A seven-coordinated manganese(II) complex with V-shaped ligand bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine: Synthesis, structure, DNA-binding properties and antioxidant activities

Article abstract of DOI:10.1016/j.jphotobiol.2012.07.005

A manganese(II) complex of the type, [MnL(pic)₂]·H ₂O, was obtained by the reaction of the V-shaped ligand bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine (L) with Mn(pic)₂ (pic = picrate). The ligand L and Mn(II) complex were confirmed on the basis of elemental analysis, molar conductivities, ¹H NMR, IR, UV-vis spectra and X-ray crystallography. Single-crystal X-ray revealed that central Mn(II) atom is seven-coordinate with a MnN₃O₄ environment, in which ligand L acts as a tridentate N-donor. The remaining coordination sites were occupied by four O atoms afforded by two picrate anion. Interaction of the free ligand L and Mn(II) complex with DNA were investigated by spectrophotometric methods and viscosity measurements. The results suggested that both ligand L and Mn(II) complex bind to DNA in an intercalative binding mode, and DNA-binding affinity of the Mn(II) complex is stronger than that of ligand L. Moreover, antioxidant assay in vitro shows the Mn(II) complex possesses significant antioxidant activities.

Full text of DOI:10.1016/j.jphotobiol.2012.07.005

Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
A seven-coordinated manganese(II) complex with V-shaped ligand
bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine: Synthesis,
structure, DNA-binding properties and antioxidant activities
⇑
Huilu Wu , Jingkun Yuan, Ying Bai, Hua Wang, Guolong Pan, 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:
Received 14 May 2012
Received in revised form 30 June 2012
Accepted 16 July 2012
Available online 26 July 2012
A manganese(II) complex of the type, [MnL(pic)2]ꢀH2O, was obtained by the reaction of the V-shaped
ligand bis(N-benzylbenzimidazol-2-ylmethyl)benzylamine (L) with Mn(pic)2 (pic = picrate). The ligand
L and Mn(II) complex were confirmed on the basis of elemental analysis, molar conductivities, 1H
NMR, IR, UV–vis spectra and X-ray crystallography. Single-crystal X-ray revealed that central Mn(II) atom
is seven-coordinate with a MnN3O4 environment, in which ligand L acts as a tridentate N-donor. The
remaining coordination sites were occupied by four O atoms afforded by two picrate anion. Interaction
of the free ligand L and Mn(II) complex with DNA were investigated by spectrophotometric methods
and viscosity measurements. The results suggested that both ligand L and Mn(II) complex bind to DNA
in an intercalative binding mode, and DNA-binding affinity of the Mn(II) complex is stronger than that
of ligand L. Moreover, antioxidant assay in vitro shows the Mn(II) complex possesses significant antiox-
idant activities.
Keywords:
Benzimidazole
Manganese(II) complex
Crystal structure
DNA-binding property
Antioxidant property
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
istry [18]. Due to their intriguing variety architectures [19,20] and
important properties that span from luminescence [21] to biologi-
Benzimidazole and its derivatives have attracted considerable
interests in recent years for their versatile properties in chemistry
and pharmacology [1,2]. As a part of the chemical structure of vita-
min B12 [3], the benzimidazole scaffold is a privileged structural
motif for displaying chemical functionality in biologically active
molecules [4]. Some of its derivatives have potent pharmacological
applications as anticancer [5], antihypertensive [6], antiviral [7],
anti-inflammatory [8], vasodilator [9] and antimicrobial agents
[10]. Moreover, as a typical heterocyclic ligand, the large benzimid-
azole rings not only can provide potential supramolecular recogni-
cal activities [22], the rational design and synthesis of the transition
metal complexes containing benzimidazole-based ligands also
gives the possibility for further research, such as design of struc-
tural probes and the development of novel therapeutics.
As a section of our program for constructing of V-shaped bis-
benzimidazol ligands containing the different substitutive groups
with corresponding transition metal complexes, we have investi-
gated the DNA binding ability of such complexes in our previous
publications [23–26]. In this contribution, the synthesis, character-
ization and DNA-binding activities of Mn(II) complex with a
V-shaped ligand are presented. Additionally, the antioxidant prop-
erty for Mn(II) complex have also been presented and discussed in
detail.
tion sites for
p p stacking interactions, but also act as hydrogen
ꢀ ꢀ ꢀ
bond acceptors and donors to assemble multiple coordination
geometry [11].
The interaction of small molecules with DNA is a dynamic, thriv-
ing field which has drawn ever increasing research interests in re-
cent decades [12–14]. An understanding that how these small
molecules bind to DNA will potentially be useful in the design of
such new compounds, which can recognize specific sites or confor-
mations of DNA [15–17]. Hence, studies on the interaction of small
molecules, such as transition metal complexes, with DNA have been
a pet subject for the researchers in the field of bioinorganic chem-
2. Experimental section
2.1. Materials and methods
Calf thymus DNA (CT-DNA), ethidium bromide (EB), nitroblue
tetrazolium nitrate (NBT), methionine (MET) and riboflavin (VitB2)
were obtained from Sigma–Aldrich Co. (USA). EDTA and safranin
were produced in China. Other reagents and solvents were reagent
grade obtained from commercial sources and used without further
purification. Tris-HCl buffer, Na2HPO4–NaH2PO4 buffer, and
⇑
Corresponding author. Tel.: +86 13893117544.
E-mail address: wuhuilu@163.com (H. Wu).
1011-1344/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotobiol.2012.07.005

14
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
EDTA–Fe(II) solution were prepared using bi-distilled water. 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 suffi-
ciently free of protein [27]. The stock solution of DNA
(2.5 ꢁ 10ꢂ3 M) was prepared in 5 mM Tris-HCl/50 mM NaCl buffer
(pH = 7.2, stored at 4 °C and used when not more than 4 days). The
DNA concentration was determined by measuring the UV absorp-
room temperature. Crystal suitable for X-ray measurement was
obtained after 3 weeks.
Yield: 246.5 mg (68%). Found (%): C, 54.48; H, 3.87; N, 14.47.
Calcd. (%) for C49H39MnN11O15: C, 54.65; H, 3.65; N, 14.31.
Km(DMF, 297 K): 39.38 S cm2 molꢂ1. UV–vis (k, nm): 270, 279,
381. FT–IR (KBr
(OANAO); 1490,
m
/cmꢂ1): 746,
m(C@N); 1618,
m
m
(o–Ar); 1267,
(C@C).
m(CAN); 1365,
m
tion at 260 nm, taking the molar absorption coefficient (e260) of
CT-DNA as 6600 Mꢂ1 cmꢂ1 [28]. Mn(pic)2 was obtained by the
reaction of the picric acid with MnCO3. The stock solution of L
and Mn(II) complex were dissolved in DMF at the concentration
3 ꢁ 10ꢂ3 M.
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
Elemental analyses were determined using a Carlo Erba 1106 ele-
mental analyzer. Electrolytic conductance measurements were
made with a DDS-307 type conductivity bridge using 3 ꢁ 10ꢂ3 mol
Lꢂ1 solutions in DMF at room temperature. The IR spectra were re-
corded in the 4000–400 cmꢂ1 region with a Nicolet FT-VERTEX 70
spectrometer using KBr pellets. Electronic spectra were taken on a
Lab-Tech UV Bluestar spectrophotometer. The fluorescence spectra
were performed on a LS-45 spectrofluorophotometer. The absor-
bance was measured with Spectrumlab 722sp spectrophotometer
at room temperature. 1H NMR spectra were recorded on a Varian
VR300-MHz spectrometer with TMS as an internal standard.
with graphite-monochromated Mo Ka radiation (k = 0.71073 Å) at
296 K. Data reduction and cell refinement were performed using
the SMART and SAINT programs [30]. The structure was solved
by direct methods and refined by full-matrix least-squares against
F2 of data using SHELXTL software [31]. 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).
2.4. DNA-binding experiments
2.4.1. Viscosity titration measurements
2.2. Synthesis
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.
2.2.1. Synthesis of the ligand L
The precursor bis(2-benzimidazol-2-ylmethyl)benzylamine
(bbb) was synthesized following a slight modification of the proce-
dure in ref [29]. bbb (7.34 g, 20 mmol) and potassium (1.56 g,
40 mmol) were put in tetrahydrofuran (150 mL), the solution was
refluxed on a water bath for 5 h with stirring. Then, benzylbromide
(6.84 g, 40 mmol) was added to this solution. With the dropping of
benzylbromide, the solution gradually becomes cream yellow.
After that, the resulting solution was concentrated and cooled, a
pale yellow solid separating out and the pale yellow precipitate
was filtered, washed with massive water, and recrystallized from
ethanol to give the ligand.
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 analysed as (g/g0)
g
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 cor-
rected from the flow time of buffer alone (t0),
g
= (t ꢂ t0) [32].
Yield: 5.56 g, 61%; m.p.: 156–158 °C. Found (%): C, 81.02; H,
6.26; N, 12.69. Calcd. (%) for C37H33N5: C, 81.14; H, 6.07; N,
12,79. 1H NMR (DMSO-d6 400 MHz) d/ppm: 3.77 (m, 2H,
ACH2AAr), 3.85 (s, 4H, ACH2-benzimidazol), 5.27 (m, 4H,
ACH2AAr), 6.89 (m, 5H, H-benzene ring), 7.19–7.26 (s, 10H, H-ben-
zene ring), 7.42–7.64 (m, 8H, H-benzimidazol ring). Km(DMF,
2.4.2. 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
297 K): 4.16 S cm2 molꢂ1. UV–vis (k, nm): 279, 286. FT–IR (KBr
m/
cmꢂ1): 740,
m
(o–Ar); 1286, (CAN); 1464, (C@N), 1612, (C@C).
m
m
m
The synthetic routine of ligand L is showed in Scheme 1.
(ligands/complexes) as constant (30 lM) while gradually increas-
ing 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. 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 [33]:
2.2.2. Synthesis of complex: [MnL(pic)2]ꢀH2O
To a stirred solution of L (273.5 mg, 0.50 mmol) in hot EtOH
(10 mL) was added Mn(pic)2 (127.8 mg, 0.25 mmol) in EtOH
(2 mL). A yellow crystalline product formed rapidly. The precipi-
tate was filtered off, washed with EtOH and absolute Et2O, and
dried in vacuo. The dried precipitate was dissolved in MeCN to
form a yellow solution into which Et2O was allowed to diffuse at
Scheme 1. Synthetic routine of ligand L.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
15
Table 1
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) [39]. The scavenging effect
for OHꢀ was calculated from the following expression:
Crystal data and structure refinement for [MnL(pic)2]ꢀH2O.
Complex
[MnL(pic)2]ꢀH2O
49H39MnN11O15
1076.85
Molecular formula
Molecular weight
Color
C
yellow
Crystal size (mm3)
Crystal system
Space group
a (Å)
0.32 ꢁ 0.28 ꢁ 0.26
Triclinic
P-1
10.6596(15)
12.9361(18)
18.509(3)
Scavenging ratioð%Þ ¼ ½ðAi ꢂ A0Þ=ðAc ꢂ A0Þꢃ ꢁ 100%
b (Å)
c (Å)
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.
a
b (°)
(°)
87.0440(10)
81.6040(10)
77.5880(10)
2465.4(6)
c
(°)
V (Å3)
Z
2
T (K)
296(2)
1.451
1110
Dcalcd (g cmꢂ3
F (000) (e)
)
2.5.2. Superoxide radical scavenging activity
The superoxide radical was investigated indirectly using the
system of MET-VitB2-NBT [40,41]. The aqueous solution contained
0.5 mL 3.3 ꢁ 10ꢂ5 M VitB2, 1 mL 2.3 ꢁ 10ꢂ4 M NBT, 1 mL 0.05 M
MET, and the complexes of various concentrations were prepared
with 0.067 M phosphate buffer (Na2HPO4–NaH2PO4, pH = 7.8).
They were illuminated by a fluorescent lamp with a constant light
intensity at 25 °C. The optical absorbance (A) of the solution at
560 nm was measured with various illumination periods (t). All
the tests were run in triplicate and are expressed as the mean
and standard deviation (SD) [39]. The scavenging effect for O2ꢂꢀ
was calculated from the following expression:
h Range for data collection (°)
hkl range
2.39–25.99
ꢂ12 6 h 6 13, ꢂ15 6 k 6 15,
ꢂ22 6 l 6 22
18758
9517 [Rint = 0.0253]
Full-matrix least-squares on F2
9517/17/685
0.0495/0.1185
0.0758/0.1358
1.015
Reflections collected
Independent reflections
Refinement method
Data/restraints/parameters
Final R1/wR2 indices [I P 2
R1/wR2 indices (all data)a
Goodness-of-fit on F2
r
(I)]a
Largest diff. peak and hole (e Åꢂ3
)
0.647 and ꢂ0.394
Scavenging ratioð%Þ ¼ ½ðA0 ꢂ AiÞ=A0ꢃ ꢁ 100%
where Ai = absorbance in the presence of the tested compound,
A0 = absorbance in the absence of the tested compound.
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,
e
a corre-
sponds to the observed extinction coefficient (Aobsd/[M]), ef corre-
sponds 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 slope to
intercept in the plot of [DNA]/(ea
of Kb.
3. Results and discussion
Caution: Picrate salts of metal complexes with organic ligands
are potentially explosive. Only small amounts of the material
should be prepared and handled with great care.
ꢂ
ef) versus [DNA] gave the value
Ligand L and Mn(II) complex are very stable in the air. Mn(II)
complex is remarkably soluble in polar aprotic solvents such as
DMF, DMSO and MeCN; slightly soluble in ethanol, methanol, ethyl
acetate and chloroform; insoluble in water, Et2O, and petroleum
ether. The molar conductivities in DMF solution indicate that li-
gand L and Mn(II) complex are nonelectrolyte compounds [42,43].
2.4.3. Competitive binding with ethidium bromide
The enhanced fluorescence of EB in the presence of DNA can be
quenched by the addition of a second molecule [34,35]. 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 concentra-
tions of the compounds. The fluorescence spectra of EB were mea-
sured using an excitation wavelength of 520 nm, and the emission
range was set between 550 and 750 nm. The influence of the addi-
tion of each compound to the DNA-EB complex solution has been
obtained by recording the variation of the fluorescence emission
spectra. The spectra were analyzed according to the classical
Stern–Volmer equation [36]:
3.1. IR and electronic spectra
The IR spectra of the Mn(II) complex are closely related to that
of the free ligand L. One of the most diagnostic changes occurs in
Table 2
Selected bond distances (Å) and angles (°).
Complex
[MnL(pic)2]ꢀH2O
I0=I ¼ 1 þ KSV
Bond distances
MnAO(1)
MnAO(2)
MnAO(8)
MnAO(9)
2.151(2) MnAN(1)
2.466(2) MnAN(3)
2.080(2) MnAN(5)
2.497(2)
2.502(2)
2.206(2)
2.221(2)
where I0 and I are the fluorescence intensities at 599 nm in the ab-
sence and presence of the quencher, respectively, Ksv is the linear
Stern–Volmer quenching constant, and [Q] is the concentration of
Bond angles
O(8)AMnAO(1) 86.47(8) N(3)AMnAO(9) 136.42(8)
O(8)AMnAN(3) 93.64(8) N(5)AMnAO(9) 86.51(8)
O(1)AMnAN(3) 147.17(8) O(2)AMnAO(9) 143.92(7)
O(8)AMnAN(5) 157.08(8) O(8)AMnAN(1) 103.31(8)
O(1)AMnAN(5) 85.51(8) O(1)AMnAN(1) 139.43(8)
N(3)AMnAN(5) 104.64(8) N(3)AMnAN(1) 72.40(7)
O(8)AMnAO(2) 101.24(9) N(5)AMnAN(1) 70.19(7)
O(1)AMnAO(2) 70.10(7) O(2)AMnAN(1) 142.15(7)
N(3)AMnAO(2) 77.74(8) O(9)AMnAN(1) 72.33(7)
N(5)AMnAO(2) 96.13(8) O(1)AMnAO(9) 74.27(8)
O(8)AMnAO(9) 70.65(8)
the quencher. In these experiments [CT-DNA] = 2.5 ꢁ 10ꢂ3 mol Lꢂ1
,
[EB] = 2.2 ꢁ 10ꢂ3 mol Lꢂ1
.
2.5. Antioxidant property
2.5.1. Hydroxyl radical scavenging activity
Hydroxyl radicals were generated in aqueous media through
the Fenton-type reaction [37,38]. The aliquots of reaction mixture
(3 mL) contained 1.0 mL of 0.10 mmol aqueous safranin, 1 mL of

16
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
the region between 1650 and 1250 cmꢂ1. The spectra of L show a
strong band at 1464 cmꢂ1 and weak bands at 1612 cmꢂ1. By anal-
ogy with the assigned bands of imidazole, two bands are attributed
to the
m
(C@N) and
m
(C@C) frequencies of the benzimidazole group,
(C@N) undergo a red shift (about
respectively [44–46]. The band
m
26 nm) in the Mn(II) complex as compared to free ligand L, which
can be attributed to the coordination of the benzimidazole nitro-
gen to the metal center [47]. As found in other metal complexes
with benzimidazoles, these are the preferred nitrogen atom for
coordination [48]. Moreover, Information regarding the possible
bonding modes of the picrate and benzimidazole rings may also
be obtained from the IR spectra [24,25].
DMF solutions of ligand L and Mn(II) complex show, as ex-
pected, almost identical UV spectra. The UV bands of ligand L
(286, 279 nm) are undergo a red-shifted about 9 nm for Mn(II)
complex, which is evidence of C@N coordination to the metal cen-
ter. These bands are assigned to n ? p⁄ and
p ?
p⁄ (imidazole)
transitions. The picrate bands (observed at 381 nm) are assigned
to
p
?
p⁄ transitions [23,24].
3.2. X-ray structures of the complex
Fig. 2. The coordination sphere around the Mn(II) atom showing the monocapped
(O9) trigonal prismaticin light blue.
Basic crystal data, description of the diffraction experiment, and
details of the structure refinement are given in Table 1. Selected
bond distances and angles are presented in Table 2.
The crystal structure of Mn(II) complex consists of a discrete
[MnL(pic)2] and a solvent water molecule. The solvent water mol-
ecules are present in the crystal lattice, but have no direct interac-
tions with the [MnL(pic)2]. The ORTEP structure (30% probability
ellipsoids) of the [MnL(pic)2] with atom-numberings is shown in
Fig. 1.
The central Mn(II) atom is seven-coordinated with a MnN3O4
coordination environment. As depicted in the Fig. 2, the coordina-
tion geometry of the central Mn(II) atom is a distorted mono-
capped trigonal prismatic, made up by four O atoms provided by
two picrate and three N atoms afforded by a tridentate ligand L.
Three N atoms constructed an undersurface while another under-
surface was constituted by three O atoms (O1, O2, O8) and the
O9 atom in the capping position [49].
As shown in Fig. 3, there are two types of the
p p interactions
ꢀ ꢀ ꢀ
in Mn(II) complex [50–53]: (i) between benzene ring and picrate,
d = 3.722 Å (d = centroid-to-centroid distance). (ii) between two
benzimidazole rings from different structure units, d = 3.884 Å.
Hence, an infinite 2-D (two dimension) layer is propagated due
to the
pꢀ ꢀ ꢀp interactions.
Due to the exist of the solvent water molecule which inlayed in
the coordination cations as a sandwich, such arrangement not only
can provide some groups for the formation of hydrogen bonds that
make the crystal structure more stable, but also contribute to con-
struct an infinite 3-D network (Fig. 4). The distinct topological
structure affords an opportunity to investigate the potential in
molecular recognition and gas absorption.
3.3. DNA binding properties
3.3.1. Viscosity titration measurements
Hydrodynamic measurements that are sensitive to the length
change (i.e., viscosity and sedimentation) are regarded as the least
ambiguous and the most critical tests of a interaction model in
solution in the absence of crystallographic structural data
[27,54]. A classical intercalative molecular interaction causes a sig-
nificant increase in viscosity of the DNA solution due to the in-
crease in separation of the base pairs at the intercalation sites
and hence an increase in the overall DNA length [55,56]. Whereas,
agents bound to DNA through groove binding do not alter the rel-
ative viscosity of DNA, and agents electrostatically bound to DNA
will bend or kink the DNA helix, reducing its effective length and
its viscosity, concomitantly [54,57].
Viscosity titration measurements were carried out to clarify the
interaction modes between the investigated compounds and CT-
DNA. The results of ligand L and Mn(II) complex on the viscosities
of CT-DNA are shown in Fig. 5. The viscosity of CT-DNA is increas-
ing steadily with the increment of ligand L and Mn(II) complex,
indicating that ligand L and Mn(II) complex can interaction with
CT-DNA in an intercalate mode, which may due to the large copla-
nar aromatic rings in two compounds that facilitate it intercalating
to the base pairs of double helical DNA [55,56].
3.3.2. Absorption spectroscopic studies
The application of electronic absorption spectroscopy is one of
the most useful techniques for DNA binding studies [33]. In
Fig. 1. Molecular structure and atom numberings of [MnL(pic)2] with hydrogen
atoms were omitted for clarity.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
17
helix by intercalation, and it is noteworthy that the hypochromicity
of Mn(II) complex is greater than that of ligand L [59,60].
To compare quantitatively the affinity of ligand L and Mn(II)
complex towards DNA, the intrinsic binding constant (Kb) of ligand
L and Mn(II) complex was calculated by plotting the changes in
the absorbance of the complex upon incremental addition of
increasing concentration of DNA. The Kb values of ligand L and
Mn(II) complex are 1.78 ꢁ 103 Mꢂ1 (R = 0.99 for 16 points),
4.35 ꢁ 104 Mꢂ1 (R = 0.99 for 16 points), respectively. Therefore,
compare with the classic DNA-intercalative reagent, such as Ethi-
dium bromide (EB), acridine orange (AO), methylene blue (MB)
[61], we speculate that ligand L and Mn(II) complex most possibly
binds to DNA in an intercalation mode. The magnitude of Kb value
is parallel to the intercalative strength and the affinity of a com-
pound binding to DNA [59,60]. Hence, the result of the absorption
spectral suggested that DNA-binding affinity of the Mn(II) complex
is stronger than that of ligand L, which is consistent with the above
viscosity titration measurements results.
3.3.3. Competitive binding with ethidium bromide
Ethidium bromide, a polycyclic aromatic dye, is the most widely
used fluorescence probe for DNA structure. It binds to DNA by
intercalation within the stacked bases [62]. It has been reported
that the enhanced fluorescence of the EB–DNA complex can be
quenched at least partially by the addition of a second molecule
and this could be used to assess the relative affinity of the molecule
for DNA [63]. No luminescence was observed for ligand L and
Mn(II) complex at room temperature in any organic solvent or in
the presence of CT-DNA. So the binding of complexes with CT-
DNA cannot be directly presented in the emission spectra. There-
fore, competitive EB binding studies could be undertaken in order
to examine the binding of each complex with DNA.
Fig. 3. 2-D layer formed via
pꢀ ꢀ ꢀp interactions in [MnL(pic)2]ꢀH2O, hydrogen atoms
and solvent molecules were omitted for clarity (different interactions are distin-
guished by different colors). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
The emission spectra of EB-DNA system in the absence and
presence of complex are displayed in Fig. 7. The behaviors of ligand
L and Mn(II) complex are in good agreement with the Stern–Vol-
mer equation, which provide further evidence that there are some
interactions between ligand L and Mn(II) complex and DNA. The
Ksv values of ligand L and Mn(II) complex are 1.67 ꢁ 104 Mꢂ1
(R = 0.98 for 11 points in the linear part), 2.91 ꢁ 104 Mꢂ1
(R = 0.97 for 10 points in the linear part), respectively. The phe-
nomena suggest that ligand L and Mn(II) complex can compete
for DNA-binding sites with EB and displace it from the EB-DNA sys-
tem [64], which is usually characteristic of the intercalative inter-
action of compounds with DNA [65]. Moreover, the affinity for DNA
is more strongly in case of Mn(II) complex when compared with Li-
gand L. Such a trend is consistent with the previous viscosity titra-
tion and absorption spectral results.
Based on the results of viscosity measurements and spectropho-
tometric methods above, we found that the affinity for DNA is
more strongly in case of Mn(II) complex when compared with
the ligand L. For this difference, we attributed to three possible rea-
sons. (i) By comparison of the molecular structure of the ligand L
and Mn(II) complex, we find the greater number of coplanar aro-
matic rings may lead to higher affinity for DNA [25,26]. (ii) The
charge transfer of coordinated ligands caused by the coordination
of the central atom, lead to the decrease of the charge density of
the plane conjugate system, which is conducive to insert [66].
(iii) This difference in their DNA binding ability also could be
attributed to the presence of an electron deficient center in the
charged Mn(II) complex where an additional interaction between
the complex and phosphate rich DNA back bone may occur [67,68].
Fig. 4. 3-D network formed in [MnL(pic)2]ꢀH2O (In order to simplify, only central
Mn(II) atoms were reserved).
general, compound binding with DNA through intercalation usually
results in hypochromism and bathochromism, due to the intercala-
tive mode involving a strong stacking interaction between an
aromatic chromophore and the DNA base pairs. The extent of the
hypochromism commonly parallels the intercalative binding
strength [58]. The absorption spectra of ligand L and Mn(II)
complex in the absence or presence of CT-DNA are given in Fig. 6.
As the DNA concentration is increased, the hypochromism reaches
as high as 28.32% at 272 nm for free ligand L; 44.39% at 271 nm for
Mn(II) complex. The kmax for ligand L and Mn(II) complex have a
slight red shifts of about 1–2 nm under identical experimental con-
ditions. The results suggested an association of the compound with
DNA and it is also likely that ligand L and Mn(II) complex bind to the
3.4. Antioxidant property
Since some similar complexes exhibit reasonable DNA-binding
affinity, it is worthwhile considered to study other potential

18
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
Fig. 5. Effect of increasing amounts of (a) ligand L and (b) Mn(II) complex on the relative viscosity of CT-DNA at 25.0 0.1 °C.
Fig. 6. Electronic spectra of (a) ligand L, (c) Mn(II) complex in Tris–HCl buffer upon addition of CT-DNA. [Compound] = 3 ꢁ 10ꢂ5 Mꢂ1, [DNA] = 2.5 ꢁ 10ꢂ5 Mꢂ1. Arrow shows
the emission intensity changes upon increasing DNA concentration. Plots of [DNA]/(ea
ꢂ ef) versus. [DNA] for the titration of (b) ligand L, (d) Mn(II) complex with CT-DNA.

H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
19
Fig. 7. Emission spectra of EB bound to CT-DNA in the presence of (a) ligand L and (c) Mn(II) complex; [Compound] = 3 ꢁ 10ꢂ5 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) ligand L and (d) Mn(II) complex. (Plots of I0/I versus
[Complex]).
aspects, such as antioxidant and antiradical activity. It is a well-
documented fact that some transition metal complexes display sig-
nificant antioxidant activity [69–71] and therefore we undertook a
systematic investigation on the antioxidant potential of free ligand
L and Mn(II) complex .
3.4.1. Hydroxyl radical scavenging activity
We compared the ability of Mn(II) complex to scavenge hydro-
xyl radicals with those of the well-known natural antioxidants
mannitol and vitamin C, using the same method as reported in a
previous paper [72]. The 50% inhibitory concentration (IC50) value
of mannitol and vitamin C are about 9.6 ꢁ 10ꢂ3 and 8.7 ꢁ 10ꢂ3
Mꢂ1, respectively. As shown in Fig. 8, according to the antioxidant
experiments, the IC50 values of Mn(II) complex is 3.56 ꢁ 10ꢂ6 Mꢂ1
,
which implies that Mn(II) complex has the preferable ability to
scavenge hydroxyl radical. It can be concluded that a much less
or no scavenging activity was exhibited by the free ligand L when
compared to that of Mn(II) complex which may be due to the che-
lation of ligand with the central metal atom [73]. The lower IC50
values observed in antioxidant assays demonstrate that Mn(II)
complex has a strong potential to be applied as scavengers to elim-
inate the radicals.
Fig. 8. A plot of scavenging percentage (%) versus concentration of the Mn(II)
complex on hydroxyl radical.

20
H. Wu et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 13–21
Appendix A. Supplementary material
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre with reference number CCDC
872018. Copies of the data can be obtained, free of charge, on
application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Tel: +44-01223-762910; fax: +44-01223-336033; e-mail: depos-
it@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.jphotobiol.2012.07.005.
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Acknowledgements
The authors acknowledge the financial support and Grant from
‘Qing Lan’ Talent Engineering Funds by Lanzhou Jiaotong Univer-
sity. The grant from ‘Long Yuan Qing Nian’ of Gansu Province also
is acknowledged.

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