LI Xio LUO Xu-Jin YANG Yn MA Li WANG Shu-Long LUO Zhi-Hui LIU Rong-Jun LIANG Wei-Jing

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Synthesis, Structure, and DNA Binding of a Platinum(II) Complex Based on the 3,4,5-Trimethoxy-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline①

LI Xiaoa, bLUO Xu-JianbYANG Yanb②MA Lia②WANG Shu-LongbLUO Zhi-HuibLIU Rong-JunbLIANG Wei-Jiangb

a(400030)b(537000)

A new platinum(II) complex of [Pt(chda)(3,4,5-triopip)]Cl·2H2O (1) based on3,4,5-trimethoxy-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (3,4,5-triopip) has been synthesized by hydrothermalmethodsand its consequences are characterized by elemental analysis, IR, single-crystal X-ray diffraction, UV-Vis absorption and fluorescent spectrum. The UV-Vis absorption studies reveal that the molecule undergoes considerable interaction with the nucleic acid. In this mononuclear structure, the platinum adopts a four-coordinated square planar geometry, which may favor the intercalation between the neighboring bases of the G-quadruplex (G4) DNA.

platinum complex, DNA interaction, UV-Vis absorption;

1 INTRODUCTION

Recently, the development of molecular sensors for biomacromolecules has received considerable attention due to their potential applications in clinical diagnosis and therapeutic advances[1]. In particular, the fluorescent sensors for detecting nucleic acids that are associated with certain disease have attracted much interest for their high sensitivity and selecti- vity[2]. To date, several types of fluorescent sensors have been reported to be able to detect nucleic acid sequences, including single-walled carbon nanotu- bes[3-5], graphene oxides[6,7], carbon and Au nano- particles[8-20]. Although these materials function well to assay nucleic acid, their preparations are often laborious and/or require high-cost instrumentation. As a consequence, it is still of urgent need to develop novel sensing platforms based on the readily ob- tained materials.

Metal-organic frameworks (MOFs), readily con- structed from metal ions/clusters and organic linkers with one-, two-, or three-dimensional (1D, 2D, 3D) extended coordination networks, have captured widespread interest due to their intriguing structural topologies and potential applications as functional materials in a wide range of fields[21, 22]. In particular, MOFs have been demonstrated to be powerful as fluorescent sensors for the detection of various cations[23-25], anions[26], vapors[27, 28]and other small molecules[29]. However, it is known that the organic linkers in MOFs usually have special functional groups and may offer a source for possiblestac-king, hydrogen bonding, and electrostatic interac- tions with negatively charged nucleic acid sequen- ces[30-36].

Recently, the interaction of polypyridyl metal complexes with DNA has been a hot topic due to the potential usage as DNA probes or other labels[37-39], as the chelate ligand partially intercalated between the adjacent base pairs of DNA. In addition, some experimental and theoretical studies of the DNA binding and related properties of Ru(II) polypyridyl complexes have been reported[40-42]. In this paper, we designed and synthesized a new chiral platinum(II) complex, and studied their interaction with G4-DNA and the complex with high DNA-binding affinity.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All reagents were brought from commercial sources and used without further purification. IR spectra were recorded in the range of 4000～400 cm-1on a Perkin-Elmer Spectrum One FT/IR spec- trometer using a KBr pellet. Elemental analysis (C, H, N) was performed on a Perkin-Elemer 2400II CHN elemental analyzer.UV-vis absorption titration was performed on a Cary 100 Conc. UV-visible spectro- photometer (Agilent Technologies, Australia).Fluorescence study was performed on a Shimadzu RF-5301/PC spectrofluorometer (Tianmei Technolo- gies, Japan). The crystal structure was determined by a Bruker APEX area-detector diffractometer (Bruker, Germany) and employing the SHELDRICK crys- tallographic software. Calf thymus DNA (ct-DNA), ds26 DNA and G4-DNA (hTel, hTel-1) were purchased from Sigma. Buffer (5mM tris(hydroxyl- methyl)aminomethane(tris)hydrochloride, 50 mM·NaCl, pH = 7.35) was used for UV-Vis absorption and FID assays. Assumed a molar absorption is 6600 M-1cm-1(260 nm)[43], the con-centration of ct-DNA was spectrophotometrically determined. In the presence of a buffer containing 1% DMSO, the UV-Vis absorption titration of complex 1was performed by using a fixed complex concentration to which increments of the DNA stock solution were added ([DNA]/[complex] ranging from 0 to 10). In DNA interaction studies, the complex was dissolved in DMSO for the preparation of stock solution at 2.0 × 10-6M. Before the UV-Vis absorption spectrum was recorded, complex-DNA solutions were allowed to incubate for 10 min. FID measurements were performed as described pre- viously[44].

2. 2 Synthesis of complex 1

K2PtCl4(8.0 mmol) was ?rst dissolved in DMSO & PEI mixed solution (10 mL, volume ratio 10: 1) and heated to near boiling. This hot solution was added to a hot solution of 3,4,5-trimethoxy-phenyl- 1H-imidazo[4,5-f][1,10]phenanthroline (5.0 mmol) in DMSO (5 mL). Finally, (1, 2)-diaminocyclo- hexane (5.0 mmol) in 30 mL MeOH/MeCN (20:1) was added to the hot solution, and the mixture was stirred at re?ux for 24 h, and finally cooled to room temperature. After ?ltration, the ?ltrate was allowed to stand at room temperature for about ten days, and red crystals were obtained(yield 40% based on 3,4,5-triopip). Anal. Calcd. (%) for [Pt(chda)(3,4,5- triopip)]Cl·2H2O (1): C, 44.01; H, 4.32; N, 10. 99. Found (%): C, 44.08; H, 4.29; N, 11.04. Selected IR data (KBr, cm-1): 3329, 1570, 1489, 1445, 1381, 1354, 1346, 1297, 1176, 1156, 1119, 1094, 1034, 1000, 936, 858, 823, 795, 756, 712, 629.

2. 3 Structure determination

A colorless single crystal of the title compound with dimensions of 0.22mm × 0.20mm × 0.18mm was mounted on a glass fiber. X-ray diffraction intensity data were collected on a Bruker APEX area-detector equipped with a graphite-monochro- matized Mo-radiation (= 0.71073 ?) by using an-2scan mode in the range of 3.05≤≤25.40o at 293(2) K. A total of 12881 reflections were measured, of which 5393 were unique (int= 0.0367) and 4011 were observed (> 2()) and used in the subsequent structure determination and full-matrix least-square refinements. Absorption correction was performed by the SADABS program[45].The struc- ture was solved by direct methods and subsequent difference Fourier syntheses, revealing the positions of all non-hydrogen atoms. The hydrogen atoms were located geometrically. All non-hydrogen atoms were refined anisotropically. All calculations were performed by using the SHELXTL package[46]. The final= 0.1436 and0.3624 (= 1/[2(F2) + (0.1588)2+ 246.7108], where= (F2+ 2F2)/3) for 4011 observed reflections with> 2(),= 1.086, (Δ/)max= 0.558, (Δ)max= 8.227 and (Δ)min= –16.715 e·?-3. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Fig. 1. View of the coordination environment of the Pt(II) center in complex 1

3 RESULTS AND DISCUSSION

3. 1 Structure

The single-crystal X-ray diffraction analysis (Fig. 1) demonstrates that complex 1 belongs to monoclinic crystal system with21/space group. And its asym- metric unit consists of one discrete [Pt(chda)(3,4,5- triopip)]Cl·2H2O, two dissociative water molecules and one Cl ion. Each Pt(II) center is four-coordinated by four N atoms from one 3,4,5-triopipand onecyclohexanediamine. The Pt–N bond lengths are in the range of 2.006(8)～2.061(8) ?, which are within the normal range. The whole molecule retains the planar mononuclear structure, which may favor the intercalation between the neighboring bases of G4-DNA. Each molecule is interlinked by weak interaction between the chloride and the N atom of hexamethylene diamine of Cl-H···N and N-H···Cl hydrogen bonds, forming a supramolecular structure. In addition, there are also weak hydrogen bonds such as O-H···N (between solvent H2O and 3,4,5- trimethoxy-phenyl-1H-imidazo-[4,5-f][1,10]phenanthroline N atoms or cyclohexanediamine N atoms). Moreover, two lattice water molecules providing hydrogen bonding interactions to strengthen the crystalline stability are included.

3. 2 DNA-binding studies

3. 2. 1 UV-vis absorption titrations

Absorption titration is the most common method for investigating the interaction of transition metal complexes with DNA. In general, transition metal complexes exhibit hypochromism and red shift in their electronic spectra when bound to DNA. The degree of hypochromism depends on the binding mode and affinity. The DNA sample was sequentially added in aliquots to the complex solutions, and the absorbance spectra were recorded after each addition. Based on previous studies[47], the UV-vis spectra of complex 1of absorption bands at= 235-290 nm are assigned to intraligand n-* and* transitions (LC) perturbed bycomplexation to the M(II) metal. The moderately intenseabsorptions around 280 nm for complex 1can be attributed to spin-allowed metal- to-ligand chargetransfer MLCT (dM-π*) transition in analogy to assemblemetal complexes containing phen ligand[48]. As shown in Figs. 2～4, the hypo- chromisms of MLCT bands of complex 1 for ct- DNA, hTel and hTel-1 were calculated to be appro- ximately 31.91%, 32.02% and 34.73%, respectively. Their standard deviation is 3.56%. The addition of the G rich sequence to the solutions of complex 1 led to the red shifts of 2, 3 and 4 nm of the band at 279, 288 and 289 nm, respectively with their standard deviation to be 2.89. These experimental data indicated that complex 1 preferred to bind with the G4-DNA.

Fig. 2. UV-Vis absorption spectrum of complex 1 with increasing concentrations of ct-DNA (The concentration of the complex is 1.0 × 10-6M, [DNA]/[complex] ranged from 0 to 10)

Fig. 3. UV-Vis absorption spectrum of complex 1 with increasing concentrations of hTel (The concentration of the complex is 1.0 × 10-6M. [DNA]/[complex] ranged from 0 to 10)

Fig. 4. UV-Vis absorption spectrum of complex 1 with increasing concentrations of hTel-1 (The concentration of the complex is 1.0 × 10-6M, [DNA]/[complex] ranged from 0 to 10)

When the complex is added into DNA solution, it is probably penetrated into adenine base stacks in DNA helix, causing the changes of base hydrophobic interaction and van der Waals forces and affecting the stability of DNA conformation and unwinding the double-helical structure of DNA. It can be considered that interaction of the complexes with the DNA base pairs may destroy DNA helix, stimulate DNA cleavage and increase the UV-Vis absorption of purine and pyrimidine bases and then hypochromic effect occurs[49]. According to literature[50], the inter- calation of complex into DNA base pairs is accompanied by bathochromism and hypochromism, while groove binding or electrostatic interaction shows no (or minor) change in the UV-Vis absorption spectra. In conclusion, complex1 may bind to ct-DNA, hTel and hTel-1 via an intercalative mode.The intrinsic binding constantKof complex 1 with DNA by UV-Vis absorption spectral analysis was calculated by the following equation[51]:

[DNA]/(εa– εf) = [DNA]/(εb–εf) + 1/[b(εb–εf)]

where [DNA] is the concentration of DNA per nucleotide in base pairs, εaindicates the extinction coefficient of complexes at a given DNA con- centration, and εf, εbrepresent the extinction coeffi- cients of complexes free in solution and those fully bound to DNA, respectively, andKis the equili- brium binding constant. In the plot of [DNA]/(εa– εf) versus [DNA],Kthe intrinsic binding constants,can be given by the ratio of the slope to intercept.Kvalues, the binding constants of complex 1 for ct-DNA, hTel and hTel-1, are 5.63 × 103, 1.27 × 106and 3.87 × 106M-1, respectively.Their standard deviation is 1.19 × 105.

3. 2. 2 Fluorescence studies

As the primary pharmacological target of many antitumour drugs, DNA and DNA binding activities of metal complexes provide important insight for the development of effective metal-based chemothera- peutic drugs. To determine whether the induced apo- ptosis is mediated through the intrinsic apoptotic pathway, the binding af?nities of complex 1 with G4-DNA (hTel and hTel-1) compared to double helix DNA (ds26 DNA) were studied using a FID assay[44](Fig. 5). As shown in Fig. 5, complex 1 is approved to be effective G4-DNA binders (1.32 μΜ 26DC50= 2.68 μΜ ).

4 CONCLUSION

A new platinum(II) complex of [Pt(chda)(3,4,5- triopip)]Cl·2H2O (1) based on3,4,5-trimethoxy- phenyl-1H-imidazo[4,5-f][1,10]phenanthroline(3,4,5-triopip) has been synthesized by hydrothermalmethods. The single-crystal X-ray diffraction analy- sis demonstrates that complex 1 belongs to mono- clinic crystal system with21/space group. And its asymmetric unit consists of one discrete [Pt(chda)(3,4,5-Triopip)]Cl·2H2O, two dissociative water molecules and one Cl ion. Each Pt(II) center is four-coordinated by four N atoms from one 3,4,5- triopipand onecyclohexanediamine. UV-vis absorp- tion titrations revealed complex 1 preferred to bind with the G4-DNA. The FID assay suggested that complex 1 exhibits better binding affinity of complex 1 with G4-DNA (hTel and hTel-1) compared to double helix DNA (ds26 DNA).

Fig. 5. FID assay for complex 1 bound with different types of DNA, includingds 26, hTel and hTel-1

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23 August 2017;

16 April 2018 (CCDC 1472979)

① Financially supported by the National Natural Science Foundation of China (Nos: 51463023 and 21461028), Guangxi key lab of agricultural resources chemistry and biotechnology and Guangxi Colleges and Universities Program of Innovative Research Team and Outstanding Talent

E-mails:mlsys607@126.com and yy135175@163.com

10.14102/j.cnki.0254-5861.2011-1812