Study on synthesis, crystal structure, antioxidant and DNA-binding of mono-, di- and poly-nuclear lanthanides complexes with bis(N-salicylidene)-3- oxapentane-1,5-diamine

Article abstract of DOI:10.1016/j.jphotobiol.2014.04.005

A Schiff base ligand bis(N-salicylidene)-3-oxapentane-1,5-diamine H ₂L have been prepared. Reaction of the shape-specific designed ligand with Ln(NO₃)₃6H₂O afforded three novel complexes, namely, Sm(L)(NO₃)(DMF)(H₂O) 1, [Eu(H ₂L)₂(NO₃)₃]_n 2 and Tb₂(L)₂(NO₃)₂ 3. The ligand and complexes were characterized by elemental analysis, UV-Vis, IR, NMR spectroscopy and X-ray crystallography. It is noteworthy that the complexes demonstrate three different types of the structure which changed according to the charge density and acidity of the lanthanide. Complex 1 is a discrete mononuclear species that Sm(III) ion is nine-coordinated in the structure and forming a distorted tricapped trigonal prism geometry. Complex 2 is possessed a 1-D ribbon framework constructed from an extended array of ten-coordinated Eu³⁺ centers and the Schiff base ligands. Complex 3 is revealed as a centrosymmetric binuclear neutral entity, in which Tb(III) ion is eight-coordinated with the coordination surround of distorted square antiprism geometry. In order to explore the relationship between the structure and biological properties, the DNA-binding properties have been investigated by electronic absorption, fluorescence, and viscosity measurements. The results suggest that the ligand and complexes bind to DNA via groove modes. The intrinsic binding constants K_b of the complexes 1-3 are (1.19 ± 0.112) × 10 ⁵, (4.22 ± 0.086) × 10⁴ and (3.89 ± 0.104) × 10⁴ M^-1 respectively. Moreover, the antioxidant activity experiments show that these compounds also exhibit good antioxidant activities against OH and O₂^- in vitro studies.

Full text of DOI:10.1016/j.jphotobiol.2014.04.005

Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
Study on synthesis, crystal structure, antioxidant and DNA-binding
of mono-, di- and poly-nuclear lanthanides complexes with
bis(N-salicylidene)-3-oxapentane-1,5-diamine
⇑
Huilu Wu , Guolong Pan, Yuchen Bai, Yanhui Zhang, Hua Wang, Furong Shi, Xiaoli Wang, Jin Kong
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Schiff base ligand bis(N-salicylidene)-3-oxapentane-1,5-diamine H2L have been prepared. Reaction of
the shape-specific designed ligand with Ln(NO3)3ꢀ6H2O afforded three novel complexes, namely,
Sm(L)(NO3)(DMF)(H2O) 1, [Eu(H2L)2(NO3)3]n 2 and Tb2(L)2(NO3)2 3. The ligand and complexes were char-
acterized by elemental analysis, UV–Vis, IR, NMR spectroscopy and X-ray crystallography. It is notewor-
thy that the complexes demonstrate three different types of the structure which changed according to the
charge density and acidity of the lanthanide. Complex 1 is a discrete mononuclear species that Sm(III) ion
is nine-coordinated in the structure and forming a distorted tricapped trigonal prism geometry. Complex
2 is possessed a 1-D ribbon framework constructed from an extended array of ten-coordinated Eu3+ cen-
ters and the Schiff base ligands. Complex 3 is revealed as a centrosymmetric binuclear neutral entity, in
which Tb(III) ion is eight-coordinated with the coordination surround of distorted square antiprism
geometry. In order to explore the relationship between the structure and biological properties, the
DNA-binding properties have been investigated by electronic absorption, fluorescence, and viscosity
measurements. The results suggest that the ligand and complexes bind to DNA via groove modes. The
intrinsic binding constants Kb of the complexes 1–3 are (1.19 0.112) ꢁ 105, (4.22 0.086) ꢁ 104 and
(3.89 0.104) ꢁ 104 Mꢂ1 respectively. Moreover, the antioxidant activity experiments show that these
compounds also exhibit good antioxidant activities against OHÅ and O2ꢂÅ in vitro studies.
Received 1 February 2014
Received in revised form 1 April 2014
Accepted 7 April 2014
Available online 18 April 2014
Keywords:
Bis(N-salicylidene)-3-oxapentane-1,
5-diamine
Lanthanide(III) complex
Crystal structure
DNA-binding property
Antioxidant activity
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
fascinating architectures and potential application as functional
materials [14–16]. Particularly, biological activity and also number
Studies on the synthesis and DNA-binding property of metal
complexes are currently receiving much attention due to their
various uses in nucleic acid chemistry, especially their potential
applications in DNA molecule probes and chemotherapeutic
reagents [1–3]. Especially, the relevance and importance of this kind
of organic [4,5] and inorganic complex [6–8] molecules intensified
extensive research in drug discovery, including cancer chemother-
apy. Basically, the complex bound to DNA through three non-cova-
lent modes: intercalation, groove binding and external static
electronic effects [9–11]. Therefore, an understanding of how these
small molecules bind to DNA will potentially be useful in the design
of such new compounds, which can recognize specific sites or con-
formations of DNA [12,13].
of other applications resulted in strongly increasing interest for
lanthanides [17,18]. One of the most studied applications is usage
of the lanthanide molecule complexes to address DNA/RNA by
non-covalent binding and/or cleavage [19–21]. In addition, the
interest in the chelation of metal ions by Schiff base macrocyclic
(coronands) and open-chain (podands) ligands has continually
increased owing to the recognition of the role of played by these
structures in bioinorganic and medicinal inorganic chemistry
[22,23]. So, well designed organic ligands enable a fine tuning of
special properties of the metal ions. The lanthanide ions are
oxophilic. They are expected to interact strongly with polarised
oxygen-bearing functional groups such as salicylide groups
[24,25]. Herein, we introduce the synthesis and structural charac-
terization of lanthanide complexes with the Schiff base ligand
bis(N-salicylidene)-3-oxapentane-1,5-diamine (Scheme 1). The
pentadentate ligand contains strong donors, namely phenoxo oxy-
gen atoms as well as imine nitrogen atoms bearing an excellent
coordination ability with metal ions through its N2O3 donor set.
In recent years, the rational design and synthesis of lanthanide
coordination complexes have attracted great interest for their
⇑
Corresponding author. Tel.: +86 13893117544.
E-mail address: wuhuilu@163.com (H. Wu).
http://dx.doi.org/10.1016/j.jphotobiol.2014.04.005
1011-1344/Ó 2014 Elsevier B.V. All rights reserved.

34
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
(%) for C4H12N2O: C, 46.25; H, 11.54; N, 26.90. FT–IR (KBr
/cmꢂ1): 1120,
(CAOAC); 3340, (ANH2) stretching frequency,
respectively.
m
m
m
2.2.2. Synthesis of H2L
For the synthesis of H2L, salicylic aldehyde (10 mmol, 1.22 g) in
EtOH(5 mL) was added dropwise to a 5 mL EtOH solution of
3-oxapentane-1,5-diamine (5 mmol, 0.52 g). After the completion
of addition, the solution was stirred for an additional 4 h at
78 °C. After cooling to room temperature, the precipitate was fil-
tered. The product was dried in vacuo, and obtained yellow crystal-
line solid. Yied: 1.19 g (68.5%). Found (%): C, 69.09; H, 6.54; N, 8.83.
Calcd. (%) for C18H20O3N2: C, 69.21; H, 6.45; N, 8.97. 1H NMR (CDCl3
400 MHz) d/ppm: 8.30 (s, 2H, N@CAH), 6.79–7.33 (m, 8H,
H-benzene ring), 3.66–3.74 (m, 8H, OA(CH2)2AN@C). UV–Vis (k,
Scheme 1. Schematic diagram showing the synthesis of ligand H2L.
On the other hand, an excess of activated oxygen species in the
forms of superoxide anion (O2ꢂÅ) and hydroxyl radical (OHÅ), gener-
ated by normal metabolic processes, may cause various diseases
such as carcinogenesis, drug-associated toxicity, inflammation,
atherogenesis, and aging in aerobic organisms [26]. Although a
variety of (OHÅ) scavengers are known, their application is limited
by: (1) the requirement of unphysiologically high scavenger con-
centrations which depend on the rate constant, (2) toxic side
effects including initiation of further radical chain reactions, or
(3) the instability of the compound in biological systems [27].
Many researchers have been working hard to develop complexes
in order to achieve the efficient scavenger. It has been recently
demonstrated that some minor groove binders for DNA are effec-
tive inhibitors of the formation of a DNA/TBP complex or topoi-
somerases [28,29]. In this paper, we give a full account of the
synthesis, crystal structure, DNA-binding properties and
antioxidant activities of three Ln(III) complexes with bis(N-salicy-
lidene)-3-oxapentane-1,5-diamine.
nm): 268, 316. FT–IR (KBr
(CAOAC); 3458, (OH) stretching frequency, respectively.
m m(C@N); 1286,
/cmꢂ1): 1637,
m
m
2.2.3. Preparation of complexes
Three complexes were prepared by a similar procedure. To a
stirred solution of H2L (156 mg, 0.5 mmol) in EtOH (10 mL) was
added Ln(NO3)3(H2O)6 (0.5 mmol; Sm, 222 mg; Eu, 223 mg; Tb,
226 mg) 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 Ln(III) complexes that suitable for X-
ray diffraction studies were obtained by vapor diffusion of diethyl
ether into the solution for few weeks at room temperature.
Sm(L)(NO3)(DMF)(H2O) 1. Yield: 224 mg, 59.3%. Found (%): C,
40.82; H, 4.71; N, 8.88. Calcd. (%) for C21H27N4O8Sm: C, 41.09; H,
4.43; N, 9.13. UV–Vis (k, nm): 268, 316. FT–IR (KBr
1290, (CAOAC); 1362, 1031, (NO3); 1632, (C@N) stretching fre-
quency, respectively.
m
/cmꢂ1):
2. Experimental
m
m
m
2.1. Materials and measurements
[Eu(H2L)2(NO3)3]n 2. Yield: 186 mg, 49.1%. Found (%): C, 44.58;
H, 4.41; N, 9.97. Calcd. (%) for C36H40EuN7O15: C, 44.91; H, 4.19;
m
/cmꢂ1): 1292,
All chemicals were of analytical grade. Calf thymus DNA
(CT-DNA), ethidium bromide (EB), nitroblue tetrazolium
nitrate (NBT), methionine (MET) and riboflavin (VitB2) were
obtained from Sigma-Aldrich Co. (USA) and used without purifica-
tion. The solution of CT-DNA gave a ratio of UV absorbance at 260
and 280 nm, A260/A280, of 1.8–1.9, indicating that the DNA was suf-
ficiently free of protein [30]. The stock solution of DNA
(2.5 ꢁ 10ꢂ3 M) was prepared in 5 mM Tris-HCl/50 mM NaCl buffer
solution (pH = 7.2, stored at 4 °C and used when not more than
4 days). The CT-DNA concentration per nucleotide was determined
spectrophotometrically by employing an extinction coefficient of
6600 Mꢂ1 cmꢂ1 at 260 nm [31]. The stock solution of complex
was dissolved in DMF at the concentration 3 ꢁ 10ꢂ3 M.
N, 10.18. UV–Vis (k, nm): 269, 316. FT–IR (KBr
(CAOAC); 1365, 1051, (NO3); 1633, (C@N) stretching fre-
quency, respectively.
m
m
m
Tb2(L)2(NO3)2 3. Yield: 199 mg, 52.1%. Found (%): C, 40.30; H,
3.73; N, 7.56. Calcd. (%) for C36H40N6O14Tb2: C, 40.69; H, 3.41; N,
7.91. UV–Vis (k, nm): 269, 316. FT–IR (KBr
(CAOAC); 1368, 1052, (NO3); 1633, (C@N) stretching fre-
m
/cmꢂ1): 1214,
m
m
m
quency, respectively.
2.3. 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 Ka radiation (k = 0.71073 Å) at
The C, H and N elemental analyses were determined using a
Carlo Erba 1106 elemental analyzer. IR spectra were recorded from
4000 to 400 cmꢂ1 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 reso-
lution used is 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.
296 K. Data reduction and cell refinement were performed using
the SMART and SAINT programs. The absorption corrections are
carried out by the empirical method [33]. The structure was solved
by direct methods and refined by full-matrix least-squares against
F2 of data using SHELXTL software [34]. The uncoordinated water
molecule was found to be disordered. Its electron density was
removed from the reflection intensities by using the routine
SQUEEZE in PLATON. All H atoms were found in difference electron
maps and subsequently refined in a riding-model approximation
with CAH distances ranging from 0.93 to 0.97 Å and Uiso(H) = 1.2
Ueq(C) 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 S1.
2.2. Synthesis
2.2.1. Synthesis of 3-oxapentane-1,5-diamine
3-Oxapentane-1,5-diamine was synthesized following the pro-
cedure in Ref. [32]. Found (%): C, 45.98; H, 11.50; N, 26.76. Calcd.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
35
Table 1
Crystal and structure refinement data for complexes 1, 2 and 3.
Complex
1
2
3
Empirical formula
Molecular weight
Crystal system
Space group
a (Å)
C
21H27N4O8Sm
C36H40EuN7O15
962.71
Monoclinic
C2/c
14.052(7)
16.276(8)
17.397(9)
90
C36H36N6O12Tb2
1062.55
Monoclinic
C2/c
29.224(20)
11.766(8)
15.117(10)
90
613.82
Monoclinic
P2(1)/n
11.564(7)
11.973(7)
18.496(11)
90
b (Å)
c (Å)
a
(°)
b (°)
106.471(6)
90
2456(2)
4
102.038(6)
90
3891(3)
4
117.552(7)
90
4608(5)
4
c
(°)
V (Å3)
Z, Dc (mg/m3)
l
(mmꢂ1
)
2.443
1.692
3.103
F (000)
1228
1952
2080
h Range for data collection(°)
Crystal size (mm)
Limiting indices, h k l
2.30–25.49
0.25 ꢁ 0.23 ꢁ 0.20
ꢂ14 to 13
ꢂ11 to 14
ꢂ22 to 22
12,959
2.39–25.50
0.28 ꢁ 0.25 ꢁ 0.22
ꢂ9 to 16
ꢂ14 to 19
ꢂ21 to 21
8886
2.65–25.50
0.26 ꢁ 0.24 ꢁ 0.21
ꢂ35 to 33
ꢂ13 to 14
ꢂ18 to 18
12,187
Reflections collected
Unique reflections
Rint
4541
0.0533
3595
0.0641
4255
0.0487
Data/restraints/parameters
4541/3/309
1.006
3595/0/268
1.008
4255/0/253
0.927
Goodness-of-fit on F2
R1/wR2 [I > 2
r
(I)]
0.0331/0.0611
0.0534/0.0672
0.562, ꢂ0.848
0.0506/0.0819
0.0699/0.0881
1.003, ꢂ0.765
0.0399/0.0932
0.0665/0.1015
1.426, ꢂ1.547
R1/wR2 (all data)
Largest diff. peak, hole (e Åꢂ3
)
2.4. DNA-binding study methods
studied by adding a certain amount of a solution of the complex step
by step into the buffer solution of the DNA–EB complex [38]. The
fluorescence spectra of EB were measured using an excitation wave-
length of 520 nm, and the emission range was set between 550 and
750 nm. The influence of the addition of each compound to the
DNA–EB complex solution has been obtained by recording the vari-
ation of the fluorescence emission spectra. The spectra were ana-
lyzed according to the classical Stern–Volmer equation [39]:
2.4.1. Electronic absorption titration
All spectrophotometric measurements were performed in
thermostated quartz sample cells at 25 °C. Solutions for analysis
were prepared by dilution of stock solutions immediately before
the experiments. Spectrophotometer slit widths were kept at
1 nm for absorption spectroscopy and 5/5 nm for emission spec-
troscopy. Electronic absorption titration experiments were per-
formed by maintaining the concentration of the test compounds
I0=I ¼ 1 þ KSV½Qꢃ ¼ 1 þ Kqs0½Qꢃ
(ligand/complexes) as constant (30 lM) while gradually increasing
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, [Q] is the concentration of the
the concentration of CT-DNA. To obtain the absorption spectra, the
required amount of CT-DNA was added to both compound solution
and the reference solution to eliminate the absorbance of CT-DNA
itself [35,36]. Each sample solution was scanned in the range of
190–500 nm, and the mixture was allowed to equilibrate for
5 min before the spectra were recorded. From the absorption titra-
tion data, the binding constant (Kb) was determined using the
equation [9,10]:
quencher, Kq is the quenching rate constant and s0 is luminescence
decay time in absence of the quencher. In these experiments
[CT-DNA] = 2.5 ꢁ 10ꢂ3 mol/L, [EB] = 2.2 ꢁ 10ꢂ3 mol/L.
2.4.3. Viscosity titration measurements
Viscosity experiments were conducted on an Ubbelodhe vis-
cometer, 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.
½DNAꢃ=ðea
ꢂ
ef Þ ¼ ½DNAꢃ=ðeb
ꢂ
ef Þ þ 1=Kbðeb
ꢂ
ef
Þ
where [DNA] is the concentration of CT-DNA in base pairs, ea
corresponds to the extinction coefficient observed (Aobsd/[M]),
ef corresponds to the extinction 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
Titrations were performed for the complexes (3–30
l
M), and each
M) present
versus the ratio
of the concentration of the compound to CT-DNA, where is the
compound was introduced into CT-DNA solution (42.5
l
1/3
in the viscometer. Data were analyzed as (g/g0)
g
slope to intercept in the plot of [DNA]/(ea
the value of Kb.
ꢂ
ef) versus [DNA] gave
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 the 5 mM Tris-HCl/50 mM NaCl
2.4.2. Fluorescence studies
buffer alone (t0),
g
= (t ꢂ t0) [40].
EB emits intense fluorescence in the presence of CT-DNA, due to
its strong intercalation between the adjacent CT-DNA base pairs. It
was previously reported that the enhanced fluorescence can be
quenched by the addition of a second molecule [37]. The extent of
fluorescence quenching of EB bound to CT-DNA can be used to deter-
mine the extent of binding between the second molecule and CT-
DNA. The effect of each complex with the DNA–EB complex was
2.5. Antioxidation study methods
2.5.1. Hydroxyl radical assay
Hydroxyl radicals were generated in aqueous media through
the Fenton-type reaction [41]. The aliquots of reaction mixture

36
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
(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 com-
pound. 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 OHÅ was calculated from the following expression:
316 nm) are marginally shifted in complexes, which provide clear
evidence for nitrogen and oxygen atom coordination to metal ions.
Two absorption bands are assigned to
p ?
pꢄ (benzene) and
p
?
pꢄ (C@N) transitions [48].
3.2. X-ray structures of the complexes
The crystal analysis unambiguously reveals that complex 1
crystallizes in monoclinic space group P2(1)/n. The crystal struc-
ture of Sm(L)(NO3)(DMF)(H2O) demonstrates a discrete mononu-
clear species with the Sm(III) center in a nine-coordinate SmN2O7
coordination geometry [two imine NAN(1), N(2) from the amine
part; one ether OAO(1); two coordinated phenoxo OAO(2) and
OAO(3); one OAO(4) from DMF; two OAO(6) and O(7) from biden-
tate univalent NOꢂ3 and one coordinated water OAO(5)] as shown
in Fig. 1(a). The coordination polyhedron around Sm(III) is a dis-
torted tricapped trigonal prism (Fig. 1(b)). The bond lengths are
in the range Sm–Ophenoxo: 2.303(3)–2.333(3), Sm–Oether: 2.619(3),
Scavenging ratio ð%Þ ¼ ½ðAi ꢂ A0Þ=ðAc ꢂ A0Þꢃ ꢁ 100%
where Ai = absorbance in the presence of the test compound;
A0 = absorbance of the blank in the absence of the test compound;
Ac = absorbance in the absence of the test compound, EDTA-Fe(II)
and H2O2.
2.5.2. Superoxide radical assay
A nonenzymatic system containing 1 mL 9.9 ꢁ 10ꢂ6 M VitB2,
1 mL 1.38 ꢁ 10ꢂ4 M NBT, 1 mL 0.03 M MET were used to produce
superoxide anion (Oꢂ2 Å), and the scavenging rate of Oꢂ2 Å under the
Sm–ODMF: 2.448(3), Sm–Onitrate: 2.550(3)–2.598(3) and Sm–Owater
:
2.439(3) Å, all of which are within the range of those observed for
other nine-coordinated Ln(III) complexes with oxygen donor
ligands [49,50].
influence of 0.1–1.0 lM tested compound was determined by
monitoring the reduction in rate of transformation of NBT to
monoformazan dye [43]. The solution of MET, VitB2 and NBT were
prepared with 0.02 M phosphate buffer (pH = 7.8) at 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 = (1ꢂk0/k) ꢁ 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 SOD mimic compound (SOD
is superoxide dismutase), respectively. The IC50 values for the com-
plexes were determined by plotting the graph of percentage inhi-
bition of NBT reduction against the increase in the concentration
of the complex. The concentration of the complex which causes
Hydrogen-bonding interactions play important roles in crystal
packing of 1 [51]. As shown in Fig. 2, the neighboring [Sm(L)(NO3)
(DMF)(H2O)] moieties are connected by two hydrogen bonds
between the metal-coordinated H2O and the ligand with
O(5)ꢀ ꢀ ꢀO(2) and O(5)ꢀ ꢀ ꢀO(3) distances of 2.713(4) Å and 2.740(4) Å
and an infinite 2-D (two dimension) layer is propagated due to
the Hydrogen-bonding interactions. Moreover, each molecule is
linked by hydrogen bonds between the coordinated ligands to form
an infinite 3-D network (Fig. S1). The distinct topological structure
affords an opportunity to investigate the potential in molecular rec-
ognition and gas absorption.
The single-crystal X-ray analysis reveals that complex 2 crystal-
lizes in monoclinic space group C2/c and possesses a 1-D ribbon
framework constructed from an extended array of ten-coordinated
Eu3+ centers and the ligands. Each europium ion is coordinated by
six oxygen atoms from three bidentate nitrate groups and the
other four oxygen atoms from the phenoxo groups of the different
ligands, as shown in Fig. 3(a). The coordination polyhedron around
Eu(III) is a distorted bicapped square antiprism (Fig. 3(b)). The four
ligands act as bridging linkers through their phenoxo oxygen
atoms, where EuꢂHLꢂEu double-stranded helices are found
(Fig. S2). Two ligands form an ellipse with two europium ions
which are located at the top of the ellipse with the Euꢀ ꢀ ꢀEu separa-
tion of 12.778 Å, and the two ellipses are linked by the europium
ion. The adjacent ellipses are related by a twofold axis which
50% inhibition of NBT reduction is reported as IC50
.
3. Results and discussion
The Ln(III) complexes were prepared by reaction of H2L with
Ln(NO3)3(H2O)6 in ethanol. They are soluble in polar aprotic sol-
vents such as DMF, DMSO and MeCN, slightly soluble in water, eth-
anol, methanol, ethyl acetate, and chloroform and insoluble in Et2O
and petroleum ether. The elemental analysis shows that their com-
position which were confirmed by the crystal structure analysis.
3.1. IR and electronic spectra
The IR spectra of the Ln(III) complexes (Ln = Sm, Eu and Tb)
were analyzed in comparison with that of their common free
ligand H2L in the region 4000–400 cmꢂ1. In the free ligand H2L, a
strong band is found at 1637 cmꢂ1 together along with a weak
band at 1286 cmꢂ1. By analogy with the assigned bands, the former
can be attributed to
(CAOAC) [45,46]. The location of the two bands was slightly
shifted for complex 1, the band at 1637 cmꢂ1 is shifted to
1632 cmꢂ1 and the band at 1286 cmꢂ1 is shifted to 1290 cmꢂ1
m(C@N), while the latter can be attributed to
m
,
which can be attributed to the coordination of the ligand to the
metal center atom. Similar shifts also appear in complexes 2
(1633, 1292 cmꢂ1) and 3 (1633, 1214 cmꢂ1), which gives the same
conclusion. Moreover, bands at 1362, 1031 cmꢂ1 in 1, 1365,
1051 cmꢂ1 in 2 and 1368, 1052 cmꢂ1 in 3 indicate that nitrate is
bidentate [47], in agreement with X-ray diffraction.
DMF solutions of the ligand H2L and Ln(III) complexes show, as
expected, almost identical UV spectra. The UV bands of H2L (268,
Fig. 1. (a) Ball and stick representation of 1; H atoms are omitted for clarity. (b)
Coordination polyhedron of Sm.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
37
Fig. 2. 2-D layer structure formed of 1 constructed by hydrogen bonding which are indicated with dashed green lines. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Fig. 3. (a) One-dimensional structure of 2; H atoms are omitted for clarity. (b) Coordination polyhedron of Eu.
passes through the Eu1 atom and nitro atoms N4, O8. The euro-
pium ions in the same chain form an infinite 1D zigzag structure
and the Euꢀ ꢀ ꢀEuꢀ ꢀ ꢀEu angle is 147.55°. The chains are packed in
the same way, and the adjacent europium atoms in the different
chains are in the same plain.
The Eu–Ophenoxo distances range from 2.368(4) to 2.373(4) Å and
the Eu–Onitrate distances range from 2.549(4) to 2.662(4) Å. The
bond length is almost equal to those found in Eu complexes
[52,53]. The ligands are disposed in a facial arrangement with
Nꢀ ꢀ ꢀEuꢀ ꢀ ꢀN angles being symmetrical, while the O(phen-
oxo)ꢀ ꢀ ꢀEuꢀ ꢀ ꢀO(phenoxo) in the same ellipse displays a narrower
angle of 69.43°. This would appear to be an electrostatic rather than
steric effect, with the repulsions from two negatively charged
oxygens per nitrate ligand dominating the increased steric
repulsions from the bulky phenoxo groups. Most interestingly,
the polymer can offer a lot of perfectly cavities to trap guest mole-
cules (Fig. 4). Thus the solid may also have the potential for practical
applications such as gas absorption and molecular recognition.
Complex 3 crystallized in the monoclinic space group C2/c, and
the crystallographic analysis reveals that 3 is a centrosymmetric

38
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
Fig. 4. Parallel arrangement Eu(III) coordination chains contain of cavities, the large orange sphere indicates the size of the cavities. Hydrogen atoms are omitted for clarity.
neutral homobinuclear entity. The structure of 3 (Fig. 5(a)) shows
that two adjacent [Tb(L)(NO3)] moieties are bridged via two
under favorable conditions, which is important in host–guest
chemistry and has applications in chemistry, biology, and
materials science.
phenoxo groups. In the
l2-diphenoxo bridged binuclear structure
both Tb(III) centers are octacoordinated (Fig. 5(b)). The local coor-
dination environment is identical for both the centers by symme-
try and is best described as a distorted square TbN2O6 antiprism
(Fig. 5(b)). Due to the flexibility of the ligand, it loses its planarity.
The bond lengths are in the range Tb(1)–Nimine 2.495(5)–2.517(6),
Tb(1)–Oether: 2.469(4) and Tb(1)–Onitrate: 2.475(4)–2.488(4) Å. The
nature of coordination of the two identical Schiff base moieties
of the same ligand is completely different. Of the two phenoxo
oxygen atoms of each ligand, one is simply monocoordinated while
the other one bridges the adjacent Tb(III) centers as reflected by
the Tb–Ophenoxo bond lengths [Tb(1)–O(2), 2.330(4) and Tb(1)–
O(3), 2.181(5) Å ]. The distance of Tb(1)–Tb(1A): 3.771(2) Å is rel-
atively too long to consider any direct intramolecular Tb–Tb
interaction.
Hydrogen-bonding interactions are also very significant for 3.
The hydrogen bonding interactions lead to the formation of a 2-D
supramolecular network as shown in Fig. S3. An interesting feature
of this structure is the intermolecular hydrogen bond that exists
among the Tb(III) complexes, which also affords approximately
10 ꢁ 8 Å2 sized voids to trap guest molecules (Fig. 6). It is worth
noting that intermolecular interactions have the potential to
assemble smaller and simpler fragments into desired cavities
3.3. DNA binding properties
3.3.1. Absorption spectroscopic studies
Absorption titration can monitor the interaction of a compound
with DNA. The obvious hypochromism and red shift are usually
characterized by the noncovalently intercalative binding of com-
pounds to DNA helix, due to the strong stacking interaction
between the aromatic chromophore of the compound and base
pairs of DNA [54]. However, the intercalation between a compound
and DNA helix cannot be excluded only by no or small red shift of
UV–Vis absorption bands. In fact, some groove binders of Hoechst
33258 family can also present red shifts or even blue shifts of
absorption bands when they bind to DNA helix by groove binding
modes, especially for multiple binders [55].
To clarify the interactions between the compounds and DNA,
the electronic absorption spectra of the ligand and its Ln(III) com-
plexes (Ln = Sm, Eu and Tb) in the absence and in the presence of
the CT-DNA (at a constant concentration of the compounds) were
obtained, which are shown in Fig. 7, respectively. As can be seen
from Fig. 7a–d the ligand and complexes exhibit intense absorption
bands at 391–393 nm and addition of increasing amounts of CT-
DNA results in hypochromism and bathocromic shift in the UV–
Vis spectra of the compounds. In the present case, with addition
of DNA, the ligand exhibit hypochromism of about 32.4% accompa-
nied by bathochromism of about a 1–2 nm shift in the absorption
maxima. Corresponding complexes exhibit hypochromism of
about 70.1%, 68.2% and 58.9%, and also accompanied by bathochro-
mism of about a 1–3 nm shift in the absorption maxima. The hyp-
ochromism observed for the
p ?
pꢄ transition bands indicating
strong binding of the compounds to DNA.
To compare quantitatively the affinity of the ligand and Ln(III) com-
plexes towards DNA, the intrinsic binding constants Kb of the two com-
pounds to CT-DNA were determined by monitoring the changes of
absorbance with increasing concentration of DNA. The data of intrinsic
binding constants Kb are shown in Table 2. The Kb values of the ligand
and complexes 1–3are (5.30 0.096) ꢁ 103 Mꢂ1 (R2 = 0.99 for 16 points),
(1.19 0.112) ꢁ 105 Mꢂ1 (R2 = 0.99 for 16 points), (4.22 0.086) ꢁ 104
Mꢂ1 (R2 = 0.99 for 16 points) and (3.89 0.104) ꢁ 104 Mꢂ1 (R2 = 0.99
for 16 points), respectively, from the decay of the absorbances. The Kb
values obtained here are lower than that reported for classical intercalator
Fig. 5. (a) Ball and stick representation of 3; H atoms are omitted for clarity. (b)
Coordination polyhedron of Tb.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
39
Fig. 6. A space-filling diagram of the nanosized holes in 3.
Fig. 7. Absorption spectra of compound in the presence of CT-DNA (the DNA absorption was subtracted). The concentration of the ligand H2L (a) and complexes 1 (b), 2 (c),
and 3 (d) was kept constant at 3 ꢁ 10ꢂ5 Mꢂ1. Arrows show the absorbance changes upon increasing DNA concentration. Inset: Plots of [DNA]/(ea
of DNA with compound; j, experimental data points; solid line, linear fitting of the data.
ꢂ ef) versus for the titration
Table 2
Based on the above results, we found that the affinity for DNA is
stronger in case of Ln(III) complexes when compared with the
ligand H2L. For this difference, we attributed two possible reasons.
(i) The electrostatics of positively charged Ln3+ ions cause an inter-
action with polyanionic DNA. (ii) The charge transfer of coordi-
nated H2L, caused by coordination of the central Ln(III), results in
reduction of charge density of the planar conjugated system; this
change will lead to complexes binding to DNA more easily [9,10].
Parameters of Kb, KSV and Kq for Ligand and complexes 1, 2 and 3.
Compound
Kb ꢁ 105
0.053 0.0001
1.19 0.112
4.22 0.086
3.89 0.104
KSV ꢁ 104
0.35 0.010
2.01 0.099
1.83 0.121
1.50 0.075
Kq
H2L
1
2
0.35 ꢁ 1012
2.01 ꢁ 1012
1.83 ꢁ 1012
1.50 ꢁ 1012
3
(for ethidium bromide and [Ru(phen)DPPZ] whose binding constants
have been found to be in the order of 106–107 Mꢂ1)[56–58].Itisclearthat
the hypochromism and Kb values are not enough evidence, but these
results can suggest an intimate association of the compounds with CT-
DNA and indicate that the binding strength of complex is in the order of
1 > 2 > 3 > H2L.
3.3.2. Competitive binding with ethidium bromide
The ability of a complex to change the fluorescence intensity of
ethidium bromide (EB) in its EB–DNA adduct has been reported as
a standard intercalating agent of DNA and it is a reliable tool to
measure the affinity of the complex for DNA, irrespective of the
binding modes. Therefore, a solution of EB has been used as a

40
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
spectral probe since it does not emit in the buffer solution due to
probable quenching of its emission by the solvent [59]. However,
intense emission is observed when EB strongly intercalates with
the adjacent DNA base pairs. But a decrease in emission intensity
results from the displacement of EB by a quencher molecule. The
extent of emission quenching could be used to determine the
extent of binding between the metal complex with DNA [59].
For the ligand H2L and Ln(III) complexes, 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
complexes 1–3 are shown in Fig. 8a–d. The quenching of EB bound
to CT-DNA by the Ln(III) complexes are in good agreement with
the linear Stern–Volmer equation, which provides further evidence
that the Ln(III) complexes bind to DNA and only one type of quench-
ing process occurs. The data of KSV are presented in Table 2. The KSV
values of the ligand H2L and complexes 1–3 are (0.35 0.010) ꢁ 104
(R2 = 0.98 for 21 points in the line part), (2.01 0.099) ꢁ 104 Mꢂ1
(R2 = 0.98 for 10 points), (1.83 0.121) ꢁ 104 Mꢂ1 (R2 = 0.98 for 10
points) and (1.50 0.075) ꢁ 104 Mꢂ1 (R2 = 0.98 for 10 points),
respectively. The Stern–Volmer dynamic quenching constants can
also be interpreted as binding affinities of the complexation reac-
tions [60]. Consequently, the data of Ksv present the order of
1 > 2 > 3 > H2L, which is consistent with the previous absorption
spectral conclusions. Moreover, the Kq values for the ligand and
level [58], we consider it is impossible for the complexes to scramble
EtBrfromDNA.Similarfluorescencequenchingeffect of EtBrboundto
DNA has been observed for the addition of several groove-binding
compounds, including netropsin and distamycin A [21,62]. The
observed results make us to suspect that the complexes may interact
with DNA through the groove binding mode or intercalation mode,
releasing some EtBr molecules from EtBr–DNA system [62–64].
3.3.3. Viscosity titration measurements
Viscosity titration measurements were carried out to further
clarify the interaction modes between the investigated compounds
and CT-DNA. 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 [65]. 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 be in close proximity to the DNA base pairs as
well [66]. In contrast, molecule that binds exclusively in the DNA
grooves by partial and/or non-classical intercalation, under the
same conditions, typically cause less pronounced (positive or neg-
ative) or no change in DNA solution viscosity [67].
The effects of the ligand and complexes 1–3 on the viscosity of
CT-DNA is shown in Fig. 9. The experimental results exhibited that
the addition of the ligand and complexes 1–3 causes no significant
viscosity change, indicating that these compounds can bind to DNA
by groove modes [67]. In addition, the lanthanide complexes bind
to DNA via groove modes, due to the conjugated effect and
hydrogen-bonding interactions. It does not alter or destroy DNA
conformation and the environment of the complexes [20,21].
complexes 1–3 are 0.35 ꢁ 1012
,
2.01 ꢁ 1012
,
1.83 ꢁ 1012 and
1.50 ꢁ 1012 L Mꢂ1 Sꢂ1
, respectively, and were far larger than
2.0 ꢁ 1010 L Mꢂ1 Sꢂ1, the maximum diffusion collision quenching
rate constant of various quenchers with the biopolymer. Conse-
quently, the probable quenching mechanism of fluorescence of the
ligand and complexes 1–3 binding reactions should follow a static
quenching process rather than a dynamic one [61].
InsertfigureinFig. 8showthe plotsofI0/Iversus[Component]. The
data of KSV are all at 104 Mꢂ1 level for the ligand and its Ln(III) com-
plexes, accordingly. In view of the strong interaction of EtBr with
DNA of which the binding constant of EtBr with DNA is at 106 Mꢂ1
3.4. Antioxidant properties
Generation of reactive oxygen species (ROS) is a normal process
in the life of aerobic organisms. It has been estimated that free
Fig. 8. Fluorescence spectra of the DMF solution of the ligand H2L (a) and complexes
1 (b), 2 (c), and 3 (d) in Tris-HCl buffer upon addition of CT-DNA.
[Complex] = 3 ꢁ 10ꢂ5 Mꢂ1. Arrow shows the intensity changing upon increasing CT-DNA concentrations. A Sterne–Volmer quenching plot of the Ln(III) complexes inserting
in their own fluorescence spectra with increasing concentrations of CT-DNA.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
41
Fig. 9. Effect of increasing amounts of Ln(III) complexes on the relative viscosity of
CT-DNA at 25.0 0.1 °C.
Fig. 11. The inhibitory effect of three Ln(III) complexes on Oꢂ2 Å radicals; the
suppression ratio increases with increasing concentration of the test compound.
(OHÅ) with those of the well-known natural antioxidants mannitol
and vitamin C, using the same method as reported in a previous
paper [70]. The 50% inhibitory concentration (IC50) value of manni-
tol and vitamin C are about 9.6 ꢁ 10ꢂ3 and 8.7 ꢁ 10ꢂ3 M, respec-
tively. The results imply that the three Ln(III) complexes have the
preferable ability to scavenge hydroxyl radical (OHÅ) than mannitol
and vitamin C. It can be concluded that a much less or no scaveng-
ing activity was exhibited by the ligand when compared to that of
Ln(III) complexes which is due to the chelation of ligand with the
central metal atom [69].
radical-induced DNA damage in humans is at biologically relevant
levels, with approximately 104 DNA bases being oxidatively mod-
ified per cell per day. Oxidative damage to DNA has been suggested
to contribute to aging and various diseases including cancer and
chronic inflammation [68]. Since among all reactive oxygen spe-
cies, the hydroxyl radical (OHÅ) and superoxide radical (O2ꢂÅ) are
by far the most potent and therefore the most dangerous oxygen
metabolite, elimination of these radicals is one of the major aims
of antioxidant administration [69]. Consequently, in this paper,
the ligand and its Ln(III) complexes (Ln = Sm, Eu and Tb) studied
for their antioxidant activity by comparing their scavenging effects
on hydroxyl radical (OHÅ) and superoxide radical (Oꢂ2 Å).
3.4.2. Superoxide radical scavenging activity
As another assay of antioxidant activity, superoxide radical (Oꢂ2 Å)scav-
enging activity has been investigated. All the three Ln(III) complexes have
good superoxide radical scavenging activity. As can be seen from Fig. 11,
complexes 1–3 show the IC50 value of (6.01 0.084) ꢁ 10ꢂ5 M,
(4.38 0.065) ꢁ 10ꢂ5 M and (3.39 0.075) ꢁ 10ꢂ5 M, respectively. The
value of IC50 of vitamin C for superoxide radical scavenging effect is
(1.12 0.116) ꢁ 10ꢂ4 M (Fig. S4). The values of IC50 present the order
3 < 2 < 1 < vitamin C. The results imply that the three Ln(III) complexes
(Ln = Sm, Eu and Tb) also have the preferable ability to scavenge superox-
ide radical (Oꢂ2 Å) than vitamin C.
3.4.1. Hydroxyl radical scavenging activity
Fig. 10 shows the plots of hydroxyl radical scavenging effects
(%) for the ligand and complexes 1–3. The values of IC50 of the
ligand H2L and complexes 1–3 for hydroxyl radical scavenging
effects are (7.13 0.102) ꢁ 10ꢂ5 M, (5.58 0.074) ꢁ 10ꢂ5 M,
(3.39 0.091) ꢁ 10ꢂ5 M and (6.01 0.085) ꢁ 10ꢂ5 M, respectively.
The values of IC50 present the order 2 < 1 < 3 < H2L. It is proved that
the hydroxyl radical scavenging effects of Ln(III) complexes are
much higher than that of the ligand. Moreover, we compared the
abilities of one present compounds to scavenge hydroxyl radical
The lower IC50 values observed in hydroxyl radical and superox-
ide radical scavenging assays did demonstrate that the three Ln(III)
complexes have some scavenging effects for hydroxyl radical (OHÅ)
and superoxide radical (Oꢂ2 Å). However, due to the labile nature of
aqueous lanthanide complexes and the toxicity of uncomplexed
lanthanide ions, the Ln(III) complexes become an inhibitor (or a
drug) to scavenge hydroxyl radical (OHÅ) and superoxide radical
(Oꢂ2 Å) in vivo need further investigation.
4. Conclusion
In conclusion, the lanthanide (Sm(III), Eu(III) and Tb(III)) nitrate
complexes of a pentadentate Schiff base ligand bis(N-salicylid-
ene)-3-oxapentane-1,5-diamine (H2L) have been synthesized and
structurally characterized. The charge density and acidity of the
lanthanide imposes evident influences on the coordination geome-
try leading to three different types of structure. The experimental
results of DNA-binding about the Ln(III) complexes suggest that
the ligand and Ln(III) complexes bind to DNA in a groove mode
and the DNA-binding affinities of these three complexes follow the
order 1 > 2 > 3, which can be attribute to the electrostatics of posi-
tively charged Ln3+ ions and the charge transfer of coordinated
Fig. 10. The inhibitory effect of the ligand H2L and three Ln(III) complexes on OHÅ
radicals; the suppression ratio increases with increasing concentration of the test
compound.

42
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 135 (2014) 33–43
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National Natural Science Foundation of Gansu Province (Grant
No. 1212RJZA037), and ‘Qing Lan’ Talent Engineering Funds for
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