Synthesis, characterization and DNA interaction studies of complexes of lanthanide nitrates with tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}amine

Article abstract of DOI:10.1016/j.saa.2007.10.023

A new tripodal, hydroxyl-rich ligand, tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}amine (L), and its complexes with lanthanide nitrates were synthesized. These complexes which are stable in air with the general formula of [LnL(NO₃)₂]NO₃·H₂O (Ln = La, Sm, Eu, Gd, Y) were characterized by molar conductivity, elemental analysis, IR spectra and thermal analysis. The NO₃^- groups coordinated to lanthanide mono-dentately, and the coordination number in these complexes may be 8. The interaction of complexes with DNA were investigated by ultraviolet and fluorescent spectra, which showed that the binding mode of complexes with DNA was intercalation, and the binding affinity with DNA were La(III) complex > Sm(III) complex > Eu(III) complex > Gd(III) complex > Y(III) complex. Based on these results, it can be shown that the La(III)complex is promising candidate for therapeutic reagents and DNA probes.

Full text of DOI:10.1016/j.saa.2007.10.023

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Spectrochimica Acta Part A 70 (2008) 1114–1119
Synthesis, characterization and DNA interaction studies of complexes
of lanthanide nitrates with
tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}amine
Min Liua, Wen-bing Yuana,b,∗, Qi Zhanga, Lan Yanb, Ru-dong Yangb
a Key laboratory of Tropic Biological Resources of Ministry of Education, Hainan University, Haikou 570228, China
b College of Chemistry and Chemical Engineering, Lanzhou University, Gansu 730000, China
Received 18 August 2007; received in revised form 17 October 2007; accepted 18 October 2007
Abstract
A new tripodal, hydroxyl-rich ligand, tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}amine (L), and its complexes with lanthanide nitrates
were synthesized. These complexes which are stable in air with the general formula of [LnL(NO3)2]NO3·H2O (Ln = La, Sm, Eu, Gd, Y) were
−
characterized by molar conductivity, elemental analysis, IR spectra and thermal analysis. The NO3 groups coordinated to lanthanide mono-
dentately, and the coordination number in these complexes may be 8. The interaction of complexes with DNA were investigated by ultraviolet and
fluorescent spectra, which showed that the binding mode of complexes with DNA was intercalation, and the binding affinity with DNA were La(III)
complex > Sm(III) complex > Eu(III) complex > Gd(III) complex > Y(III) complex. Based on these results, it can be shown that the La(III)complex
is promising candidate for therapeutic reagents and DNA probes.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Lanthanide; Tripodal ligand; Complexes; Synthesis; DNA interaction
1. Introduction
is observed at 226 and 258 nm after the interaction of Eu-
phenylalanine and DNA, and fluorescent intensity is decreased
obviously. It shows that Eu-phenylalanine can strengthen the
interaction with DNA [7]. However, compared with transition
metal complexes, the studies of the interaction of lanthanide
complexeswithcalfthymusDNAarefew, especiallywhentripo-
dal ligands are used as hosts. As our studies on complexes with
tripodal ligands, a new polydentate, hydroxyl-rich tripodal lig-
and tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}amine (L)
and its complexes with lanthanide are synthesized and charac-
terized, and the interaction with calf thymus DNA is investigated
by ultraviolet and fluorescent spectra.
DNA binding studies are very important for the development
of new therapeutic reagents and DNA probes [1,2]. Generally,
there are three kinds of binding models for small molecules with
DNA, i.e., (I) intercalation, (II) groove binding, and (III) external
electrostatic binding. In these binding models, the intercalative
binding is strongest, because it is a type of binding in which
the intercalative molecule surface is sandwiched between the
aromatic, heterocyclic base pairs of DNA [3,4].
The luminescent characteristics of lanthanide complexes and
their binding affinity with DNA have led to their general appli-
cation as spectroscopic probes for nucleic acids [5,6]. And
compared with lanthanide metal ions, their complexes may
have different binding models and stronger binding affinity. The
interaction of Eu(III) and DNA have a small effect on the charac-
teristic absorption peak of DNA in UV spectra. Hypochromicity
2. Experimental
2.1. Reagents and apparatus
Lanthanide nitrates and tris(2-aminoethyl)amine (abbrevi-
ated as tren in the following) were prepared according to Refs.
[8,9]. All other solvents and reagents used were of analyt-
ical grade. All the experiments involving interaction of the
complex with DNA were performed in Tris–HCI (0.01 M, pH
∗
Corresponding author at: Key laboratory of Tropic Biological Resources of
Ministry of Education, Hainan University, Haikou 570228, China.
Tel.: +86 898 66257271; fax: +86 898 66257271.
E-mail address: lzyuanwb@163.com (W.-b. Yuan).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.023

M. Liu et al. / Spectrochimica Acta Part A 70 (2008) 1114–1119
1115
Table 1
Elemental analysis and molar conductance data
Complexes
Yield (%)
C% found (calc.)
H% found (calc.)
N% found (calc.)
Ln% found (calc.)
Λm (S cm2 mol−1
)
[LaL(NO3)2]NO3·H2O
[SmL(NO3)2]NO3·H2O
[EuL(NO3)2]NO3·H2O
[GdL(NO3)2]NO3·H2O
[YL(NO3)2]NO3·H2O
73
71
72
75
75
11.25 (11.54)
11.67 (11.40)
11.61 (11.37)
10.74 (11.30)
12.58 (12.27)
37.84 (38.17)
37.34 (37.67)
37.62 (37.59)
37.32 (37.37)
40.87 (40.56)
3.91 (3.77)
3.67 (3.72)
3.68 (3.71)
3.24 (3.69)
4.23 (4.00)
16.84 (16.36)
17.84 (17.44)
17.23 (17.63)
18.54 (18.11)
10.85 (11.13)
78.5
86.0
79.0
76.5
78.6
7.4). Solutions of calf thymus DNA in buffer gave a ratio of
UV absorbance at 260 and 280 nm of ca. 1.9:1, indicating
that DNA was free from protein [10]. The DNA concentra-
tion per nucleotide was determined by absorption spectroscopy
using the molar absorption coefficient (6600 M−1 cm−1) at
260 nm [11]. Stock solutions were stored at 4 ◦C and used in
no more than 4 days. Redistilled water was used to prepare
buffers.
Fig. 1. The procedure of ligand (L) synthesis.
2.3. Preparation of complexes
Carbon, nitrogen and hydrogen were determined by using
an Elementer Vario-EL elemental analyzer. Molar conductivity
measurement was carried out with a DDSJ-308 conductome-
ter (made in China) at 25 ◦C. The IR spectra was recorded
on a Nicolet NEXUS 670 FT-IR spectrometer in the region of
4000–400 cm−1 in KBr pellets. Thermal analysis was performed
on a PCT-2A thermobalance analyzer (Beijing optics appara-
0.5 mmol tris{2-[(3,4-dihydroxybenzylidene)imino]ethyl}
amine (L) was dissolved in 10 mL methanol. To this solution
was added slowly a 10 mL methanol solution of 0.5 mmol
lanthanide nitrates under stirring. The solution was continually
stirred for 2 h at room temperature, and then the precipitate was
collected by filtration, washed with methanol for three times,
and dried in a vacuum.
1
tus factory). H NMR spectra were recorded on a Varian VR
300 MHz spectrometer in CDCl3. The FAB mass spectrum was
obtained on a VG ZAB-HS mass spectrometer. UV–vis spectra
were recorded on a UV 757 CRT spectrophotometer (Shanghai
Precision Scientific Instrument Co., Ltd.). Fluorescence spectra
were recorded on a 970 CRT spectrometer (Shanghai Precision
Scientific Instrument Co., Ltd.).
3. Results and discussion
3.1. Characterization of ligand (L)
The new polydentate, hydroxyl-rich ligand which
derived from the 1:3 condensation of tren with 3,4-
dihydroxybenzaldehyde was characterized by IR, 1H NMR,
and FAB-MS. The IR spectrum of the ligand shows strong
band at 1650 cm−1, which is attributable to stretch vibration
of the ν(C N). And the ligand exhibits band at 3378 cm−1
in the infrared spectrum, which is assigned to ν(ArO–H). In
addition, the peak at 1280 cm−1 can be assigned to ν(Ar–O)
of hydroxybenzene [12]. And the integral intensities of each
2.2. Synthesis of ligand (L) Fig. 1
A solution of 1 mmol tren in 5 mL dichloromethane
was added dropwise to
a solution of 3 mmol 3,4-
dihydroxybenzaldehyde in 15 mL of mixed solution of
2:1 (V:V) dichloromethane and methanol. The mixture was
stirred at room temperature for 5 h. The precipitated solid was
filtered, washed with the mixed solution of dichloromethane
and methanol three times, and dried in a vacuum. Yield, 83%;
FAB-MS: m/z = 507.4 (M + H). 1H NMR (CDCl3, ppm): δ
8.02(3H, d, –CH N), 7.16–6.69(9H, m, Ar–H), 4.24(6H,
s, ArOH), 3.52(6H, s, NCH2–). IR(KBr pellets), ν/(cm−1):
3378(ArO–H), 1648(C N), 1280(Ar–O).
1
signal in the H NMR spectra measured in CDCl3 at room
temperature were found to agree with the number of different
types of protons present. The signal of –CH N proton was
observed at 8.02 ppm [13], and signals appeared at 7.16–6.69,
4.24 and 3.52 ppm are assigned to protons of Ar–H, ArOH, and
NCH2–, respectively. According to the FAB-MS, we can know
the molecule weight of this new ligand is 506.
Table 2
Major infrared spectral data for the [LnL(NO3)2]NO3·H2O (cm−1
)
−
−
−
−
−
ν5(NO3 )
Compounds
ν(OH)
ν(C N)
ν(Ar–O)
ν1(NO3
)
ν2(NO3
)
ν3(NO3
)
ν4(NO3
)
ν5-ν1
Ligand
3378
3421
3423
3424
3426
3419
1649
1651
1650
1651
1650
1650
1280
1229
1231
1231
1232
1232
[LaL(NO3)2]NO3·H2O
[SmL(NO3)2]NO3·H2O
[EuL(NO3)2]NO3·H2O
[GdL(NO3)2]NO3·H2O
[YL(NO3)2]NO3·H2O
1312
1311
312
1314
1314
1033
1034
1033
1035
1033
1383
1383
1383
1383
1383
759
764
763
755
768
1422
1421
1422
1422
1422
110
110
110
108
108

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M. Liu et al. / Spectrochimica Acta Part A 70 (2008) 1114–1119
Table 3
Data of TG–DTA spectra of some complexes
Complexes
Tendo
(◦C)
T1exdo
(◦C)
T2exdo
(◦C)
Residue (calc.)%
[LaL(NO3)2]NO3·H2O
[GdL(NO3)2]NO3·H2O
97
79
260
270
481
374
20.24 (19.20)
21.75 (20.96)
3.2. Composition of complexes
The metal ion was determined by EDTA titration using
xylenol orange as indicator. The results of elemental analy-
ses (Table 1) indicate that the composition of the complexes
is Ln(NO3)3·L·H2O (Ln = La, Sm, Eu, Gd, Y) and the ratio of
Ln/L in all complexes are 1:1.
Fig. 2. The possible structure of the complex Ln3+-L.
molar conductance values of the complexes measured in
DMF (1 × 10−3 mol L−3) at 25 ◦C are in the range of
76–86 S cm2 mol−1 (Table 1), indicating 1:1 electrolytes [14].
3.3. Solubility and molar conductance
3.4. Infrared spectra
These complexes are soluble in DMSO, DMF, and poorly
soluble in methanol, ethanol, acetone or chloroform. The
The infrared spectral data of ligand L and its complexes
are listed in Table 2. IR spectra of complexes are similar to
Fig. 3. (a) Absorption spectra of the DNA solution in the absence and presence of Eu(III) complex. CDNA: 2.1 × 10−4 mol/L. CEuL: a, 0; b, 1.47; c, 4.40; d, 7.33; e,
10.3; f, 14.7; g, 22.0; h, 29.4 × 10−5 mol/L. (b) Absorption spectra of the DNA solution in the absence and presence of Gd(III) complex. CDNA: 2.1 × 10−4 mol/L.
CGdL: a, 0; b, 1.47; c, 4.40; d, 7.33; e, 10.3; f, 14.7; g, 22.0; h, 29.4 × 10−5 mol/L. (c) Absorption spectra of the DNA solution in the absence and presence of La(III)
complex. CDNA: 2.1 × 10−4 mol/L. CLaL: a, 0; b, 1.47; c, 4.40; d, 7.33; e, 10.3; f, 14.7; g, 22.0; h, 29.4 × 10−5 mol/L. (d) Absorption spectra of the DNA solution
in the absence and presence of Sm(III) complex. CDNA: 2.1 × 10−4 mol/L. CSmL: a, 0; b, 1.47; c, 4.40; d, 7.33; e, 10.3; f, 14.7; g, 22.0; h, 29.4 × 10−5 mol/L. (e)
Absorption spectra of the DNA solution in the absence and presence of Y(III) complex. CDNA: 2.1 × 10−4 mol/L. CYL: a, 0; b, 1.47; c, 4.40; d, 7.33; e, 10.3; f, 14.7;
g, 22.0; h, 29.4 × 10−5 mol/L.

M. Liu et al. / Spectrochimica Acta Part A 70 (2008) 1114–1119
1117
Fig. 3. (Continued )
each other, indicating that the complexes have similar struc-
3.5. Thermal analyses
tures. In the complexes, the bands due to ν(OH) are observed
at 3419–3425 cm−1, so it is apparent that ν(ArO–H) bands
in these complexes are shifted to lower frequencies about
41–48 cm−1, indicating that the oxygen atoms in hydroxy-
benzene take part in coordination to the lanthanide ions. In
comparison with the free ligand, the ν(C N) bands in the
complexes shift only 1 or 2 cm−1 to lower frequencies, indi-
cating that the nitrogen atoms do not coordinate to the metal
ions.
The absorption bands assigned to the coordinated nitrate
groups (C2ν) were observed at about 1310 cm−1 (ν1), 1031 cm−1
(ν2), 1420 cm−1 (ν5) in these complexes, respectively. In addi-
tion, the separation of two strongest frequency bands |ν5–ν1|
is approximately 110 cm−1, clearly establishing that the coor-
dinated NO3− groups in the solid complexes coordinate to the
lanthanide ions mono-dentately [15]. However, in the majority
of the structurally characterized lanthanide nitrate complexes,
the nitrato ligands are bidentate [16]. And the noteworthy band
at 1380 cm−1 in the spectra of complexes indicates that free
nitrate group (D3 h) is present [17], in agreement with the
results of the molar conductivity experiments. Furthermore,
there are no bands that show H2O coordinate to metal ions
in the complexes, which is in accordance with the results of
TG–DTA.
The TG–DTA curves of all the complexes are similar. The
thermal analysis data of some complexes are listed in Table 3.
The combined TG and DTA studies were performed on the
La(III) complex. The TG curve shows weight loss in the temper-
ature of 97 ◦C, attributed to elimination of the water molecules
which is accompanied by an endothermic peak at 97 ◦C in the
DTA thermogram. The complex has no melting point. The com-
plex decomposes since there are two exothermic peaks at 260
and 481 ◦C, losing weight about 79.76% (Ca. 80.80%) in the TG
curve. Beyond the temperature of 750 ◦C, the curves of TG and
DTA have no change, possibly due to the complex decomposed
to Ln2O3.
According to the data of the elemental analyses, molar
1
conductivity, H NMR spectra, IR spectra and TG–DTA, the
possible molecular structures of the complexes are shown in
Fig. 2. The coordination number for L complexes may be 8.
4. Interaction of complexes with DNA
4.1. UV–vis spectra
The calf thymus DNA absorbs at 260–280 nm and all the
complexes have no absorption at 240–300 nm. So addition com-

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M. Liu et al. / Spectrochimica Acta Part A 70 (2008) 1114–1119
Fig. 3. (Continued ).
plexes to the DNA solution directly can be used to examine the
reaction of DNA with the complexes by the change of absorption
of DNA. When these complexes in different concentration are
added into DNA solution, respectively, the absorptivity of the
DNA solution increases considerably. Furthermore, the emis-
sion peak of DNA shift to lower wavelength step by step in the
course of adding complexes into DNA. The results can be seen
in Fig. 3(a)–(e).
The shift of the emission peak and the enhancement of its
intensity may be largely due to the fact that the complexes inter-
calated into the base group pairs of DNA and destroyed the
H-binding of purine bases and pyrimidine bases, which may
cause the slight change of the conformation of DNA [18]. Based
on the above investigation, it can be found that all the com-
plexes can bind to DNA, and have strong binding affinity with
DNA.
it bind to DNA by intercalation [19]. In contrast to DNA, the
addition of complexes to the solution of EB has no effect on the
emission intensity and emission peak of EB.
Fig. 4 shows the emission spectra of the EB–DNA system in
the absence and presence of La(III) complexes. The emission
intensity of EB–DNA system decreases when the concentration
of La(III) complex increases. This phenomenon indicates that
there is a strong binding and a large affinity in the complex with
the calf thymus DNA.
The addition of complexes caused the quenching fluores-
cence of the EB–DNA system. This case can be considered as
the complexes directly reacted with the DNA of DNA–EB sys-
tem, which leads to the EB molecules left the EB–DNA system,
and the emission intensity of EB–DNA system decreased [20].
So the binding constant K of complexes to DNA can be deter-
mined from the following equation: F0/F = 1 + K[M]. Where,
[M], F0 and F correspond to the concentration of quenching
agent, the emission intensity of the EB–DNA system before and
after the addition of complexes, respectively. K is the appar-
ent binding constant, which is given by the ratio of the slop
to intercept [21]. According to the equation, in the plots of
F0/F versus [M], K = 3.55 × 103[M−1] for La(III) complex is
the highest, and the lowest binding constant K is 2.10 × 103
4.2. Fluorescence spectra
Further exploration of the binding of complexes with DNA,
fluorescence spectra studies were carried out on DNA pretreated
with EB by varying the concentration of the added complexes.
EB is employed as a probe in examination of the reaction, and

M. Liu et al. / Spectrochimica Acta Part A 70 (2008) 1114–1119
1119
[M−1] for the Y(III)complex. The binding constant K of com-
plexes to DNA are shown in Fig. 5. By above investigation,
it can be shown that the binding affinity with DNA is La(III)
complex > Sm(III) complex > Eu(III) complex > Gd(III) com-
plex > Y(III) complex.
5. Conclusion
The present studies show that the novel tripodal ligand and its
complexes have been synthesized and characterized. The results
of these characterization methods show that these complexes are
1:1 (Ln/L), and their coordination number is 8.
The interaction studies of complexes with calf-thymus DNA
suggest that the complexes intercalated into the base group pairs
of DNA, have strong binding affinity with DNA. And accord-
ing to fluorescence data, the binding constant for the reaction
at 298 K are obtained, which show that the binding affinity
with DNA is La(III) complex > Eu(III) complex > Sm(III) com-
plex > Gd(III)complex > Y(III) complex. Basedonthoseresults,
it can be shown that the La(III)complex is promising candidate
for therapeutic reagents and DNA probes.
Acknowledgements
The authors thank NSFC (no. 20761003, 20762003) and the
National Natural Science Foundation of Hainan Province (no.
20602) for financial support.
References
[1] P.J. Dandlier, R.E. Holmlin, J.K. Barton, Science 274 (1997) 1465.
[2] D.S. Sigman, A. Mazumder, D.M. Perrin, Chem. Rev. A 93 (1993)
2295.
[3] C.V. Kumar, H.E. Asuncion, J. Am. Chem. Soc. 115 (1993) 8547.
[4] X.Q. He, Q.Y. Lin, R.D. Hu, X.H. Lu, Spectrochim. Acta A 68 (2007)
184–190.
[5] P. Yang, R. Ren, B.Sh. Yang, Chem. Res. Appl. 6 (1994) 22.
[6] Y.L. Zhao, F.Y. Zhao, Chin. J. Rare Earths 21 (2000) 5.
[7] H.B. Shen, L.H. Kuai, L.H. Ni, et al., J. Rare Earths 15 (1997) 300.
[8] Q.D. Tan, W.G.M.G. He, et al., Chem. J. Chin. Univ. 7 (1986) 1067.
[9] S. Kimura, S. Young, J.P. Collman, Inorg. Chem. 9 (1970) 1183.
[10] J. Marmur, J. Mol. Biol. 3 (1961) 208.
Fig. 4. Emission spectra of the EB–DNA system in the absence and presence
of La(III) complex, CEB:6.34 × 10−6 mol/L.CDNA:2.25 × 10−4 mol/L. Excited
at 540 nm, CLaL: a, 0; b, 1.47; c, 2.94; d, 4.41; e, 5.88; f, 7.35; g, 11.0; h,
14.7 × 10−5 mol/L.
[11] M.F. Reichmann, S.A. Rice, C.A.J. Am. Chem. Soc. 76 (1954) 3047.
[12] W.B. Yuan, L. Yan, R.D. Yang, Chin. J. Appl. Chem. 21 (2004) 829.
[13] H. Yang, W.H. Sun, Z. Li, L. Wang, Synth. Comm. 32 (2002) 2395.
[14] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[15] E. Katsoulakoua, V. Bekiarib, C.P. Raptopoulouc, Spectrochim. Acta A 61
(2005) 1627–1638.
[16] X.L. Tang, W. Dou, W.Sh. Liu, Spectrochim. Acta A 68 (2007) 349–
353.
[17] K. Nakamoto, Infrared and Raman Spectroscopy of Inorganic and Coordi-
nation Compounds, John Wiley & Sons, New York, 1986.
[18] Ch.Q. Jiang, J.X. He, J.Sh. Wang, Spectrosc. Spect. Anal. 22 (2002) 103.
[19] J.K. Veal, R.L. Rill, J. Biochem. 80 (1991) 1132.
[20] Y.Zh. Kang, H.B. Shen, Y.Q. Luo, et al., J. Chin. Rare Earths 22 (2002)
23.
[21] Y.M. Song, X.L. Lu, M.L. Yang, et al., J. Transit. Met. Chem. 30
(2005) 4.
Fig. 5. The fluorescence quenching curve of the complexes to DNA–EB.