Characterization and DNA Interaction of Lanthanide Complexes Based on Thiourea Ligand①

REN Y-Wen HU Hng-N ZHANG Jun ZHUANG Xin-Jing LI Dong-Ping② LI Yong-Xiu

a (Department of Chemistry, Nanchang University, Nanchang 330031, China) b (Key Laboratory of Functional Molecule Design and Interface Process, Anhui Jianzhu University, Hefei 230022, China)

ABSTRACT Three isostructural lanthanide complexes, namely [Ln(L)3·CH3OH] (Ln = Tb(1), Dy(2), Ho(3), HL = N-(pyridin-2-ylcarbamothioyl)benzamide), were successfully synthesized and characterized by IR, UV, elemental analysis, single-crystal X-ray diffraction and DNA-biding analysis. X-ray single-crystal diffractions show that the family of lanthanide complexes crystallizes in monoclinic C2/c space group. Each lanthanide atom is coordinated with three carbonyl-O atoms, three pyridyl-N atoms and three imine-N atoms from three distinct L-1 ligands, forming a distorted tricapped trigonal prism. The binding properties of complexes with ct-DNA were studied by simulating the physiological environment of human body. The results suggest that compounds could bind with ct-DNA through interaction under a spontaneous process.

Keywords: lanthanide complex, crystal structure, DNA-binding activity;

1 INTRODUCTION

Schiff-base ligands have wide applications in the fields of food, dye industry, analytical chemistry, catalysis, antifungal, agricultural chemical, biological activity and electroche- mistry[1-10]. Especially, the thiourea derivatives are promising ligands with biological activities and strong coordination abilities[11-13]. Commonly, the complexes based on thiourea ligands can show stronger biological activity than the ligand itself[14,15]. Thus, in the past decades, various Schiff base metal complexes, such as Co(II), Cu(II) and Zn(II) complexes, have been prepared and their biochemical properties have been studied in detail[14-18].

Moreover, many scientists have devoted to research on the application of lanthanide complexes in medicine since the late of 19thcentury[19,20]. Studies have shown that lanthanide complexes have lower toxicity and better biological activity than some transition metal complexes or other organic synthetic drugs[21-23]. However, the lanthanide complexes based on thiourea ligands are reported rarely. Herein, we present the synthesis, crystal structure and characterization of a family of lanthanide complexes, [Ln(L)3·CH3OH] {Ln = Tb (1), Dy (2), Ho (3)}. In the simulated physiological enviro- nment, the DNA binding properties of complexes 1～3 are also examined by UV spectroscopy.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All chemicals of analytical grade were purchased from suppliers and used without further treatment. ct-DNA (deoxyribonucleic acid sodium salt from calf thymus) was purchased from Sigma, and used as received. ct-DNA (50 mg) was dissolved in 50 mL tris-HCl (pH = 7.4) buffer to prepare a stock solution at room temperature[24,25]. The concentration of ct-DNA stock solution was determined by UV absorption spectra at 260 nm (ε = 6600 μmol-1·cm-1)[26]. UV-Vis absorption titration experiments were performed by varying the concentration of ct-DNA while maintaining the concentration of complex (1.0 × 10-5mol·L-1). Ligand (HL) was synthesized according to the reported literature[27]. FT-IR spectra were measured on a BRUKER ALPHA Fourier infrared spectrometer. The UV absorption spectra were tested at a UV/VIS-2550 Spectrometer. The single-crystal X-ray diffraction measurement was recorded on a Bruker D8 QUEST CCD diffractometer. Elemental analysis data were performed on a Perkin-Elmer 240C analyzer.

2. 2 Synthesis of the complexes

Synthesis of 1: A mixture of TbCl3·6H2O (0.1 mmol, 37.2 mg), HL ligand (0.3 mmol, 77.1 mg) and KOH (0.3 mmol, 16.8 mg) was dissolved in 10 mL methanol, and then stirred at room temperature for 2 hours. The resulting yellow deposit was filtrated and washed with methanol several times. Then the yellow powder was dissolved in dichloromethane and N-propanol (2:3) and diffused by ether. Yellow block crystals of 1 were obtained after one day, washed with ethanol and dried in air. Yield: 40.3% based on TbCl3·6H2O. Anal. Calcd. for C40H34N9O4S3Tb: C, 50.05; H, 3.57; N, 13.13%. Found: C, 50.08; H, 3.62; N, 13.11%. IR: 3405(w), 1631(s), 1587(m), 1536(s), 1460(m), 1409(s), 1289(w), 1149(m), 1059(m), 844(w), 780(w), 704(m), 646(w).

Synthesis of 2: Compound 2 was synthesized similarly to 1, but using DyCl3·6H2O instead of TbCl3·6H2O in the solution. Yield: 32.7% based on DyCl3·6H2O. Anal. Calcd. for C40H34N9O4S3Dy: C, 49.87; H, 3.56; N, 13.08%. Found: C, 49.88; H, 3.60; N, 13.07%. IR: 3418(w), 1635(s), 1588(m), 1534(s), 1454(m), 406(s), 1285(w), 1144(m), 1077(m), 842(w), 781(w), 700(m), 626(w).

Synthesis of 3: Compound 3 was synthesized similarly to 1, but using HoCl3·6H2O rather than TbCl3·6H2O in the solution. Yield: 30.9% based on DyCl3·6H2O. Anal. Calcd. for C40H34N9O4S3Ho: C, 49.74; H, 3.55; N, 13.05%. Found: C, 49.78; H, 3.61; N, 13.06%. IR: 3418(w), 1629(s), 1582(m), 1534(s), 1454(m), 1414(s), 1285(w), 1144(m), 1077(m), 848(w), 781(w), 707(m), 619(w).

2. 3 X-ray structural determination

The X-ray diffraction experiments for complexes were made on a Bruker diffractometer using graphite-monochromatic radiation (λ = 0.71073 Å) with the φ-ω scan mode at room temperature. The crystal structures were solved by direct methods. Non-hydrogen atoms were refined by full-matrix least-squares methods on F2using the SHELXS97 and SHELXL97 program packages, respectively[28]. Hydrogen atoms were generated geometrically and refined isotropically using a riding mode. Details of the crystal parameters, selected bond lengths and bond angles for complexes 1～3 are summarized in Tables 1 and 2, respectively.

Table 1. Crysta1 Data of Complexes 1～3

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) in Complexes 1～3

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

Crystallographic analyses reveal that compounds 1～3 are isostructural and crystallized in the monoclinic space group of C2/c. So, only the structure of 1 is selected to describe in the following discussion. As shown in Fig. 1a, the mononuclear lanthanide complex contains three L-ligands, one Tb atom, and one distorted methanol molecule. Each Tb atom is nine-coordinated by three imine-N atoms, three pyridyl-N atoms and three carbonyl-O atoms from three distinct L-ligands, which forms distorted tricapped trigonal prism geometry. It is worth noting that the thiolate sulfur atom in ligand does not coordinate with the Tb3+cation. As illustrated in Fig. 1b, the N(1), N(4), N(7) and O(1), O(2), O(3) atoms construct the top and bottom triangle planes of the trigonal prism with dihedral angles of 3.16°, while N(2), N(5) and N(8) occupy the capped positions of the geometry. The dihedral angles of O(2), O(3), N(1), N(7) and O(1), O(3), N(1), N(4) atoms are 12.82° and 19.15°, respectively, and that of O(1), O(2), N(4), N(7) is 21.63°. It indicates that the parallelogram shapes of the tricapped trigonal prism are distorted seriously. The average Tb-O bond length is 2.350 Å (range from 2.343(9) to 2.359(1) Å), which is longer than the average Dy-O (2.332 Å) and Ho-O (2.302 Å) bond lengths in 2 and 3. The average Tb-N bond length is 2.492 Å, ranging from 2.475 to 2.501 Å, while the Dy-N (average 2.486 Å) and Ho-N (average 2.460 Å) bond distances vary from 2.468 to 2.498 Å in 2 and from 2.446 to 2.479 Å in 3. The intramolecular Tb···Tb distance is 10.333 Å (The intramolecular Dy···Dy distance is 10.349 Å in 2 and Ho···Ho is 10.275 Å in 3).

Fig. 1. (a) Structure of complex 1. (b) Distorted tricapped trigonal prism geometry of complex 1. (c) Packing diagram of complex 1. (d) Hydrogen bonds and C-H···π interaction in complex 1 Solvent molecules and selected hydrogen atoms are omitted for clarity. Symmetry codes: (a) x, -y, -0.5 + z; (b) x, -y, 0.5 + z; (c) 1 - x, -y, 1 - z; (d) -x, -y, -z

As illustrated in Fig. 1c and 1d, there are abundant intramolecular hydrogen bonds between the imine groups and thiolate sulfur atoms of L-ligands in adjacent molecules. The mononuclear complexes are interconnected by hydrogen bonds, forming a ladder-type one-dimensional chain with the hole about 3.76 × 7.52 Å2. And then, the adjacent ladder-type chains are further linked by C-H···π interactions (C-H···Cg2.752 Å) between the phenyl and pyridyl groups of molecules to form an open two-dimensional framework[29]. However, the holes are occupied by lattice methanol molecules. Interestingly, there are two kinds of double hydrogen bond chains constructed by N(6)-H(6)···S(3a) and N(9)- H(9A)···S(2b) or N(3)-H(3A)···S(1c). The d(D···A) distances and D-H···A angles of hydrogen bonds range from 3.386 to 3.512 Å and 155 to 161° in 1, from 3.399 to 3.533 Å and 154 to 160° in 2, and from 3.487 to 3.522 Å and 155 to 160° in 3, respectively (Table 3).

Table 3. Hydrogen-bonding Geometry Parameters (Å, °) for 1～3

3. 2 UV spectra

As shown in Fig. 2, the solid state absorption spectra of free ligand HL and complexes 1～3 are recorded in the near ultraviolet region (200～400 nm) at room temperature. For both complexes and ligand, two main absorption bands with various intensities can be observed. The intense absorption at 264 nm is assignable to π-π* transition of benzene rings for both compounds and ligand. The broad absorption at 303 nm of free ligand may be due to the π-π* transition of -C=N group which shifts electrons to the delocalized π orbital on the benzene ring[30]. Complexes 1～3 show red-shift absorption at 308～312 nm probably due to the coordination of ligand to the metallic center.

Fig. 2. UV spectra of ligands and complexes 1～3

3. 3 DNA binding studies

Electronic absorption spectroscopy is effective to examine the binding modes of metal complexes. Generally, the hypochromism and red-shift are associated with the intercalative binding mode of compound to the DNA, while the hyperchromic effect and blue-shift may be attributed to the electrostatic interaction of compound to DNA. The absorption spectra of complexes 1～3 in the absence and presence of ct-DNA are shown in Fig. 3. With gradual addition of ct-DNA, the absorbance of compounds rises obviously at about 265 nm due to the π-π* transition of the aromatic ring. It reveals that all compounds exhibited significant hyperchromism, accompanied with the slight blue-shifts in the absorbance maxima (from 265 to 263 nm in 1, from 265 to 263 nm in 2 and from 266 to 263 nm in 3). Commonly, these hyperchromic effects indicate that compounds 1～3 bind with ct-DNA through the van deer Waals contacts, such as π-π accumulation or hydrogen bond between the ligand and the base pairs of ct-DNA molecules[31].

To quantitatively compare the binding ability of compounds towards ct-DNA, the intrinsic binding constant (Kb) is obtained by monitoring the changes in the absorbance of compounds using the following equation[32]:

[DNA]/(εa- εf) = [DNA]/(εb- εf) + 1/Kb(εb- εf)

In this equation, [DNA] is the concentration of ct-DNA, εais the apparent extinction coefficient, and εband εfare the extinction coefficients of the free compound and the free and fully bound forms, respectively[32]. On the plot of [DNA]/(εa- εf) versus [DNA] (inset in Fig. 3), the binding constants Kbfor compounds are given by the ratio of the slope to the interception. As shown in Table 3, the Kbvalues are estimated to be 5.50 × 104M-1for 1, 5.39 × 104M-1for 2 and 5.63 × 104M-1for 3, respectively. Obviously, this family of lanthanide complexes shows comparable binding ability to each other owing to their similar structures and properties of metallic centers. Furthermore, the Kbvalues have the same level as those reported lanthanide complexes[33-35].

Fig. 3. (a) UV-vis spectra of the interactions of 1～3 with ct-DNA. [Complex] = 1.0 × 10-5 mol·L-1, [DNA] = 0～7.15 × 10-5 mol·L-1. The arrow shows that the absorption intensity increases as the DNA concentration increases; (b) [DNA]/[εa - εf] for [DNA]

The corresponding thermodynamic parameter can also be calculated according to the Gibbs equation: ΔGb= -RTlnKb. As illustrated in Table 4, the Gbvalues of the lanthanide complexes-ct-DNA system are all negative, which reveals that the interaction between compounds and ct-DNA are spontaneous process.

Table 4. Binding Constant (Kb) and Free Energy (ΔG) for Complexes 1～3

4 CONCLUSION

A family of mononuclear lanthanide complexes (Ln = Tb, Dy, Ho) have been successfully synthesized and charac- terized by single-crystal X-ray diffraction. Electronic absorption spectra reveal that these lanthanide compounds can bind with ct-DNA and may be potential candidates as antitumor reagents.

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