DNA-binding properties studies and spectra of a novel Cu(II) complex with a new coumarin derivative

Article abstract of DOI:10.1248/cpb.56.452

A new coumarin derivative (8-methylcoumaro-4a,10a-pyrone-3-carbaldehyde benzoyl hydrazone) ligand and its novel Cu(II) complex have been synthesized and characterized on the basis of elemental analyses, molar conductivities, ¹H-NMR, IR spectra, UV-visible spectroscopy and thermal analyses. In addition, the interactions of the Cu(II) complex and the ligand with calf-thymus DNA were investigated by spectrometric titrations, ethidium bromide displacement experiments and viscosity measurements. It was found that both the two compounds, specially the Cu(II) complex, strongly bind with calf-thymus DNA, presumably via an intercalation mechanism.

Full text of DOI:10.1248/cpb.56.452

452
Chem. Pharm. Bull. 56(4) 452—456 (2008)
Vol. 56, No. 4
DNA-Binding Properties Studies and Spectra of a Novel Cu(II) Complex
with a New Coumarin Derivative
Gao-fei QI, Zheng-yin YANG,* Dong-dong QIN, Bao-dui WANG, and Tian-rong LI
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou
University; Lanzhou 730000, P. R. China.
Received October 7, 2007; accepted January 17, 2008; published online January 23, 2008
A new coumarin derivative (8-methylcoumaro-4a,10a-pyrone-3-carbaldehyde benzoyl hydrazone) ligand
and its novel Cu(II) complex have been synthesized and characterized on the basis of elemental analyses, molar
conductivities, 1H-NMR, IR spectra, UV–visible spectroscopy and thermal analyses. In addition, the interactions
of the Cu(II) complex and the ligand with calf-thymus DNA were investigated by spectrometric titrations, ethid-
ium bromide displacement experiments and viscosity measurements. It was found that both the two compounds,
specially the Cu(II) complex, strongly bind with calf-thymus DNA, presumably via an intercalation mechanism.
Key words Cu(II) complex; coumarin; interaction; calf-thymus DNA; intercalation mechanism
300-MHz spectrometer in DMSO-d6 (dimethyl sulfoxide) with TMS
(tetramethyl silane) as internal standard. Conductivity measurements were
performed in DMF (N,N-dimethylformamide) with a DDS-11A conduc-
tometer at 25.0 °C. UV–visible spectra were recorded on a Shimadzu UV-
Binding studies of small molecules to DNA are very im-
portant in the development of DNA molecular probes and
new therapeutic reagents.1—3) Over the past decades, the
DNA-binding metal complexes have been extensively studied
240 spectrophotometer. Thermal behavior was monitored on a PCT-2 differ-
as DNA structural probes, DNA-dependent electron transfer ential thermal analyzer. FAB-MS (fast atom bombardment mass spectrome-
try) was obtained on a VG ZAB-HS mass spectrometer. The fluorescence
spectra were recorded on a Hitachi RF-4500 spectrofluorophotometer.
Materials and Methods Calf thymus DNA (CT-DNA) and EB (ethid-
ium bromide) were purchased from Sigma Chemical Co. (Saint Louis, MO,
probes, DNA footprinting and sequence-specific cleaving
agents and potential anticancer drugs.4—6) The interaction of
metal complexes with DNA has been an area of intense inter-
est to both inorganic chemists and biochemists. Many transi-
tion metal complexes, especially copper(II) complexes have
been used as probes of DNA structure in solution.7,8) So de-
sign of small copper(II) complexes that bind and react at spe-
cific sequences of DNA becomes important. Basically, metal
complexes interact with the double helix DNA in either a
non-covalent or a covalent way. The former way includes
three binding modes: intercalation, groove binding, and ex-
ternal static electronic effects. Among these interactions, in-
tercalation is one of the most important DNA binding modes
as it invariably leads to cellular degradation. Intercalators
usually have planar aromatic ring systems that occupy the
space between two adjacent DNA base pairs. Examples of
intercalators include ethidium bromide (EB), acridine or-
ange, and doxorubicin.9) Moreover, it was reported that the
U.S.A.). EDTA and CuCl2·2H2O were produced in China. All chemicals
used were of analytical grade. All the experiments involving the interaction
of the complexes with CT-DNA were carried out in doubly distilled water
buffer containing 5 mM Tris [Tris(hydroxymethyl)-aminomethane] and
50 mM NaCl and adjusted to pH 7.1 with HCl. The solution of CT-DNA in
the buffer gave a ratio of UV absorbance of about 1.8—1.9 : 1 at 260 and
280 nm, indicating that the CT-DNA was sufficiently free of protein.16) The
CT-DNA concentration per nucleotide was determined spectrophotometri-
cally by employing an extinction coefficient of 6600 Mꢀ1 cmꢀ1 at 260 nm.17)
The compounds were dissolved in a mixture solvent of 1% DMF and 99%
Tris–HCl buffer (5 mM Tris–HCl, 50 mM NaCl, pH 7.1) at the concentration
1.0ꢁ10ꢀ5 M. Absorption titration experiments were performed with fixed
concentrations drugs (10 mM) while gradually increasing the concentration of
CT-DNA. While measuring the absorption spectra, an equal amount of CT-
DNA was added to both the compounds solution and the reference solution
to eliminate the absorbance of CT-DNA itself. Viscosity experiments were
conducted on an Ubbelodhe viscometer, immersed in a thermostated water-
bath maintained to 25.0 °C. Titrations were performed for the Cu(II) and the
intercalating ability increases with the planarity of inter- ligand (0.5—3 mM), and each compound was introduced into a CT-DNA so-
lution (5 mM) present in the viscometer. Data were presented as (h/h0)1/3 ver-
sus the ratio of the concentration of the compound and CT-DNA, where h is
the viscosity of CT-DNA in the presence of the compound and h0 is the vis-
cosity of CT-DNA alone. Viscosity values were calculated from the observed
flow time of CT-DNA containing solution corrected from the flow time of
buffer alone (t0), hꢂtꢀt0.18,19)
calators.10,11) As a result, the elucidation of non-covalent
interactions with DNA by small natural products and their
synthetic derivatives have drawn a lot of attention from many
researchers.12—15)
In this paper, our work stems from our interest in synthe-
sizing and evaluating the key DNA-binding interactions of a
new coumarin derivative containing a large planar aromatic
ring systems and its novel copper(II) complex. In our studies,
the interaction of the two compounds with calf-thymus (CT)
DNA was investigated using a host of physical methods like
spectrometric titrations, ethidium bromide displacement ex-
periments and viscosity measurements.
To compare the binding affinity of the two compounds bound to DNA,
fluorescence titration method was used. Fixed amounts of compound were
titrated with increasing amounts of DNA, over a range of DNA concentra-
tions from 2.5 to 20 mM. An excitation wavelength of 330 nm was used.
Further support for the Cu(II) complexes and the ligand binding to DNA
via intercalation is given through the emission quenching experiment. EB is
a common fluorescent probe for DNA structure and has been employed in
examinations of the mode and process of metal complex binding to DNA.20)
A 2 ml solution of 10 mM DNA and 0.33 mM EB (at saturating binding levels)
was titrated by 5—25 mM the Cu(II) and ligand (lexꢂ500 nm, lemꢂ520.0—
Experimental
650.0 nm). According to the classical Stern–Volmer equation21)
:
Instrumentation Melting points were determined on a Beijing XT4-
100X microscopic melting point apparatus. Elemental analyses (C, H, N)
F0/FꢂKq[Q]ꢃ1
were carried out on an Elemental Vario EL analyzer. IR spectra were ob- where F0 is the emission intensity in the absence of quencher, F is the emis-
tained in KBr discs on a Therrno Mattson FTIR spectrophotometer in the
sion intensity in the presence of quencher, Kq is the quenching constant, and
4000—400 cmꢀ1 region. 1H-NMR spectra were recorded on a Varian VR [Q] is the quencher concentration. The shape of Stern–Volmer plots can be
∗ To whom correspondence should be addressed. e-mail: yangzy@lzu.edu.cn © 2008 Pharmaceutical Society of Japan

April 2008
453
Fig. 1. Scheme of the Synthesis of the Ligand (L)
used to characterize the quenching as being predominantly dynamic or
static. Plots of F0/F versus [Q] appear to be linear and Kq depends on tem-
perature.
Preparation of Two Compounds Synthesis of the Ligand: The com-
pounds of 1 and 2 (Fig. 1) were prepared according to the literature.22,23)
Synthesis of the ligand L was in accordance with the following method: an
ethanol solution (10 ml) containing benzoyl hydrazine (1.36 g, 10 mmol) was
added dropwise to another ethanol solution (10 ml) containing the com-
pound 2 (2.56 g, 10 mmol). After stirring at room temperature for 4 h, the
mixture became clear, and soon the yellow precipitate solid was formed after
continuing stirring. The precipitate was collected by filtration and washed
with ethanol. Recrystallization from anhydrous ethanol gave the ligand L,
which was dried in a vacuum. Yield, 86.5%. mp 163—165 °C. 1H-NMR
(DMSO-d6, ppm) d: 11.00 (1H, s, NH), 8.83 (1H, s, CHꢂN), 8.33 (1H, s, 2-
H), 8.03 (1H, d, Jꢂ8.6 Hz, H-10), 6.95 (1H, d, Jꢂ8.6 Hz, H-9), 7.53—7.76
(4H, t, Jꢂ8.8 Hz, ph-H), 8.0 (1H, d, Jꢂ8.8 Hz, ph-H(p)), 6.18 (1H, s, 7-H),
2.40—2.47 (3H, s, CH3). FAB-MS: m/zꢂ375 [MꢃH]ꢃ. IR nmax (cmꢀ1) 1714
(CꢂO of ring b), 1657 (O–CꢂO of ring a), 1617 (CHꢂN), 1600 (CꢂO of
phCO).
Synthesis of the Cu(II) Complex: The ligand (1 mmol, 0.37 g) was dis-
solved in anhydrous ethanol (10 ml) and a solution of CuCl2·2H2O (1 mmol,
0.17 g) in anhydrous ethanol (10 ml) was then added dropwise with stirring.
Then the mixture solution was refluxed on an oil-bath at 80 °C for 4 h with
stirring. After cooling to room temperature, a large amount of green precipi-
tate appeared. It was separated from the solution by suction filtration, puri-
fied by washing several times with ethanol, and dried for 24 h in vacuum.
[CuL·Cl2·H2O]: green. Yield: 83%. IR nmax (cmꢀ1) 3259 (H2O), 1691
(CꢂO of ring b), 1659 (O–CꢂO of ring a), 1598 (CHꢂN), 1544 (CꢂO of
phCO). Lm (S cm2 molꢀ1): 40. Anal. Calcd for C21H16N2O6Cl2Cu: C, 47.91,
H, 3.06, N, 5.32. Found: C, 47.96, H, 3.23, N, 5.78.
Fig. 2. The Suggested Structure of the Complex
solution. The molar conductivity of the Cu (II) complex is
39.5—40 (S cm2 molꢀ1) in DMF, showing that it is non-elec-
trolytes in DMF.24) This means that the chloridion takes part
in co-ordination in coordinative bond.
IR Spectra: The IR of the ligand and its Cu(II) complex is
presented in the experimental section. The IR spectra of the
ligand exhibit bands of the (CꢂO of ring b) and (CꢂO of
phCO) vibrations at the 1714 and 1600 cmꢀ1, but in its
Cu(II) complex they move to the 1691 and 1544 cmꢀ1 re-
spectively, D (ligand-complex) is equal to 23—56 cmꢀ1.
These shifts indicate that the group loses its original charac-
teristics and forms coordinative bonds with metal. The IR
spectra of the Cu(II) complex indicates that the n (CꢂN)
band of the ligand at 1617 cmꢀ1 due to the azomethine link-
age is shifted towards lower frequency 1598 cmꢀ1 (19 cmꢀ1),
indicating that the ligand coordinates to metal ions via the
Results and Discussion
Characterization of the Compounds Properties of the azomethine nitrogen.25) The aqueous n (OH) band of the
Compounds and Structure of the Cu(II) Complex: The ligand Cu(II) complex appears at 3259 cmꢀ1, showing that there is
is soluble in methanol and ethanol, while the Cu(II) complex some water in the complex. In the two compounds, the n
is slightly soluble in methanol, insoluble ethanol. The two (O–CꢂO of ring a) appears at the 1657 and 1659 cmꢀ1 re-
compounds are soluble in DMF; DMSO; insoluble in water; spectively, which means that (–O–CꢂO) of ring ‘a’ does not
benzene and diethyl ether. But they are air stable for ex- take part in co-ordination.
tended periods. Since the crystal structure of the Cu(II) com-
UV–Vis Spectra: The study of the electronic spectra in the
plex has not been obtained yet, we characterized the complex ultraviolet and visible ranges for the ligand and its Cu(II)
and determined its possible structure by elemental analyses, complex was carried out in a buffer solution. The electronic
molar conductivities, IR data, thermogravimetry-differential spectra of ligand has a strong band at lmaxꢂ361 nm, a
thermal analysis (TG–DTA), and UV–vis measurements. The medium band at lmaxꢂ338 nm. The complex also yields two
likely structure of the Cu(II) complex is shown in Fig. 2.
bands, but the two bands at 338 and 361 nm in the ligand are
Stability and Molar Conductivity of the Cu(II) Complex in shifted to 340 and 364 nm or so in its Cu(II) complex. These
Solution: The stability of Cu(II) complex in an aqueous solu- indicate that the Cu(II) complex has been formed.
tion has been studied by observing the UV–vis spectrums
Thermal Analyses: The Cu(II) complex begins to decom-
and estimating the molar conductivities at different time in- pose at 278 °C and there are two exothermic peaks appearing
tervals for any possible change. The tested Cu(II) complex around 278 and 474 °C. The corresponding TG curves show
was prepared in DMF and for experiments freshly diluted in a series of weight loss. Under 278 °C, there are no endother-
phosphate buffer system (at pH 7.4, 7.8). Then, the UV–vis mic peak and no weight loss on corresponding TG curves. It
spectrums and molar conductivities were researched at dif- indicates that there are no crystal solvent molecules. While
ferent time intervals. The investigations reveals that the being heated to 600 °C, the complex becomes its correspon-
UV–vis spectra have remained unaltered for the solutions ding oxide and maintains a constant weight.
and its molar conductance values have no obvious change for
DNA-Binding Mode and Affinity Electronic Absorp-
very freshly prepared and for over the whole experiment tion Titration: Electronic absorption spectroscopy is an effec-
(12 h). It indicates that the Cu(II) complex is quite stable in tive method to examine the binding mode of DNA with metal

454
Vol. 56, No. 4
complex.20,26,27) If the binding mode was intercalation, the p* tion titration data for the ligand at 361 nm and its Cu(II)
orbital of the intercalated ligand can couple with the p or- complex at 364 nm as a function of DNA addition. The ex-
bital of the base pairs, thus, decreasing the p→p* transition tent of hypochromicity in the charge-transfer band as a func-
energy and resulting in the bathochromism. On the other tion of DNA binding, plotted reciprocally as Ao/A versus
hand, the coupling p orbital is partially filled by electrons, [M]/[DNA], is found to provide a good measure of relative
thus, decreasing the transition probabilities and concomi- binding affinity. Informatiom from Fig. 4, we can deduce
tantly resulting in hypochromism.28) Figure 3 shows the ab- easily that the Cu(II) complex intercalates deeply into the
sorption spectra variations of the ligand and its Cu(II) com- DNA base pairs than the free lignad.
plex in the absence and presence of the CT-DNA (at a con-
Fluorescence Spectra: The ligand and its Cu(II) complex
stant concentration of the compounds). The electronic spec- can emit weak luminescence in Tris-buffer with a max wave-
tra of ligand has a strong band at 361 nm, a medium band at length of about 373 nm and 443 nm. The results of the emis-
338 nm. While in the spectra of the Cu(II) complex, the sion titrations for the two compounds with DNA are illus-
strong band and the medium band were shifted to 364 and trated in the titration curves (Fig. 5). Upon addition of DNA,
340 nm respectively. In the presence of CT-DNA, the absorp- the emission intensities at about 443 nm of the two com-
tion bands of Cu(II) complex at about 340 nm and 364 nm pounds grow to around 1.30 and 1.53 times larger, respec-
exhibits hypochromism of about 8.81% and 14.9%, and tively, than those in the absence of DNA. The results of the
bathochromism of about 2 and 3 nm, respectively. The ligand emission titrations suggest that both the compounds are pro-
at 338 nm and 361 nm exhibits hypochromism of about tected from solvent water molecules by the hydrophobic en-
4.61% and 6.89%, and bathochromism of about 1 and 3 nm. vironment inside the DNA helix, and that the Cu(II) complex
It is noteworthy that the hypochromicity of the complex is can be protected more efficiently than the ligand. This im-
greater than that of the present ligand.
plies that both the compounds can insert between DNA base
The degree of hypochromism generally correlates well pairs and that the Cu(II) complex can bind to DNA more
also with overall binding strength.28) Figure 4 shows absorp- strongly than the ligand. In order to further illustrate this
point clearly, changes in emission intensities for the ligand
and the Cu(II) complex have been plotted against the added
Fig. 3. (a) Electronic Spectra of the Ligand (10 mM) in the Presence of In-
creasing Amounts of CT-DNA
[CT-DNA]ꢂ0—22.5 mM. Arrow shows the absorbance changes upon increasing CT-
Fig. 4. Hypochromism in the Visible Charge-Transfer Band as a Function
of [Compound]/[DNA]
DNA concentration.
(b) Electronic Spectra of the Cu(II) Complex (10 mM) in the Presence of In-
creasing Amounts of CT-DNA
Ao/A represents the ratio of absorbance of free compound (in the absence of DNA)
to the absorbance as a function of increasing concentrations of added DNA (0—
22.5 mM).
[CT-DNA]ꢂ0—22.5 mM. Arrow shows the absorbance changes upon increasing CT-
DNA concentration.
Fig. 5. (a) The Emission Enhancement Spectra of the Ligand (10 mM) in the Presence of 0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20 mM CT-DNA
Arrow shows the emission intensity changes upon increasing DNA concentration.
(b) The Emission Enhancement Spectra of the Cu(II) Complex (10 mM) in the Presence of 0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20 mM CT-DNA
Arrow shows the emission intensities upon increasing DNA concentration.

April 2008
455
Fig. 8. Effect of Increasing Amounts of the Ligand and the Cu(II) Com-
plex on the Relative Viscosity of CT-DNA at 25.0 °C
Fig. 6. Changes in Emission Intensities (at about 443 nm) for the Ligand
(10 mM) and Its Cu(II) Complex (10 mM) in the Presence of Calf Thymus
DNA (0—20 mM) in Buffer Solutions
provide indirect evidence for the DNA-binding mode. Figure
7 shows the emission spectra of the DNA–EB system with
increasing amounts of the ligand and the Cu(II) complex.
The emission intensity of the DNA–EB system decreases as
the concentration of the two compounds increased, which in-
dicated that two compounds could displace EB from the
DNA–EB system. The resulting decrease in fluorescence was
caused by EB changing from a hydrophobic environment to
an aqueous environment.29) The quenching plots illustrate
that the quenching of EB bound to DNA by the compounds
are in good agreement with the linear Stern–Volmer equa-
tion. The plots of F0/F versus [Q], Kq is given by the ratio of
the slope to the intercept. The Kq values for the ligand and its
Cu(II) complex are 4.7ꢁ103 and 1.26ꢁ104 Mꢀ1, respectively.
The data show that the interaction of the Cu(II) complex with
DNA is stronger than that of the ligand, which is consistent
with the above absorption.
Viscosity Measurements: As optical photophysical probes
generally provide necessary, but not sufficient, clues to fur-
ther clarify the interactions between the study on complex
and DNA, viscosity measurements were carried out. Hydro-
dynamic measurements that are sensitive to length change
(i.e. viscosity and sedimentation) are regarded as the least
ambiguous and the most critical tests of binding in solution
in the absence of crystallographic structural data. A classical
intercalation model demands that the DNA helix lengthen as
base pairs are separated to accommodate the binding ligand,
leading to an increase in DNA viscosity. In contrast, a partial,
non-classical intercalation of compound could bend (or kink)
the DNA helix, reducing its effective length and, concomi-
tantly, its viscosity.19,30) Viscosity experimental results clearly
show that both the compounds can intercalate between adja-
Fig. 7. (a) The Emission Spectra of DNA–EB System (10 mM DNA and
0.32 mM EB), lexꢂ500 nm, lemꢂ520.0—650.0 nm, in the Presence of 0, 5,
10, 15, 20, 25, and 30 mM Ligand
Arrow shows the emission intensity changes upon increasing ligand concentration.
ꢀ1
Inset: Stern–Volmer plot of the fluorescence titration data of ligand, Kqꢂ4.7ꢁ103
M
.
(b) The Emission Spectra of DNA–EB System (10 mM DNA and 0.32 mM cent DNA base pairs, causing an extension in the helix, and
EB), lexꢂ500 nm, lemꢂ520.0—650.0 nm, in the Presence of 0, 5, 10, 15,
20, 25, and 30 mM Cu(II) Complex
thus increase the viscosity of DNA. The effects of both com-
pounds on the viscosity of DNA are shown in Fig. 8.
Arrow shows the emission intensity changes upon increasing Cu(II) complex con-
centration. Inset: Stern–Volmer plot of the fluorescence titration data of Cu(II) com-
ꢀ1
plex, Kqꢂ1.26ꢁ104
M
.
Conclusions
Taken together, we have synthesized and characterized a
DNA concentration per mole compounds at about 443 nm in new coumarin derivative (8-methylcoumaro-4a,10a-pyrone-
Fig. 6. 3-carbaldehyde benzoyl hydrazone) ligand and its novel
Steady-state emission quenching experiments are also Cu(II) complex. DNA-binding studies indicate that the
used to observe the binding mode of the compounds to DNA. Cu(II) complex and its free ligand can interact with calf thy-
It is well known that EB can intercalate nonspecifically into mus DNA by intercalation mechanism (Fig. 9). Furthermore,
DNA, which causes it to fluoresce strongly. Competitive various comparative experiments show that the Cu(II) com-
binding of other drugs to DNA and EB will result in dis- plex can bind to DNA more strongly than the free ligand,
placement of bound EB and a decrease in the fluorescence which attributes to chelating effect of the copper(II) ion to
intensity. This fluorescence-based competition technique can the free ligand. Chelating effect (metal ion to free ligand) can

456
Vol. 56, No. 4
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