II) nitrate stock
13
solution was prepared in deionized water. A concentration of 2.5×10-5 M of
14
curcumin in aqueous solution of 0.5% ethanol was prepared by pipetted 50 μL
15
of 5.0×10-3 M curcumin stock solution into a 10 mL volumetric flask, and
16
adjusted using deionized water. A concentration of 2.5×10-5 M of curcumin in
17
aqueous solution was kept constant whereas the copper(II) nitrate stock
18
solution was varied from 0 to 7.5×10-5 M. The pH was controlled using a
19
buffer system consisting of 4% of 1.0 M ammonium acetate-acetic acid buffer
20
pH 5.5 which was added to a mixture of copper(II) and curcumin dye. In order
21
to reach the complexation equilibrium, the absorption spectra of the solution
22
was recorded after standing for 20 minutes. An Agilent 8453 UV-Vis
23
spectrophotometer was employed for measuring absorbance values using
24
quartz cells of a path length of 1 cm.
5
1
Preparation of Cu(II)-Curcumin Solid Complex
2
The synthesis of the solid 1:2 complexes between copper(II) ion and
3
curcumin was carried out by the addition of 10 mL MeOH to 0.25 mmol
4
curcumin, after which 240 mL deionized water was added, followed by the
5
drop-wise addition of copper(II) nitrate 0.125 mmol in aqueous solution,
6
which was stirred for 2 h at room temperature. After standing overnight, a
7
yellow green solution, a gelatinous residual product appeared. This was then
8
isolated by vacuum filtration, washed with cold MeOH and dried overnight in
9
a desiccator. The solid crude was then recrystalized in MeOH and acetic acid.
10
The crystal was separated and washed with cold MeOH-water. Diffuse
11
reflectance UV-Vis spectrophotometry was used to characterize the solid
12
complex. Diffuse reflectance a measurement from 9,090 to 20,000 cm-1 was
13
recorded as polycrystalline sample using a Perkin–Elmer Lambda 2S
14
spectrophotometer
15
sphere attachment. Sample preparation for IR study was done by using KBr
16
disk technique. The Bio-Rad FTS 175C FTIR spectrophotometry was used to
17
observe the stretching frequency of the carbonyl group of the β-diketone
18
moiety in curcumin and the Cu(II)-curcumin complex.
equipped
with
an
integrating
19
20
Results and Discussion
21
UV-Visible Spectra of Curcumin in Aqueous Solution and at pH 5.5
22
It was found that increasing the Cu(II) concentration decreases the
23
absorption band at 427 nm of curcumin and also results in the appearance of a
24
new band at 361 nm of new compound. The effect of the Cu(II) concentration
25
on the visible spectra (λmax) of Cu(II) curcumins with pH control at 5.5 is
6
1
shown in Figure 2 and indicates a large hypsochromic shift of 66 nm in band
2
427 nm correspond with the study of Barik (Barik, et.al., 2007) and Zhao
3
(Zhao, et.al., 2005).
4
Barik reported that the UV-Vis region showed 2 absorption bands of
5
curcumin ligands: an n→π* transition at ~360-430 nm and π→π* transition at
6
~240-290 nm (Barik, et.al., 2007). The absorption band showed decay of the
7
parent Cu(II)-curcumins band at 427 nm after being mixed with copper(II)
8
ions. The UV-Vis spectrum of curcumin in aqueous solution at pH 5.5 exhibits
9
a maximum adsorption band at 427 nm with an extinction coefficient of
10
31,528 M-1cm-1. A new band which increases with the amount of added
11
copper(II) appeared at 361 nm with an extinction coefficient of
12
M-1cm-1.
18,772
13
The 427 nm peak due to curcumin in aqueous solution is attributed to
14
the n→π* transition of curcumin. A past study of the stoichiometry of various
15
metals with curcumin showed different ratio of 1:1, 1:2 and most 1:2
16
complexes were arranged in nearly planar form (Shen, et.al., 2005). A 361 nm
17
band (Figure 2) is attributed to the curcumin →Cu(II) charge transfer band
18
(TØnnesen and Karlsen, 1992). This study corresponds to Barik, et.al. (2007)
19
in which the β–diketone of curcumin chelated with copper in DMSO and
20
Zhao, et.al. (2005) showing a large blue shift of the β–diketone of trans-
21
ASTX with copper from 480 nm to 373 nm in ethanol.
22
23
24
7
1
Complex Stoichiometry of Curcumin and Copper(II) in Aqueous Solution
2
at pH 5.5
3
In this study, the stoichiometry of the complex was determined by
4
using the molar ratio method. Increasing the copper(II) ion resulted in a
5
decrease of absorbance at 427 nm (Figure 3), which showed inflection at
6
[Cu]/[curcumin] = 0.45, and indicates stoichiometric of Cu(II) to curcumin of
7
1:2 for the complex.
8
The diffuse reflectance UV-Vis spectrum of the solid crystal of Cu(II)-
9
curcumin (Figure 4) showed 3 major absorption bands with wavelength
10
maxima at 316 nm (band I), 472 nm (band II), and 701 nm (band III),
11
respectively. An intense band I which occurred in the UV region is considered
12
to be associated with the absorption due to the π→π* transition of curcumin,
13
and band II at 400-550 nm was due to the n→π* transition of curcumin ligand
14
(Annaraj, et.al. 2004; Barik, et.al., 2007). A broad band III centered on 701
15
nm is usually assigned to a d-d transition band (Welch and Chapman, 1993),
16
indicating that the solid was a compound of copper(II) with curcumin. Cu(II)
17
was present in the complex and coordinated to the curcumin ligand (Figure 4).
18
The IR spectra of the curcumin copper(II) complex showed the
19
υ (C=O) a large intense band at 1,509 cm-1 which is different from band at
20
1,513 cm-1 and 1,588 cm-1 of free curcumin (Figure 5-6). The copper(II)
21
curcumin complex showed a symmetrically large broad band at 3,448 cm-1 of
22
O-H stretching indicating a symmetrical structure of complex compound
23
(Shen, et.al., 2005). The υ (C-O-Cu) band of the complex at 478 cm-1
24
suggested that curcumin forms a complex with copper(II) and using β-
25
diketone part as the binding site (Barik et.al., 2007). Figure 7 showed the
8
1
proposed structure of the Cu(Cur)2 complex in aqueous solution and in solid
2
state.
3
4
Conclusions
5
The interaction of curcumin and Cu(II) was studied by UV-Vis spectroscopy
6
and the significant large hypsochromic shift was observed for the absorption
7
band of the Cu(II)-curcumin complex. Using the molar ratio method, it was
8
found that the stoichiometric composition of the complex in aqueous solution
9
was Cu(curcumin)2. The diffuse reflectance UV-Vis spectra and IR spectra
10
indicated that the complex was reasonably stable both in aqueous solution and
11
in solid state.
12
13
Acknowledgements
14
We gratefully acknowledge support from Suranaree University of Technology,
15
and Kumuang Wittaya School, Chaiyaphum, Thailand.
16
17
References
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5
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13
1
2
3
OH
R 1 3 ''
2 ''
4
2'
1 ''
1'
1
2
4 ''
5HO
O
3
4
5
6
6 ''
3' R 2
7
6'
5 ''
R 1 3 ''
5'
2 ''
HO
O
3
5
2'
1'
1
2
4 ''
4'
OH
O
1 ''
4
6
6 ''
5 ''
5'
Compounds
R1
R2
MW
7
Cur
OCH3
OCH3
368
8
DC
OCH3
H
338
9
BDC
H
H
307
10
12
13
14
15
4'
6'
6
11
3' R 2
7
Figure 1. The equilibrium of keto-enol formations of curcuminoid
OH
14
1
2
3
4
0.8
5
0.7
6
7
8
Absorbance
0.6
427 nm
361 nm
0.5
0.4
0.3
0.2
9
10
11
0.1
200
300
400
500
600
700
800
Wavelength (nm)
12
13
14
Figure 2. Electronic absorption spectra of curcumins (2.5×10-5 M) in
15
aqueous solution in the absence and presence of copper(II)
16
from 0 to 2.5×10-5 M at pH 5.5
17
18
19
15
1
2
0.60
4
5
6
7
8
Absorbance at 427 nm
3
0.55
0.50
0.45
0.40
0.35
[Cu2+]/[Cur] >1 precipitated
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
9
2+
[Cu ] / [Cur]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Figure 3. Absorbance versus [Cu]/[curcumin] molar ratio plots at
427 nm at pH 5.5
16
Absorbance
2.0
1.5
316 nm(band I)
472 nm(band II)
1.0
701 nm(band III)
0.5
0.0
200 300 400 500 600 700 800 900 1000 1100
Wavelength (nm)
1
2
3
4
5
Figure 4. The diffuse reflectance UV-Vis spectrum of solid crystal of
Cu-curcumin complex
17
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Figure 5. FT-IR spectrum of curcumin alone
18
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Figure 6. FT-IR spectrum of of the Cu(curcumin)2 complex
19
1
2
3
4
OCH
5
OCH
3
HO
OH
6
O
H 2O
7
3
OCH
3
3
OH
O
O
OH2
Cu
O
OCH
HO
O
Cu
O
O
O
8
HO
9
10
OH
OCH
3
aqueous solution
OCH
3
HO
OH
OCH
OCH
3
solid state
11
12
13
14
Figure 7. The proposed structure of the Cu(curcumin)2 complexes
3