Synthesis, structure, antioxidation, and DNA-binding studies of a binuclear ytterbium(III) complex with bis(N-salicylidene)-3-oxapentane-1,5-diamine

Article abstract of DOI:10.1007/s11164-013-1440-5

A new complex of ytterbium(III) nitrate with bis(N-salicylidene)-3-oxapentane-1,5-diamine (H₂L), with the composition Yb₂(L)₂(NO₃)₂·2H₂O, was synthesized and characterized by physic-chemical and spectroscopic methods. The crystal structure of the ytterbium(III) complex has been determined by single-crystal X-ray diffraction. It reveals a centrosymmetric binuclear neutral entity where Yb(III) metal centers are bridged by two phenoxo oxygen atoms. Electronic absorption titration spectra, ethidium bromide displacement experiments, and viscosity measurements indicate that both the ligand and the Yb(III) complex can bind calf thymus DNA, presumably via a groove binding mechanism. Furthermore, the antioxidant activities of the Yb(III) complex were determined by a superoxide and hydroxyl radical scavenging method in vitro, which indicates that it is a scavenger for OH and O₂ ^- radicals.

Full text of DOI:10.1007/s11164-013-1440-5

Res Chem Intermed
DOI 10.1007/s11164-013-1440-5
Synthesis, structure, antioxidation, and DNA-binding
studies of a binuclear ytterbium(III) complex
with bis(N-salicylidene)-3-oxapentane-1,5-diamine
•
Huilu Wu Guolong Pan Yuchen Bai Hua Wang
•
•
•
•
Jin Kong Furong Shi Yanhui Zhang Xiaoli Wang
•
•
Received: 10 June 2013 / Accepted: 8 October 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A new complex of ytterbium(III) nitrate with bis(N-salicylidene)-3-ox-
apentane-1,5-diamine (H2L), with the composition Yb2(L)2(NO3)2ꢀ2H2O, was
synthesized and characterized by physic-chemical and spectroscopic methods. The
crystal structure of the ytterbium(III) complex has been determined by single-crystal
X-ray diffraction. It reveals a centrosymmetric binuclear neutral entity where
Yb(III) metal centers are bridged by two phenoxo oxygen atoms. Electronic
absorption titration spectra, ethidium bromide displacement experiments, and vis-
cosity measurements indicate that both the ligand and the Yb(III) complex can bind
calf thymus DNA, presumably via a groove binding mechanism. Furthermore, the
antioxidant activities of the Yb(III) complex were determined by a superoxide and
hydroxyl radical scavenging method in vitro, which indicates that it is a scavenger
-
for OH and O2 radicals.
Keywords Bis(N-salicylidene)-3-oxapentane-1,5-diamine ꢀ Ytterbium(III)
complex ꢀ Crystal structure ꢀ DNA-binding property ꢀ Antioxidant
activity
Introduction
It is well known that deoxyribonucleic acid (DNA) is an important genetic
substance in organism. The regions of DNA involved vital processes, such as gene
expression, gene transcription, mutagenesis, and carcinogenesis [1, 2]. Numerous
biological experiments have demonstrated that DNA is the primary intracellular
H. Wu (&) ꢀ G. Pan ꢀ Y. Bai ꢀ H. Wang ꢀ J. Kong ꢀ F. Shi ꢀ Y. Zhang ꢀ X. Wang
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070,
People’s Republic of China
e-mail: wuhuilu@163.com
123

H. Wu et al.
target of anticancer drugs. Interaction between small molecules and DNA can cause
damage in cancer cells, blocking the division, and resulting in cell death [3, 4]. The
biological activities shown by DNA binder metal complexes are various and are
often related to their specific DNA-binding mechanism, ranging from intercalation
to covalent and groove binding [5, 6]. Therefore, research on the mechanism of the
interaction of complexes with DNA is attracting more and more attention. The
results will potentially be useful in the design of new compounds that can recognize
specific sites or conformations of DNA [7–9].
Lanthanide metal complexes have been used as biological models to understand
the structures of biomolecules and biological processes [10]. One of the most
studied applications is the use of lanthanide complexes to address DNA/RNA by
non-covalent binding and/or cleavage [11, 12]. In addition, Schiff bases are able to
inhibit the growth of several animal tumors, and some metal chelates have shown
good antitumor activities against animal tumors [13]. So, well-designed organic
ligands enable a fine tuning of special properties of the metal ions. In this study, a
Schiff base ligand and its ytterbium(III) complex were synthesized and character-
ized. Their DNA-binding behavior and antioxidant ability were investigated.
Experimental
Materials and methods
All chemicals were of analytical grade. Calf thymus DNA (CT-DNA), EB, nitroblue
tetrazolium nitrate (NBT), methionine (MET), and riboflavin (VitB2) were obtained
from Sigma-Aldrich (USA) and used without purification. The stock solution of ligand
and complex were dissolved in DMF at the concentration 3 9 10-3 M. All experiments
involving interaction of the ligand and the complex with CT-DNA were carried out in
double-distilled water buffer containing5 mM Tris and 50 mMNaCl adjusted to pH7.2
with hydrochloricacid. A solution ofCT-DNA gavea ratio ofUV absorbanceat260 and
280 nm of about 1.8–1.9, indicating that the CT-DNA was sufficiently free of protein
[34]. The CT-DNA concentration per nucleotide was determined spectrophotometri-
cally by employing an extinction coefficient of 6,600 M-1 cm-1 at 260 nm [35].
C, H, and N elemental analyses were determined using a Carlo Erba 1106
elemental analyzer. IR spectra were recorded from 4,000 to 400 cm-1 with a
Nicolet FT-VERTEX 70 spectrometer using KBr pellets. Electronic spectra were
taken on Lab-Tech UV Bluestar and Spectrumlab 722sp spectrophotometers and the
spectral resolution used was 0.2 nm. Fluorescence spectra were recorded on a LS-45
spectrofluorophotometer. 1H NMR spectra were obtained with a Mercury plus
400 MHz NMR spectrometer with TMS as internal standard and CDCl3 as solvent.
The synthetic route for the ligand precursor H2L is shown in Scheme 1.
3-Oxapentane-1,5-diamine
3-Oxapentane-1,5-diamine was synthesized following the procedure in Ref. [36].
Calcd. (%) for C4H12N2O: C, 46.25; H, 11.54; N, 26.90. Found (%): C, 45.98; H,
123

Synthesis, structure, antioxidation and DNA-binding studies
Scheme 1 Schematic diagram showing the synthesis of ligand H2L
11.50; N, 26.76. FT–IR (KBr m/cm-1): 1,120, m(C–O-C); 3,340, m(–NH2) stretching
frequency, respectively.
Bis(N-salicylidene)-3-oxapentane-1,5-diamine (H2L)
For the synthesis of H2L, salicylic aldehyde (10 mmol, 1.22 g) in EtOH(5 mL) was
added dropwise into a 5-mL EtOH solution of 3-oxapentane-1,5-diamine (5 mmol,
0.52 g). Then, the solution was stirred for 4 h at 78 °C. After cooling to room
temperature, the precipitate was filtered. The product was dried in vacuo, and a
yellow crystalline solid obtained. Yield: 1.19 g (68.5 %). Calcd. (%) for
C18H20O3N2: C, 69.21; H, 6.45; N, 8.97. Found (%): C, 69.09; H, 6.54; N, 8.83.
1H NMR (CDCl3 400 MHz) d/ppm: 8.30 (s, 2H, N=C–H), 6.79–7.33 (m, 8H,
H-benzene ring), 3.66–3.74 (m, 8H, O-(CH2)2-N=C). UV–Vis (k, nm): 268, 316.
FT–IR (KBr m/cm-1): 1,637, m(C=N); 1,286, m(C–O–C); 3,458, m(OH) stretching
frequency, respectively.
Yb2L2(NO3)2ꢀ2H2O
To a stirred solution of H2L (0.156 g, 0.5 mmol) in EtOH (10 mL) was added
Yb(NO3)3(H2O)6 (0.234 g 0.5 mmol) and triethylamine (0.3 mL) in EtOH (10 mL).
The yellow sediment generated rapidly. The precipitate was filtered off, washed
with EtOH and absolute Et2O, and dried in vacuo. The dried precipitate was
dissolved in DMF to form a yellow solution. The yellow block crystals of the
Yb(III) complex suitable for X-ray diffraction studies were obtained by vapor
diffusion of diethyl ether into the solution for few weeks at room temperature.
Yield: 0.204 g (54.7 %). Calcd. (%) for C36H40N6O14Yb: C, 38.19; H, 3.55; N,
7.45. Found (%): C, 38.19; H, 3.73; N, 7.21. UV–Vis (k, nm): 269, 317. FT–IR (KBr
m/cm-1): 1,244, m(C–O–C); 1,388, 1,057, m(NO3); 1,633, m(C=N) stretching
frequency, respectively.
X-ray crystallography
A suitable single crystal was mounted on a glass fiber, and the intensity data were
collected on a Bruker Smart CCD diffractometer with graphite-monochromated Mo
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H. Wu et al.
˚
-Ka radiation (k = 0.71073 A) at 296 K. Data reduction and cell refinement were
performed using the SMART and SAINT programs [37]. The structure was solved
by direct methods and refined by full-matrix least-squares against F2 of data using
SHELXTL software [38]. All H atoms were found in difference electron maps and
subsequently refined in a riding-model approximation with C–H distances ranging
˚
from 0.93 to 0.97 A and Uiso (H) = 1.2 or 1.5 Ueq(C). The crystal data and
experimental parameters relevant to the structure determination are listed in
Table 1. Selected bond lengths and angles are presented in Table 2.
Crystallographic data (excluding structure factors) for the structure in this paper
have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK. Copies of the data can be obtained free of
charge on quoting the depository number CCDC-929560 (Fax: ?44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Table 1 Crystal and structure refinement data for Yb2L2(NO3)2ꢀ2H2O
Complex
Yb2L2(NO3)2ꢀ2H2O
C36H40N6O14Yb2
Molecular formula
Molecular weight
Color, habit
Crystal size, mm3
Crystal system
Space group
1,126.82
Yellow, block
0.26 9 0.24 9 0.22
Monoclinic
C2/c
˚
a, A
29.108(7)
11.666(3)
15.195(4)
118.842(2)
4,519.7(19)
4
˚
b, A
˚
c, A
b, °
3
˚
V, A
Z
Absorption coefficient, mm-1
4.178
T, K
Dcalcd, g cm-3
296(2)
1.656
F (000), e
2,200
h range for data collection, °
hkl range
2.20-25.50
35, 14, 18
14,161
Reflections collected
Independent reflections/Rint
Data/restraints/parameters
Final R1/wR2 indices [I [ 2r(I)]
R1/wR2 indices (all data)a
Goodness-of-fit on F2
4,184/0.0346
4,184/3/262
0.0285/0.0713
0.0433/0.0776
1.108
-3
˚
Largest diff. peak/hole, e A
0.955/-0.797
123

Synthesis, structure, antioxidation and DNA-binding studies
˚
Table 2 Selected bond lengths (A) and bond angles (°) for Yb2L2(NO3)2ꢀ2H2O
Bond lengths
Yb(1)-O(3)
Yb(1)-O(2)#1a
2.134(4)
2.247(3)
2.277(3)
2.424(4)
Yb(1)-O(1)
Yb(1)-N(2)
Yb(1)-O(4)
Yb(1)-N(1)
2.428(4)
2.442(5)
2.460(4)
2.463(5)
Yb(1)-O(2)
Yb(1)-O(5)
Bond angles
O(3)-Yb(1)-O(2)#1
O(2)#1-Yb(1)-O(2)
O(2)#1-Yb(1)-O(5)
O(3)-Yb(1)-O(1)
O(2)-Yb(1)-O(1)
O(3)-Yb(1)-N(2)
O(2)-Yb(1)-N(2)
O(1)-Yb(1)-N(2)
O(2)#1-Yb(1)-O(4)
O(5)-Yb(1)-O(4)
N(2)-Yb(1)-O(4)
O(2)#1-Yb(1)-N(1)
O(5)-Yb(1)-N(1)
N(2)-Yb(1)-N(1)
a
96.77(14)
71.05(14)
156.71(13)
141.44(14)
132.36(13)
75.23(17)
146.56(15)
66.90(17)
149.88(12)
51.98(13)
122.62(14)
90.54(14)
88.27(16)
133.86(17)
O(3)-Yb(1)-O(2)
O(3)-Yb(1)-O(5)
O(2)-Yb(1)-O(5)
O(2)#1-Yb(1)-O(1)
O(5)-Yb(1)-O(1)
O(2)#1-Yb(1)-N(2)
O(5)-Yb(1)-N(2)
O(3)-Yb(1)-O(4)
O(2)-Yb(1)-O(4)
O(1)-Yb(1)-O(4)
O(3)-Yb(1)-N(1)
O(2)-Yb(1)-N(1)
O(1)-Yb(1)-N(1)
O(4)-Yb(1)-N(1)
84.37(13)
95.60(16)
129.94(14)
87.29(12)
70.86(14)
85.17(14)
79.00(15)
81.31(15)
78.86(12)
112.97(13)
150.67(15)
71.20(14)
67.01(15)
78.31(15)
Symmetry transformations used to generate equivalent atoms: #1 - x ? 1/2, -y ? 1/2, -z
DNA-binding experiments
Viscosity experiments were conducted on an Ubbelohde viscometer, immersed in a
water bath maintained at 25.0 0.1 °C. The flow time was measured with a digital
stopwatch and each sample was tested three times to get an average calculated time.
Titrations were performed for the complexes (3–30 lM), and each compound was
introduced into the CT-DNA solution (42.5 lM) present in the viscometer. Data
were analyzed as (g/g0)1/3 versus the ratio of the concentration of the compound to
CT-DNA, where g is the viscosity of CT-DNA in the presence of the compound and
g0 is the viscosity of CT-DNA alone. Viscosity values were calculated from the
observed flow time of CT-DNA-containing solutions corrected from the flow time
of buffer alone (t0), g = (t-t0) [7].
Absorption titration experiments were performed with fixed concentrations of the
complexes, while gradually increasing the concentration of CT-DNA. To obtain the
absorption spectra, the required amount of CT-DNA was added to both the
compound and reference solutions, in order to eliminate the absorbance of CT-DNA
itself. From the absorption titration data, the binding constant (Kb) was determined
using the equation [39]:
½DNAꢁ=ðea ꢂ efÞ ¼ ½DNAꢁ=ðeb ꢂ efÞ þ 1=Kbðeb ꢂ efÞ;
where [DNA] is the concentration of CT-DNA in base pairs, ea corresponds to the
observed extinction coefficient (Aobsd/[M]), ef corresponds to the extinction
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H. Wu et al.
coefficient of the free compound, eb is the extinction coefficient of the compound
when fully bound to CT-DNA, and Kb is the intrinsic binding constant. The ratio of
slope to intercept in the plot of [DNA]/(ea-ef) versus [DNA] gave the value of Kb.
The enhanced fluorescence of EB in the presence of DNA can be quenched by
the addition of a second molecule [40]. The extent of fluorescence quenching of EB
bound to CT-DNA can be used to determine the extent of binding between the
second molecule and CT-DNA. Competitive binding experiments were carried out
in the buffer by keeping [DNA]/[EB] = 1 and varying the concentrations of the
compounds. The fluorescence spectra of EB were measured using an excitation
wavelength of 520 nm, and the emission range was set between 550 and 750 nm.
The spectra were analyzed according to the classical Stern–Volmer equation [9]:
I0=I ¼ 1 þ KSV½Qꢁ;
where I0 and I are the fluorescence intensities at 599 nm in the absence and presence
of the quencher, respectively, KSV is the linear Stern–Volmer quenching constant,
and [Q] is the concentration of the quencher. In these experiments, [CT-
DNA] = 2.5 9 10-3 mol/L and [EB] = 2.2 9 10-3 mol/L.
Antioxidant study methods
Hydroxyl radicals were generated in aqueous media through the Fenton-type
reaction [41]. The aliquots of reaction mixture (3 mL) contained 1 mL of 0.1 mmol
aqueous safranin, 1 mL of 1.0 mmol aqueous EDTA–Fe(II), 1 mL of 3 % aqueous
H2O2, and a series of quantitative microadditions of solutions of the test compound.
A sample without the tested compound was used as the control. The reaction
mixtures were incubated at 37 °C for 30 min in a water bath. The absorbance was
then measured at 520 nm. All the tests were run in triplicate and are expressed as the
mean and standard deviation (SD) [42]. The scavenging effect for OHꢀ was
calculated from the following expression:
Scavenging ratio ð%Þ ¼ ½ ðAi ꢂ A0Þ = ðAc ꢂ A0Þ ꢁ ꢃ 100 %;
where Ai is the absorbance in the presence of the test compound, A0 is the absor-
bance of the blank, Ac is the absorbance in the absence of the test compound,
EDTA-Fe(II), and H2O2.
A
nonenzymatic system containing 1 mL 9.9 9 10-6
M VitB2, 1 mL
1.38 9 10-4 M NBT, and 1 mL 0.03 M MET was used to produce superoxide anion
(O2-), and the scavenging rate of O2- under the influence of 0.1–1.0 lM of the tested
compound was determined by monitoring the reduction in rate of transformation of
NBT to monoformazan dye [43]. The solutions of MET, VitB2, and NBT were prepared
with 0.02 M phosphate buffer (pH 7.8) in the condition of avoiding light. The reactions
were monitored at 560 nm with a UV–Vis spectrophotometer, and the rate of absorption
change was determined. The percentage inhibition of NBT reduction was calculated
using the following equation [44]: percentage inhibition of NBT reduction = (1 - k0/
k) 9 100, where k0 and k present the slopes of the straight line of absorbance values as a
function of time in the presence and absence of superoxide dismutase mimic compound,
respectively. The IC50 values for the complexes were determined by plotting the graph
123

Synthesis, structure, antioxidation and DNA-binding studies
of percentage inhibition of NBT reduction against the increase in the concentration of
the complex. The concentration of the complex which causes 50 % inhibition of NBT
reduction is reported as IC50.
Results and discussion
The Yb(III) complex, Yb2(L)2(NO3)2ꢀ2H2O, was prepared by reaction of H2L with
Yb(NO3)3(H2O)6 in ethanol. It is soluble in polar aprotic solvents such as DMF,
DMSO, and MeCN, slightly soluble in ethanol, methanol, ethyl acetate, and
chloroform, and insoluble in water, Et2O, and petroleum ether. Elemental analysis
shows that its composition is Yb2(L)2(NO3)2ꢀ2H2O which was confirmed by the
crystal structure analysis.
X-ray structure of the complex
The prepared pentadentate ligand contains strong donors, namely phenoxo oxygen
atoms as well as imine nitrogen atoms, with an excellent coordination ability for
transition/inner-transition metal ions through its N2O3 donor set. The crystal structure of
the Yb(III) complex consists of discrete Yb2(L)2(NO3)2 and two water-solvent
molecules. Single-crystal X-ray structure determination has revealed that the complex
has a centrosymmetric neutral homobinuclear entity. An ORTEP illustration of the
complex (Fig. 1) shows that two adjacent [Yb(L)(NO3)] moieties are bridged via two
phenoxo groups. In the l2-diphenoxo-bridged binuclear structure, both Yb(III) centers
are octa-coordinated (Fig. 2a). The local coordination environment is identical for both
the centers by symmetry, and is best described as a distorted square YbN2O6 antiprism
(Fig. 2b). Due to the flexibility of the ligand, it loses its planarity. The bond lengths are
in the range Yb(1)–Nimine 2.442(5)–2.463(5), Yb(1)–Oether 2.428(4), and Yb(1)–Onitrate
˚
2.424(4)–2.460(4) A. The nature of coordination of the two Schiff base moieties of the
same ligand is completely different. Of the two phenoxo oxygen atoms of each ligand,
one is simply mono-coordinated while the other one bridges the adjacent Yb(III)
centers, as reflected by the Yb–Ophenoxo bond lengths [Yb(1)–O(2) 2.277(3) and Yb(1)–
˚
˚
O(3) 2.134(4) A]. The distance Yb(1)–Yb(1A) of 3.6824(10) A is too long to consider
any direct intramolecular Yb–Yb bonding.
An interesting feature of this structure is the intermolecular hydrogen bond that
exists among the Yb(III) complexes, which also leads to large solvent-accessible voids
which might trap guest molecules (Fig. 3). Thus, the solid may have the potential for
practical applications such as gas absorption [14]. It is worth noting that intermolecular
interactions have the potential to assemble smaller and simpler fragments into desired
cavities under favorable conditions, which is important in host–guest chemistry and has
applications in chemistry, biology, and materials science.
IR and electronic spectra
For the free ligand H2L, a strong band is found at 1,637 cm-1 together with a weak
band at 1,286 cm-1. By analogy with previous assignments, the former can be
123

H. Wu et al.
Fig. 1 The molecular structure of the Yb(III) complex in the crystal with displacement ellipsoids at the
30 % probability level; H atoms are omitted for clarity
Fig. 2 a Perspective disposition of the donor sites around each Yb3? center shown in Neumann
projection. b A distorted square antiprism geometry is formed by donor atoms around the Yb3? center as
illustrated in the polyhedral view
attributed to m(C=N), while the latter can be attributed to m(C–O–C). These bands
were shifted to lower frequencies by ca. 4–42 cm-1 for the Yb(III) complex [15],
which implies direct coordination of the nitrogen and oxygen atoms to the metal ion.
Bands at 1,388 and 1,057 cm-1 indicate that nitrate is bidentate [16], in agreement
with the result of X-ray diffraction.
The electronic spectra of the ligand H2L and the Yb(III) complex were recorded
in DMF solution at room temperature. The UV bands of H2L (268 and 316 nm) are
marginally shifted in the complex. The two absorption bands are assigned to p?p*
(benzene) and p?p* (C=N) transitions [17].
123

Synthesis, structure, antioxidation and DNA-binding studies
Fig. 3 A space-filling diagram of the nanosized holes in Yb(III) complex (in order to simplify, water-
solvent molecules are omitted for clarity)
DNA binding properties
In order to clarify the interaction nature between the investigated compounds and
DNA, viscosity measurement was carried out. Hydrodynamic measurements that are
sensitive to changes in the length of DNA (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 [18]. The classic intercalation model
involves the insertion of a planar molecule between DNA base pairs, which results
in a decrease in the DNA helical twist and lengthening of the DNA; the molecule
will also be in close proximity to the DNA base pairs [19]. In contrast, a partial and/
or non-classical intercalation of the complex in the DNA grooves typically causes
low (positive or negative) or no change in DNA solution viscosity [20].
The effects of the ligand and of the Yb(III) complex on the viscosity of CT-DNA
are shown in Fig. 4. The experimental results show that the addition of H2L and
complex causes no significant viscosity change. Therefore, according to the
previously reported relationship to DNA-binding lanthanide complexes [5, 20, 21],
we can deduce that the ligand and the Yb(III) complex most probably bind to DNA
in a groove mode.
The application of electronic absorption spectroscopy in DNA-binding studies is
one of the most useful techniques [8]. To clarify the interactions between the
compounds and DNA, the electronic absorption spectra of the ligand and the Yb(III)
complex in the absence and presence of the CT-DNA (at a constant concentration of
the compounds) were obtained, which are shown in Fig. 5. With increasing DNA
concentrations, the absorption bands at 394 nm of H2L show a hypochromism of
32.35 %; the absorption bands at 393 nm of the Yb(III) complex show a
hypochromism of 60.82 %. The hypochromism observed for the p?p* transition
bands indicating strong binding of H2L and complex to DNA.
To quantitatively compare the affinity of H2L and the Yb(III) complex towards
DNA, the intrinsic binding constants Kb of the two compounds to CT-DNA were
determined by monitoring the changes of absorbance with increasing concentration
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H. Wu et al.
Fig. 4 Effect of increasing amounts of a H2L and b Yb(III) complex on the relative viscosity of CT-
DNA at 25.0 0.1 °C
Fig. 5 Electronic spectra of a H2L, c Yb(III) complex in Tris–HCl buffer upon addition of CT-DNA.
[Compound] = 3.0 9 10-5 M, [DNA] = 2.5 9 10-5 M. Arrow shows the absorbance intensity changes
upon increasing DNA concentration. Plots of [DNA]/(ea-ef) vs. [DNA] for the titration of b H2L,
d Yb(III) complex with CT-DNA
of DNA. The intrinsic binding constant Kb of H2L and of the Yb(III) complex were
5.30 9 103 M-1 (R = 0.99 for 16 points) and 3.17 9 104 M-1 (R = 0.99 for 16
points), respectively, from the decay of the absorbances. The results indicate that the
binding strength of the complex is higher than for H2L. Moreover, the Kb value
123

Synthesis, structure, antioxidation and DNA-binding studies
obtained here is lower than that reported for classical intercalator {for EB and
[Ru(phen)DPPZ]2? and some Ln complexes whose binding constants have been
found to be in the order of 106–107 M} [22–25]. The observed binding constants are
more in keeping with the groove binding with DNA, as observed in the literature
[26].
Based on the above results, the affinity for DNA is stronger for Yb(III) complex
than ligand. We attribute the possible reason is the charge transfer of coordinated
H2L caused by coordination of the central atom results in the decrease of charge
density of the planar conjugated system. This change will lead to complexes binding
to DNA more easily [7, 8]. Moreover, the helix structure of the Yb(III) complex is
able to provide lots of grooving positions to stack more strongly with the base pairs
of the DNA helix [27].
In order to further study the binding properties of the compounds with DNA,
competitive binding experiment was carried out. Relative binding of H2L and
Yb(III) complex to CT-DNA was studied by the fluorescence spectral method using
ethidium bromide (EB)-bound CT-DNA solution in Tris–HCl/NaCl buffer (pH 7.2).
As a typical indicator of intercalation, EB is a weakly fluorescent compound. But, in
the presence of DNA, emission intensity of EB is greatly enhanced because of its
strong intercalation between the adjacent DNA base pairs [7]. In general,
measurement of the ability of a complex to affect the intensity of EB fluorescence
in the EB–DNA adduct allows determination of the affinity of the complex for
DNA, whatever the binding mode may be. If a complex can displace EB from DNA,
the fluorescence of the solution will be reduced due to the fact that free EB
molecules are readily quenched by the solvent water [28]. For ligand H2L and the
Yb(III) complex, no emission was observed either alone or in the presence of CT-
DNA in the buffer. The fluorescence quenching of DNA-bound EB by the ligand
and complex are shown in Fig. 6. The behavior of H2L and Yb(III) complex are in
good agreement with the Stern–Volmer equation, which provides further evidence
that the two compounds bind to DNA. The Ksv values for H2L and Yb(III) complex
are 0.35 9 104 (R = 0.98 for 21 points in the line part) and 1.48 9 104 M-1
(R = 0.99 for 12 points), respectively, reflecting the higher quenching efficiency of
the Yb(III) complex relative to that of H2L. This result suggests that the DNA-
binding of the Yb(III) complex is stronger than that of H2L. Such a trend is
consistent with the previous absorption spectral results.
Figure 6 show the plots of I0/I versus [Component]. The data of Ksv are all at
104 M-1 level for the ligand and the Yb(III) complex, accordingly. In view of the
strong interaction of EtBr with DNA of which the binding constant of EtBr with
DNA is at 106 M-1 level [29], we consider it is impossible for the ligand and the
complex to scramble EtBr from DNA. A similar fluorescence quenching effect of
EtBr bound to DNA has been observed for the addition of several groove-binding
compounds, including netropsin and distamycin A [27, 30]. The observed results
make us believe that the ligand and the Yb(III) complex may interact with DNA
through the groove binding mode, releasing some EtBr molecules from the EtBr–
DNA system [30–32].
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H. Wu et al.
Antioxidant activities
We compared the abilities of the present compounds to scavenge hydroxyl radicals
with those of the well-known natural antioxidants mannitol and vitamin C, using the
same method as reported in a previous paper [33]. The 50 % inhibitory
concentration (IC50) values of mannitol and vitamin C are about 9.6 9 10-3 and
8.7 9 10-3 M-1, respectively. As shown in Fig. 7a, according to the antioxidant
experiments, the IC50 value of the Yb(III) complex is 3.35 9 10-5 M-1 which
implies that the Yb(III) complex exhibits better scavenging activity than mannitol
and vitamin C. It can be concluded that much less scavenging activity was exhibited
by H2L when compared to that of the Yb(III) complex which is due to the chelation
of the ligand with the central metal atom. Due to the observed IC50 values, the
Yb(III) complex can be considered as a potential drug to eliminate the hydroxyl
radical.
As another assay of antioxidant activity, superoxide radical (O2-) scavenging
activity has been investigated. The Yb(III) complex has good superoxide radical
scavenging activity. The Yb(III) complex shows an IC50 value of 4.57 9 10-5 M-1
(Fig. 7b), which indicates that it has potent scavenging activity for the superoxide
Fig. 6 Emission spectra of EB bound to CT-DNA in the presence of a H2L and c Yb(III) complex;
[Compound] = 3.0 9 10-5 M; kex = 520 nm. The arrows show the intensity changes upon increasing
concentrations of the complexes. Fluorescence quenching curves of EB bound to CT-DNA by b H2L and
d Yb(III) complex. (Plots of I0/I versus [Complex])
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Synthesis, structure, antioxidation and DNA-binding studies
Fig. 7 Plots of antioxidation properties for the Yb(III) complex. a The hydroxyl radical scavenging
effect (%) for the Yb(III) complex. b The superoxideradical scavenging effect (%) for the Yb(III)
complex
radical (O2-). This indicates that the Yb(III) complex exhibits good superoxide
radical scavenging activity and may be an inhibitor (or a drug) to scavenge
superoxide radicals (O2-) in vivo, an action which needs further investigation.
Conclusions
In this paper, a pentadentate schiff base ligand bis(N-salicylidene)-3-oxapentane-
1,5-diamine and its Yb(III) complex were synthesized and characterized. The
crystal structure of the Yb(III) complex was determined by X-ray crystallography.
The binding modes of these compounds with CT-DNA have been studied by
electronic absorption titration, EB-DNA displacement experiments, and viscosity
measurements. The results indicate that the Yb(III) complex shows higher affinities
than the free ligand and interacts with CT-DNA through the groove mode. In
-
addition, the Yb(III) complex also has active scavenging effects on OH and O2
radicals. These findings indicate that the Yb(III) complex has many potential
practical applications for the development of nucleic acid molecular probes and new
therapeutic reagents for diseases on the molecular level. However, its pharmaco-
dynamical, pharmacological, and toxicological properties should be further studied
in vivo.
Acknowledgments The present research was supported by the National Natural Science Foundation of
China (Grant No. 21367017), the Fundamental Research Funds for the Gansu Province Universities
(212086), National Natural Science Foundation of Gansu Province (Grant no. 1212RJZA037), and ‘Qing
Lan’ Talent Engineering Funds for Lanzhou Jiaotong University.
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