Synthesis, crystal structure, DNA binding properties and antioxidant activities of transition metal complexes with 3-carbaldehyde-chromone semicarbazone

Article abstract of DOI:10.1016/j.inoche.2010.07.005

3-Carbaldehyde-chromone semicarbazone (L) and its Cu(II), Zn(II), Ni(II) complexes were synthesized and characterized on the basis of crystal structure and other structural characterization methods. The metal ions and Schiff base ligand can form mononuclear five-coordination complexes with 1:1 metal-to-ligand stoichiometry at the metal ions as centres. The transition metal complexes may be used as potential anticancer drugs, because they bind to calf thymus DNA via an intercalation binding mode with the binding constants at the order of magnitude 10⁵-10⁶ M^{- 1}, and the metal complexes present stronger DNA binding affinities than the free ligand alone. In addition, the antioxidant activities of the ligand and its metal complexes were investigated through scavenging effects for superoxide anion and hydroxyl radical in vitro, indicating that the compounds show stronger antioxidant activities than some standard antioxidants, such as mannitol and vitamin C.

Full text of DOI:10.1016/j.inoche.2010.07.005

Inorganic Chemistry Communications 13 (2010) 1213–1216
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, crystal structure, DNA binding properties and antioxidant activities of
transition metal complexes with 3-carbaldehyde-chromone semicarbazone
⁎
Yong Li, Zheng-Yin Yang , Zhen-Chuan Liao, Zi-Chao Han, Zeng-Chen Liu
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Carbaldehyde-chromone semicarbazone (L) and its Cu(II), Zn(II), Ni(II) complexes were synthesized and
characterized on the basis of crystal structure and other structural characterization methods. The metal ions
and Schiff base ligand can form mononuclear five-coordination complexes with 1:1 metal-to-ligand
stoichiometry at the metal ions as centres. The transition metal complexes may be used as potential
anticancer drugs, because they bind to calf thymus DNA via an intercalation binding mode with the binding
constants at the order of magnitude 105–106 M−1, and the metal complexes present stronger DNA binding
affinities than the free ligand alone. In addition, the antioxidant activities of the ligand and its metal
complexes were investigated through scavenging effects for superoxide anion and hydroxyl radical in vitro,
indicating that the compounds show stronger antioxidant activities than some standard antioxidants, such as
mannitol and vitamin C.
Received 15 May 2010
Accepted 5 July 2010
Available online 24 July 2010
Keywords:
Transition metal complexes
Schiff base ligand
Crystal structure
DNA binding properties
Antioxidant activities
© 2010 Elsevier B.V. All rights reserved.
Cancer is a disease that has tormented man throughout history. A
manuscript named Bhrigu Samhita in almost 3000 B.C., which is
perhaps one of the earliest mentions of cancer that can be found,
describes the origin and treatment of cancer [1]. Continuing through
the ages, cancer was researched and described by numerous historical
figures including Hippocrates, Galen, and Morgagni. It was not until
the second half of the 20th century that cancer has been more fully
researched and its molecular and cellular basis began to be
understood. And then many effective treatment regiments have
been developed. However, cancer continues to be a major killer due to
a great number of limitations [2,3]. Cisplatin was initially synthesized
by Peyrone in 1844, and its biological effects were accidentally
discovered by B. Rosenberg and co-workers in 1965. And then, many
succedent experiments indicate that cisplatin can be used as efficient
anticancer drugs through binding to DNA via an intercalation mode
[4,5]. In recent years, drug researches suggest that many anticancer
agents, antiviral agents and antiseptic agents take action through
binding to DNA [6,7]. However, its usefulness limited in the
development of resistance in tumor cells and the significant side
effects exhibited have generated new areas of research, which mainly
focused on the search for new metal-based complexes with low
toxicity and improved therapeutic properties [8]. DNA is generally the
primary intracellular target of anticancer drugs, so the interaction
between small molecules and DNA can often cause DNA damage in
cancer cells, blocking the division of cancer cells and resulting in cell
death [9,10]. Small molecules, such as transition metal complexes, can
interact with DNA through the following three non-covalent modes:
intercalation, groove binding and external static electronic effects
[11,12]. Among these interactions, intercalation is the most important
one, because small molecules binding to DNA via intercalation often
possess potential anticancer activities, searching for drugs binding to
DNA via intercalation rouse many scientists' interests during the past
several decades [13].
Chromones (1-benzopyran-4-one) are a group of naturally and
widely distributed compounds which are ubiquitous to green plant
cells and, therefore, they could be expected to participate in the
photosynthetic process [14]. Molecules containing the chromone
structure receive considerable attention in the literatures recently,
mainly due to their biological and physiological activities including
killing many bacterial strains, inhibiting important viral enzymes,
destroying some pathogenic protozoans, antimycobacterial, antifun-
gal, anticonvulsant, antimicrobial and so on [15–17]. And semicarba-
zones derivatives also have aroused with significant interests in the
field of chemistry and biology due to their antibacterial, antifungal,
antimalarial, antineoplastic and antiviral activities. Schiff-bases are an
important class of ligands in coordination chemistry and are found to
possess extensive applications in different fields, which are able to
inhibit the growth of several animal tumors [18]. In addition, the
biological activities of semicarbazones are considered to be related to
their ability to form chelates with metals [19], so the preparation of
their metal complexes is necessary. On the other hand, over
production of activated oxygen species in the forms of superoxide
anion (O−2 •) and hydroxyl radical (HO•), generated by normal
metabolic process, is considered to be the main contributor to
⁎
Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582.
E-mail address: yangzy@lzu.edu.cn (Z.-Y. Yang).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.07.005

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Y. Li et al. / Inorganic Chemistry Communications 13 (2010) 1213–1216
oxidative damages to biomolecules such as DNA, lipids and proteins,
thus accelerating cancer, aging, inflammation, cardiovascular and
neurodegenerative diseases [20]. The potential value of antioxidants
has already prompted scientists to search for the cooperative effects of
compounds for improving antioxidant activity and cytotoxicity [21].
Herein, a new kind of chromone-based ligand (L, Fig. 1) and its
transition metal complexes have been synthesized and characterized.
Furthermore, comparative studies of the interactions of the ligand and
its complexes with calf thymus DNA (CT-DNA) as well as their
antioxidatant activities were investigated systematically. The experi-
mental results suggested that the compounds above would have
potential application for developing new drugs for cancers and efficient
antioxidants.
The transition metal complexes [Cu(II) complex, Zn(II) complex
and Ni(II) complex] were prepared from the novel ligand and
equimolar amounts of M(NO3)2·nH2O (M=Cu, n=2; M=Zn,
n=6; M=Ni, n=4) (see Supplementary material Experimental
Section), and the yields were good to moderate. All the metal
complexes are stable in atmospheric conditions for extended periods
and easily soluble in DMF and DMSO; slightly soluble in ethanol,
methanol and acetone; insoluble in benzene, water and diethyl ether.
The melting points of the metal complexes all exceed 300 °C.
Elemental analyses indicate that the metal complexes are of 1:1
metal-to-ligand stoichiometry complexes at the metal ions as centres.
The data of molar conductivities of the complexes in DMF solution
indicate that all of them are formed as non-electrolytes [22].
be an indicator of the intercalation behavior of the complexes binding to
DNA helix. Their DNA binding mode and affinities were investigated on
the basis of electronic absorption spectroscopy, fluorescence spectra, EB
quenching assay and viscosity measurements.
Titration with UV absorption spectroscopy is an effective method
to examine the binding mode of DNA with compounds. After
compounds intercalating into the DNA base pairs, the π* orbital of
the intercalated compounds can couple with the π orbital of the DNA
base pairs, thus, decreasing the π→π* transition energy and resulting
in the bathochromism. On the other hand, the coupling π orbital is
partially filled by electrons, thus, decreasing the transition probabil-
ities and concomitantly resulting in the hypochromism. Generally, the
binding of an intercalative molecule to DNA is always accompanied by
hypochromism and/or significant bathochromism in the absorption
spectra due to the strong stacking interactions between the aromatic
chromophore of the compounds and DNA base pairs [23]. Therefore,
in order to obtain the evidence for the binding mode of compounds to
DNA, spectroscopic titrations of compounds solutions with DNA have
been performed. The absorption spectra of the ligand and metal
complexes in the absence and presence of DNA are similar and
absorption spectra of the investigated compounds are given (Fig. S1).
In the presence of DNA, the absorption bands at about 308 nm for the
ligand, Cu(II), Zn(II) and Ni(II) complexes exhibited hypochromism of
about 9.14%, 29.99%, 41.46%, and 39.97%, respectively (Table S3),
which were all accompanied by a small red shift of no more than
3 nm. All these indicated that the investigated compounds interact
with DNA via an intercalation binding mode.
The Zn(II) complex crystallized in the monoclinic lattice with a
space group P21/c. Each unit cell contains four molecules (X-ray
diffraction data shown in Table S1 and Table S2). The crystal structure
in Fig. 2 shows that the composition of Zn(II) complex is ZnL(NO3)2,
where L is 3-carbaldehyde-chromone semicarbazone. In the Zn(II)
complex, 3-carbaldehyde-chromone semicarbazone acted as a tri-
dentate ligand, binding to Zn(II) through the oxygen atoms of 3-
carbaldehyde chromone and semicarbazone and nitrogen atom of the
C N group, forming a six- and a five-membered chelate rings with O
(3)–Zn(1)–N(1) and N(1)–Zn(1)–O(1) bite angles of 75.98(6) and
84.88(6), respectively. The coordinated nitrate molecules were
monodentated with Zn(II), and their diatances were 2.0677(16) and
2.0375(16)Å for Zn(1)–O(5) and Zn(1)–O(7), respectively. In
conclusion, elemental analyses, molar conductivities, IR spectra,
mass spectra (data shown in Supplementary material Experimental
Section) and crystal structure analyses suggest that all the powder
metal complexes are quite structurally similar to each other and
structures of Cu(II) and Ni(II) complexes are CuL(NO3)2 and NiL
(NO3)2 (L=3-carbaldehyde-chromone semicarbazone), respectively.
The crystal structure and other structural characterization methods
show that the metal complexes have one parallel ML plane, which may
In order to further test if the compounds bind to DNA via intercalation,
ethidium bromide (EB) was employed, as EB interacts with DNA as a
typical indicator of intercalation. In the UV–vis spectroscopy, the
maximal absorption peak of EB at 479 nm decreases and shifts to
485 nm in the presence of CT DNA (Fig. S2) which is the characteristic of
intercalation mode. Curve (B) shows the absorption of a mixture solution
of EB, DNA and the ligand, Cu(II), Zn(II) or Ni(II) complexes. It is also
found that the absorption at 494 nm has increased comparing with curve
(C). These may result from two reasons: (1) EB bound to the free ligand
and complexes strongly, resulting in the decrease of the amount of EB
intercalated into DNA; (2) there exists competitive intercalation between
the compounds and EB with DNA, so releasing some free EB from DNA–
EB system. However, the former reason could be precluded because there
were no new absorption peaks appearing [24].
The ligand, Cu(II), Zn(II) and Ni(II) complexes can emit fluorescence
in Tris–HCl buffer solution at ambient temperature with a maximum
wavelength at about 446 nm (Fig. S3). The fluorescence emission
intensity of every investigated compound increases steadily with the
increasing DNA concentrations. According to the Scatchard equation, a
plot of r/Cf verus r gave the intrinsic binding constants Kb of (2.29
Fig. 1. The synthetic route of the ligand (L).

Y. Li et al. / Inorganic Chemistry Communications 13 (2010) 1213–1216
1215
DNA solution viscosity [28]. The specific viscosity of the DNA samples
increased obviously with the addition of the compounds, and the
viscosity of DNA samples with the metal complexes increased more
quickly than that of the free ligand (Fig. S5). Consequently, it can be
concluded that the ligand and its metal complexes bind to DNA via an
intercalation mode and the complexes bind to DNA more strongly than
the free ligand, which is consistent with the above spectral results.
On the basis of all the spectroscopic studies together with the
viscosity measurements, we find that the free ligand and metal
complexes can bind to DNA via an intercalative binding mode (Fig. S6)
and the Zn(II) complex bind to DNA more strongly and deeply than
the free ligand and other complexes.
Because the novel ligand and its transition metal complexes
exhibit good DNA binding affinity, it is considered worthwhile to
investigate their other biological activities, such as antioxidant
activity. Antioxidant investigations are usually regarded as the
potential method to cure various life-style related diseases [29]. And
in this paper, the antioxidant activity of the ligand and its transition
metal complexes was studied by comparing their scavenging effects
on superoxide anion and hydroxyl radical. The inhibitory effects of the
tested compounds on O•2− and HO• are concentration related and the
suppression ratio increases with the increasing sample concentration
in the range of the tested concentrations. The antioxidant activity of
the compounds is expressed as 50% inhibitory concentration (IC50 in
μM). IC50 values of the ligand, Cu(II), Zn(II) and Ni(II) complexes for
HO• are 10.170, 1.195, 2.521 and 4.952 μM, respectively (Fig. S7a and
Table S4). The metal complexes clearly show higher scavenging
hydroxyl radicals activity compared with that of standard antiox-
idants like mannitol (IC50: 10.19 μM) [30]. In comparison with all the
compounds, Cu(II) complex shows the higher scavenging effect than
the other compounds. IC50 values for scavenging superoxide anions of
the ligand, Cu(II), Zn(II) and Ni(II) complexes are 32.810, 0.943, 1.630
and 3.318 μM, respectively (Fig. S7b and Table S5). The metal
complexes are better antioxidants than the ligand and the scavenging
superoxide anion ability of Cu(II) complex is stronger than other
complexes. It is reported that the value of ascorbic acid (Vc, a standard
agent for non-enzymatic reaction) for HO• is 8.727 mmol and the
scavenging effect of Vc for O•2− is only 25% at 9.94 mmol [31]. Notably,
the ligand and its metal complexes possess much stronger antioxidant
activities than the standard antioxidants. In addition, we can find that
the prepared compounds exert differential and selective effects on
scavenging radicals in the biological system due to the chelations of
organic molecules to the metal ions. It was believed that the
information obtained from present work would be useful to develop
new potential antioxidants and therapeutic agents for some diseases.
Fig. 2. ORTEP view of Zn(II) complex showing the atom numbering of scheme and 30%
thermal ellipsoids probability for the non-hydrogen atoms.
0.16) × 105, (6.53 0.72) × 105, (2.51 0.28) × 106, (2.04
0.19)×106 M−1 from the fluorescence data for the ligand, Cu(II), Zn
(II) and Ni(II) complexes, respectively (Table S3) and the Stern–Volmer
plot is linear, indicating that only one type of binding process occurs. This
phenomenon of fluorescence intensities increasing process of the
compounds binding to DNA may be attributed to that all the compounds
are protected from solvent water molecules by the hydrophobic
environment inside the DNA helix [25]. As shown from Kb values of
the ligand and metal complexes, we found that the metal complexes
interact more strongly than the free ligand alone, and the Zn(II) complex
can bind to DNA more deeply than the free ligand and other metal
complexes, and this is probably attributed to the extension of the π
system of the intercalated ligand and the coordination of metal ions,
which can lead to a planar area greater than that of the free ligand and
the coordinated ligand penetrating more deeply into and stacking more
strongly with the DNA base pairs.
By measuring the ability of compounds to affect EB fluorescence
intensity in the EB-DNA adduct, fluorescence quenching method can
be used. The emission spectra of EB bound to DNA in the absence and
increasing amounts of every compound have been recorded for
[EB]=4×10−7 M, [DNA]=5×10−6 M (Fig. S4). The emission spectra
showed that the emission band at 589 nm of the DNA–EB system
decreases in fluorescence intensity upon addition of every compound
at diverse r (= [Compound]/[DNA]) values, which indicated the
competition of the compounds with EB in binding to DNA. The
observed quenching of DNA–EB fluorescence intensity for the
compounds suggested that they can displace EB from the DNA–EB
system and interact with DNA probably via the intercalative mode
[26]. The quenching constants Kq are often used to evaluate the
quenching efficiency for every compound and vary with experimental
conditions. Kq values are given by the ratio of the slope to the intercept
and Kq values for the ligand, Cu(II), Zn(II) and Ni(II) complexes are
(1.25 0.03)×104, (1.88 0.02)×104, (2.78 0.09)×104 and (2.44
0.06)×104 M−1, respectively (Table S3). Experimental results indicated
that all the investigated compounds can bind to DNA via the same
binding mode (intercalation) and the complexes bind to DNA more
strongly than the free ligand alone.
Acknowledgement
This work is supported by the National Natural Science Foundation
of China (20975046).
Appendix A. Supplementary material
Supplementary material associated with this article can be found
in the online version. CCDC No. 775664 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
A useful technique to test the DNA binding mode and affinity is
viscosity measurements, which are sensitive to DNA length changes and
regarded as the least ambiguous and the most critical tests in the
absence of crystallographic structural data or NMR spectra [27]. Under
appropriate conditions, interaction of drugs like EB causes a significant
increase in viscosity of DNA solution due to the increase in separation of
base pairs of intercalation sites and hence results an increase in overall
DNA contour length. On the other hand, drug molecules binding
exclusively in the DNA grooves cause less pronounced or no changes in
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.07.005.
References
[1] P. Garodia, H. Ichikawa, N. Malani, G. Sethi, B.B. Aggarwal, J. Soc. Integr. Oncol. 5
(2007) 25–37.
[2] M.K. Arnold, J. Card. Fail. 3 (1997) 319–334.
[3] M. Peter, F. John, Transf. Med. Rev. 14 (2000) 223–233.

1216
Y. Li et al. / Inorganic Chemistry Communications 13 (2010) 1213–1216
[4] F. Arnesano, G. Natile, Coord. Chem. Rev. 253 (2009) 2070–2081.
[5] B.H. Lawrence, B.M. Virendra, D.B.J. Edwin, E.E. Douglas, Mutat, Res.-Fundam. Mol.
Mech. Mutagen. 623 (2007) 53–71.
[6] B.N. Trawick, A.T. Daniher, J.K. Bashkin, Chem. Rev. 98 (1998) 939–941.
[7] S.C. Zhang, Y. Shao, Y. Chen, J. Inorg. Chem. 22 (2006) 1733–1739.
[8] J. Reedijk, Pure Appl. Chem. 59 (1987) 181–192.
[9] V.S. Li, D. Choi, Z. Wang, L.S. Jimenez, M.S. Tang, H. Kohn, J. Am. Chem. Soc. 18
(1996) 2326–2331.
[10] G. Zuber, J.C. Quada Jr., S.M. Hecht, J. Am. Chem. Soc. 120 (1998) 9368–9369.
[11] S.M. Hecht, J. Nat. Prod. 63 (2000) 158–168.
[12] J.G. Liu, B.H. Ye, H. Li, Q.X. Zhen, L.N. Ji, Y.H. Fu, J. Inorg. Biochem. 76 (1999)
265–271.
[13] X. Sheng, X. Guo, X.M. Lu, G.Y. Lu, Y. Shao, F. Liu, Q. Xu, Bioconjugate Chem. 19
(2008) 490–498.
[14] Y. Mukohata, S. Nakabayashi, M. Higashida, FEBS Lett. 85 (1978) 215–218.
[15] B.H. Havsteen, Pharmacol. Ther. 96 (2002) 67–202.
[16] G. Singh, R. Singh, N.K. Girdhar, M.P.S. Ishar, Tetrahedron 58 (2002) 2471–2480.
[17] L.Z. Piao, H.R. Park, Y.K. Park, S.K. Lee, J.H. Park, M.K. Park, Chem. Pharm. Bull. 50
(2002) 309–311.
[18] M.M.B. Pessoa, G.F.S. Andrade, V.R.P. Monteiro, M.L.A. Temperini, Polyhedron 20
(2001) 3133–3141.
[19] J.R. Dimmock, S.C. Vashishtha, J.P. Stables, J. Med. Chem. 35 (2000) 241–248.
[20] S.Y. Yu, S.X. Wang, Q.H. Luo, L.F. Wang, R.P. Zhou, G. Xing, Polyhedron 12 (1993)
1093–1096.
[21] S.Y. Chiang, J. Welch, F.J. Rauscher, T.A. Beerman, Biochemistry33 (1994) 7033–7040.
[22] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81–122.
[23] A.M. Pyle, J.P. Rehmann, R. Meshoyrer, C.V. Kumar, N.J. Turro, J.K. Barton, J. Am.
Chem. Soc. 111 (1989) 3051–3058.
[24] Y. Li, Z.Y. Yang, M.F. Wang, Eur. J. Med. Chem. 44 (2009) 4585–4595.
[25] H.N. Jocelyn, L.I. Leodevico, Biotechnol. Ann. Rev. 9 (2003) 1–149.
[26] W.D. Wilson, L. Ratmeyer, M. Zhao, L. Strekowski, D. Boykin, Biochemistry 32
(1993) 4098–4104.
[27] S. Satyanaryana, J.C. Daborusak, J.B. Chaires, Biochemistry 31 (1992) 9319–9324.
[28] S. Satyanaryana, J.C. Daborusak, J.B. Chaires, Biochemistry 32 (1993) 2573–2584.
[29] P.H. Proctor, E.S. Reynolds, Physiol. Chem. Phys. Med. NMR 16 (1984) 175–195.
[30] Y. Li, Z.Y. Yang, T.R. Li, Chem. Pharm. Bull. 56 (2008) 1528–1534.
[31] R. Xing, H. Yu, S. Liu, W. Zhang, Q. Zhang, Z. Li, P. Li, Bioorg. Med. Chem. 13 (2005)
1387–1392.