DNA Binding Studies and Cytotoxicity of the Novel 1,10-phenanthroline Palladium(II) Complexes of Dithiocarbamate Derivatives

디티오카르바메이트 유도체의 새로운 1,10-페난트롤린 팔라디움(II) 착물의 DNA 결합 성질 및 세포독성에 관한 연구

Abstract

Two new palladium (II) complexes, [Pd (phen)(pip-dtc)]$NO_3$ and [Pd(phen)(mor-dtc)]$NO_3$, (where phen is 1,10-Phenantroline, pip-dtc is piperidinedithiocarbamate anion and mor-dtc is morpholinedithiocarbamate anion) have been synthesized and characterized by elemental analysis, spectroscopic studies (FT-IR, $^1H$ NMR, UV-Vis) and conductance measurement. In these complexes, the dithiocarbamate ligands coordinate with Pd (II) center as bidentate with two sulfur atoms. These two complexes have been tested against chronic myelogenous leukemia cell line, K562. They show $IC_{50}$ values less than cisplatin and thus the mode of binding of the complexes to calf thymus DNA (CT-DNA) were investigated by ultraviolet difference and fluorescence spectroscopy. They can denature DNA, exhibit cooperative binding and intercalate into DNA. Several binding and thermodynamic parameters are also described.

두 종류의 새로운 팔라디움(II) 착물인 [Pd(phen)(pip-dtc)]$NO_3$와 [Pd(phen)(mor-dtc)]$NO_3$ (여기서 phen은 1,10-페난트롤린, pip-dtc은 피페리딘디티오카르바메이트 음이온, mor-dtc은 모르폴린디티오카르바메이트 음이온을 가리킨다)를 합성하고 원소분석, 분광법(FT-IR, $^1H$ NMR, UV-Vis) 및 전기전도도 측정을 이용하여 특성을 조사하였다. 이 착물들에서, 디티오카르베이트 리간드는 팔라디움 (II) 중심과 두 개의 황 원자와 이중으로 배위 결합을 하고 있다. 이 착물의 세포독성을 K562 세포주를 이용하여 조사하였을 때 $IC_{50}$ 값이 시스플라틴보다 작았기 때문에 착물들의 송아지 흉선 DNA(CT-DNA)와의 결합 모드를 UV 흡광도 차이와 형광 분광법으로 조사하였다. 착물들은 DNA를 변형시키고, 협동결합을 하고, DNA에 끼어 들어간다. 또한 몇 가지 결합 및 열역학적인 변수들이 기술되었다.

INTRODUCTION

cis-Diamminedichloroplatinum(II) (cisplatin) is one of the most effective anticancer drugs currently available for the treatment of testicular, lung and bladder carcinomas.1-3

Despite the success of cisplatin and platinum-based drugs, the market is still accessible for new advantageous metal-based drugs that offer better viability, such as oral administration, which might help to diminish severe side effects and clinical costs. Additionally, research is focused on drugs with higher efficacy, i.e. drugs that interact differently with the target, CT-DNA, can overcome inherent or acquired cisplatin resistance in many tumors, and are active towards tumors which are non-responsive to current chemotherapy.4

Generally, there are three kinds of binding models for small molecules with CT-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 CT-DNA.5-7

A substantial investigation of other metals (Ti, Ga, Ge, Pd, Au, Co, and Sn) is underway that may help to avoid, or improve, the problems associated with the use of platinum compounds as therapeutic agents.8,9 The interest in platinum(II) and palladium(II) complexes containing N and S donor ligands has increased in recent years, with the aim to synthesize antitumor drugs having a reduced toxicity with respect to cisplatin and analogues. The molecules containing sulfur are currently under study as chemoprotectants in platinum-based chemotherapy. In particular, thiocarbonyl and thiol donors have shown promising properties for chemical use in modulating cisplatin nephrotoxicity.10,11 A result suggesting decrease of cisplatin or carboplatin nephrotoxicity has been observed in combined therapy with glutathione12 which substitutes ammonia ligands, as generally observed with thiol.10,13,14

In order to investigate the effect of combined nitrogen and sulfur donor chelating ligands on the activity of palladium(II) complexes, we designed, synthesized and characterized two palladium complexes with the 1,10-phenanthroline and S,S chelated dithiocarbamete containing the piperidine and morpholine residue. The cytotoxicities of these complexes were tested against chronic myelogenous leukemia cell line, K562. Gel filtration, electronic absorption titration and fluorescence experiments were employed to determine the modes of binding of the above Pd(II) complexes with CT-DNA. In these interaction studies, binding parameters, thermodynamic parameters, and the type of binding between these agents and CT-DNA are also described. These parameters may throw light on the interaction mechanisms of these types of complexes with DNA of cells and possible side effects of these agents. It is speculated that this mechanism must be different from that reported for cisplatin.

 

EXPERIMENTAL

Material and general methods

Palladium(II) chloride anhydrous, sodium chloride, NaOH and KBr were obtained from Fluka (Switzerland). Piperidine, morpholine, AgNO3 and carbon disulfide were purchased from Aldrich (England). 1,10-phenathroline, Sephadex G-25 and Tris-HCl buffer were obtained from Merck (Germany). Calf thymus DNA and Ethidium bromide (EB) were obtained from Sigma Chemical Co. (USA) and used as received. All other solvents and reagents used were of analytical grade and purified before use by the standard methods. The doubly distilled water was used all along. [Pd(phen)Cl2] were prepared by the procedures described in the literature.15 Piperidine dithiocarbamate sodium salt (pip-dtcNa) and morpholine dithiocarbamate sodium salt (mor-dtcNa) was synthesized in our laboratory. 16,17 All the experiments involving interaction of the complexes with CT-DNA were performed in Tris-HCl buffer (20 mM) of pH=7.0 medium containing 20 mM NaCl. Sodium choloride was used to adjust the ionic strength of the solution. Monitoring the ratio of the absorbance at 260 to that of 280 nm checks purity of CT-DNA. The solution gave a ratio of >1.8 at A260/A280, indicating that CT-DNA is sufficiently free from protein.18,19 The CT-DNA concentration per nucleotide was determined by absorption spectroscopy using the known molar extinction coefficient value of 6600 M-1 cm-1 at 260 nm.20

Microchemical analysis of carbon, hydrogen, and nitrogen for the complexes was carried out on Herause CHNORAPID elemental analyzer. 1H NMR spectra were recorded on a Brucker DRX-500 Avance spectrometer at 500 MHz in DMSO-d6 using tetramethylsilane as internal reference. Electronic absorption spectra of the title metal complexes were measured on a JAS.CO UV/Vis-7850 recording spectrophotometer. Infrared spectra of the metal complexes were recorded on a JAS.CO- 460 Plus FT-IR spectrophotometer in the range of 4000 to 400 cm-1 in KBr pellets. Fluorescence intensity measurements were carried out on a Hitachi spectrofluorimeter model MPF-4. Conductivity measurements of the above palladium complexes were carried out on a Systronics conductivity bridge 305, using a conductivity cell of cell constant 1.0. Melting points were measured on a Unimelt capillary melting point apparatus and here reported uncorrectedly. The following spectrometric measurements were all performed in a quartz cuvette of 1 cm path length.

Synthesis of metal complexes

Synthesis of [Pd(phen)(pip-dtc)]NO3: A well suspension of [Pd(phen)Br2] (0.444 g, 1 mmol) in 120 mL water/acetone (1:3 v/v) was treated with AgNO3 (0.34 g, 2 mmol) dissolved in 30 mL doubly distilled water. The mixture was left under continuous magnetic stirring in dark under reflux at 50-55 ℃ for 8 h and at room temperature (30 ℃) for 10 h. The AgCl precipitate was filtered through Whatman 42 filter paper and resulting yellow colored filtrate containing [Pd(bpy)(H2O)2](NO3)2 was kept at 40 ℃ and then 0.183 g (1 mmol) piperidinedithiocarbamate sodium salt dissolved in water/acetone (1:1) was added slowly. The solution mixture was stirred and concentrated to 5 mL at 40 ℃. The precipitate formed was filtered off, washed with acetone and dried in an oven at 40-45 ℃. Yield: 0.372 g (70%) and decomposes at 335-338 ℃. Anal. Calcd. for C18H18N4O3S2Pd (508): C, 42.51; H, 3.54; N, 11.02%. Found: C, 42.55; H, 3.51; N, 11.06%. 1H NMR (500 MHz, DMSO-d6, ppm, s=singlet, sb=singlet broad, d=doublet, t=triplet and q=quartet):21 1.82 (sb, 6H, H-a), 3.96 (sb, 4H, H-b), 8.10 (q, 2H, H-c), 8.28 (s, 2H, H-d), 8.79 (t, 2H, He), 8.98 (d, 2H, H-f) (Scheme 1(1)). Solid state FT-IR spectroscopy of the complex shows three main characteristic bands at 1536, 1007 and 2983 cm-1 assigned to ν (N-CSS), ν (SCS) and ν (N-H) modes respectively.22,23 The sharp band at 1335 cm-1 is assigned to uncoordinated NO3-ion.24 Molar conductance of the complex is 93 Ω-1 mol-1 cm2 indicating 1:1 electrolytes.16,25 Electronic spectra exhibit four bands. The band at 460 (log ε = 3.17) is assigned to metal to 1,10-phenanthroline charge transfer and the other bands at 341 (ε = 3.58), 277 (ε = 4.1) and 237 (ε = 4.11) may be assigned to first, second and higher intraligand π-π* transitions of 1,10-phenanthroline ligand as well as -CSS-group of dithicarbamate ligand.31

Scheme 1.Proposed structures and proton nmr numbering schemes of [Pd(phen)(pip-dtc)]NO3 (1) and the inset for [Pd(phen) (mor-dtc)]NO3 (2) complexes.

Synthesis of [Pd(phen)(mor-dtc)]NO3

This complex was prepared by the method described for [Pd(phen)(pip-dtc)]NO3, which was obtained by adding morpholinedithiocarbamate sodium salt to [Pd(phen(H2O)2] (NO3)2 in molar ratio of 1:1. The product was collected as yellow powder. Yield:0.400 g (78%) and decomposes at 340-342 ℃. Anal. Calcd. for C17H16N4O4S2Pd (510): C, 40.00; H, 3.14; N, 10.98%. Found: C, 40.03; H, 3.14; N, 10.94%. 1H NMR (500 MHz, DMSO-d6, ppm, s=singlet, sb=singlet broad, d=doublet, t=triplet and q=quartet):21 3.81 (t, 4H, H-a), 3.94 (t, 4H, H-b), 8.06 (q, 2H, H-c), 8.28 (s, 2H, H-d), 8.68 (d, 2H, H-e), 8.97 (d, 2H, H-f) (Scheme 1(2)). Solid state FT-IR spectroscopy of the complex shows three main characteristic bands at 1520, 1023 and 2924 cm-1 assigned to ν (N-CSS), ν (SCS) and ν (N-H) modes respectively.22,23 The sharp band at 1383 cm-1 is assigned to uncoordinated NO3- ion.24 Molar conductance of the complex is 96 Ω-1 mol-1 cm2 indicating 1:1 electrolytes.16,25 Electronic spectra exhibit four bands. The band at 341 (log ε = 6.53) is assigned to metal to 1,10-phenanthroline charge transfer and the other bands at 274 (ε = 6.44), 246 (ε = 6.39) and 208 (ε = 6.32) may be assigned to first, second and higher intraligand π-π* transitions of 1,10-phenanthroline ligand as well as -CSS- group of dithicarbamate ligand.31

Cytotoxicity assay

Cell proliferation was evaluated by using a system based on the tetrazolium compound [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, MTT] which is reduced by living cells to yield a soluble formazan product that can be assayed colorimetrically.26-28 The MTT assay is dependent on the cleavage and conversion of the soluble yellowish MTT to the insoluble purple formazan by active mitochondrial dehydrogenase of living cells. The leukemia cell line K562 was maintained in a RPMI medium supplemented with 10% heat-inactivated fetal calf serum and 2 mM L-glutamine, streptomycin and penicillin (5 μg/ml), at 310 K under a 5% CO2/95% air atmosphere. Harvested cells were seeded into 96-well plate (2×104 cell/mL) with varying concentrations of the sterilized drug (0-250 μM) and incubated for 24 hours. At the end of four hours incubations 25 μL of MTT solution (5 mg/mL in PBS) was added to each well containing, fresh and cultured medium.29 The insoluble formazan produced was then dissolved in solution containing 10% SDS and 50% DMF (under dark condition for 2 h at 310 K), and optical density (OD) was read against reagent blank with multi well scanning spectrophotometer (ELISA reader, Model Expert 96, Asys Hitchech, Austria) at a wavelength of 570 nm. Absorbance is a function of concentration of converted dye. The OD value of study groups was divided by the OD value of untreated control and presented as percentage of control (as 100%). Results were analyzed for statistical significance using two tailed Student’s t-test. Changes were considered significant at p < 0.05. In this experiment, the clear stock solution (2 mM, in deionized water) was sterilized by filtering through sterilizing membrane (0.1 nm) and then varying concentrations of the sterilized drug (0-25 μM) were added to harvested cells.

Spectroscopic studies on CT-DNA interaction

The experiments were carried out in Tris-HCl buffer using a solution of calf thymus DNA (4 mg/mL) at 277 K until homogenous. The stock solutions of Pd(II) complexes (0.05 mM for [Pd(phen)(pip-dtc)]NO3 complex and 0.1 mM for [Pd(phen)(mor-dtc)]NO3 complex) were made in Tris-HCl buffer by gentle stirring and heating at 308 K. The CT-DNA-metal complex solutions were incubated at 300 K and 310 K, separately. Then, the spectrophotometric readings at λmax of the palladium complexes (300 nm) where CT-DNA has no absorption were measured. Using trial and error method, the incubation time for solutions of DNA-metal complex at 300 K and 310 K was found to be 6 hour. No further changes were observed in the absorption readings after longer incubation, indicating that interaction is completed. All the experiments repeated to get the constant results.

CT-DNA-denaturation studies and determination of thermodynamic parameters

The application of UV absorption method to the study of denaturation of CT-DNA Pd(II) complexes were similar to that reported earlier.30,31 In this experiment, the sample cell was filled with 1.8 mL CT-DNA (~0.20 mM for [Pd(phen)(pip-dtc)]NO3 and ~0.17 mM for [Pd(phen) (mor-dtc)]NO3 complexes). In these concentrations, the absorption of CT-DNA is around 1.32 and 1.12, respectively. However, reference cell is filled with 1.8 mL Tris-HCl buffer only. Both cells were set separately at constant temperature of 300 K or 310 K and then 10 μL stock solution of [Pd(phen)(pip-dtc)]NO3 or 50 μL stock solution of [Pd(phen)(mor-dtc)]NO3 complexes, were added to each cell. After 3 min., the absorption was recorded at 260 nm for CT-DNA and at 640 nm to eliminate the interference of turbidity. Addition of metal complex to both cells was continued until no further changes in the absorption readings were observed. In these studies, the concentration of each metal complex at midpoint of transition, [L]1/2, was determined. Also, thermodynamic parameters such as: ΔGo(H2O), conformational stability of CT-DNA in the absence of metal complex; ΔGo(H2O), the heat needed for CT-DNA denaturation in the absence of metal complex; ΔSo(H2O), the entropy of CT-DNA denaturation by metal complex as well as m, measure of the metal complex ability to denature CT-DNA were found out using Pace method.32

Electronic spectroscopy

Electronic absorption titration experiments were performed with fixed concentration of each metal complex (10 μL from [Pd(phen)(pip-dtc)]NO3 complex of 0.05 mM stock and 50 μL from [Pd(phen)(mor-dtc)]NO3 complex of 0.1 mM stock), while gradually increasing the concentration of CT-DNA (20-55 μL for [Pd(phen)(pip-dtc)]NO3 complex from 1.9 mM stock and 10-90 μL for [Pd(phen)(mordtc)] NO3 complex from 2.1 mM stock) in total volume of 2 mL for obtaining the maximum ΔA (ΔAmax), i.e. change in the absorbance when all binding sites on CT-DNA were occupied by metal complex. To affirm quantitatively the affinity of the complexes bound to CT-DNA, the binding parameters (n, K and g) of metal complexes to CT-DNA were obtained. n is Hill coefficient, g is the number of binding sites per 1000 nucleotides of CT-DNA and K is apparent binding constant. In this experiment, a fixed amount of CT-DNA (25 μL for [Pd(phen)(pip-dtc)]NO3 from 1.8 mM stock and 25 μL for [Pd(phen)(mor-dtc)]NO3 complex from 2.9 mM stock) was titrated with varying concentrations of each of Pd (II) complexes (125-258 μL for [Pd(phen)(pip-dtc)]NO3 from 0.05 mM stock and 50-150 μL for [Pd(phen)(mor-dtc)]NO3 from 0.1 mM stock) in total volume of 2 mL. Each sample solution was scanned at 300 nm for two complexes.

Fluorescence spectroscopy

Quantitatively the affinity of the compounds bound to CT-DNA compared by the luminescence titration method. In this experiment, a series of solutions with various concentrations of each of the two complexes (1.78-2.55 for [Pd(phen)(pip-dtc)]NO3 and 2.44-4.67 for [Pd(phen)(mordtc)] NO3) were added to a solution of CT-DNA (60 μM) and ethidium bromide (2 μM). The samples were excited at 471 nm and emission was observed between 540 and 700 nm at 300 K and 310 K temperatures. The fluorescence intensities of the Pd(II) complexes at the highest denaturant concentration at 471 nm excitation wavelength have been checked, and the emission intensities of these compounds were very small and negligible.

Further studies to characterize the mode of binding of Pd(II) to CT-DNA were carried out by Scatchard analysis.33 In this experiment, the wavelengths of excitation and emission were set at 540 nm and 700 nm respectively. Both have 0.5 nm slit widths. Solutions of CT-DNA, EB and metal complexes were prepared in Tris-HCl buffer of pH 7.0. In this medium solutions of Pd(II) complexes were interacted with CT-DNA by incubating them at 300 K for 6 h, appropriate amount of EB was then added to them and further incubated at room temperature (300 K) for 6 h and finally processed for fluorescence spectral measurement. Saturation curves of fluorescence intensity for [Pd(bpy) (pip/mor-dtc)]+-CT-DNA system at different rf values (0.03, 0.035 and 0.042 for [Pd(phen)(pip-dtc)]NO3 complex and 0.041, 0.055 and 0.078 for [Pd(phen)(mor-dtc)]NO3 complex) were obtained in the presence of increased concentrations of EB (2, 4, …, 20 μM).

Gel filtration

Each of the above mentioned Pd(II) complexes (500 μM) was incubated with calf thymus DNA (77.4 μM for [Pd(phen) (pip-dtc)]NO3 complex and 230.95 μM for [Pd(phen) (mor-dtc)]NO3 complex) for 6 h at 301 K in Tris-HCl buffer (pH 7.0). It was then passed through a Sephadex G-25 column equilibrated with the same buffer. The elusion of the column fraction of 2.0 mL was monitored at 300 nm and 260 nm for DNA-Pd(II) complexes. Gel chromatograms are obtained by plotting of absorbance readings at the two wavelengths versus column fractions in the same plot. In this plot, the two peaks obtained may resolve or not. The former indicates that the CT-DNA is separated from the metal complex and the binding of CT-DNA to metal complex is weak and reversible. However, if the two peaks are not resolved, it indicates that the CT-DNA is not separated from the metal complex and the binding is stronger and irreversible.

 

RESULTS AND DISCUSSION

Cytotoxicity screenings

The effect of prepared complexes on human leukemic K562 cells was evaluated by MTT assay. In this study, various concentrations of Pd (II) complexes ranging from 0 to 25 μM of stock solution (2 mM) were used to culture the tumor cell lines for 24 h. The Ic50 value of the [Pd(phen) (pip-dtc)]NO3 and the [Pd(phen)(mor-dtc)]NO3 complexes are 0.22 mM. As shown in Fig. 1 for [Pd(phen)(pip-dtc)]NO3 complex and the inset of [Pd(phen)(mor-dtc)]NO3 complex, cell growing after 24 h was significantly reduced in the presence of various concentrations of the palladium complexes. Furthermore, Ic50 value of cisplatin under the same experimental conditions was determined. This value (154 μM) is much higher as compared to the Ic50 value of the above two complexes. However, the Ic50 values of these complexes are comparable with those of Pt(II) and Pd(II) dithiocarbamate complexes reported earlier.16,30,31 These results suggest that the two complexes may be potential antitumor agent.

Fig. 1.The growth suppression activity of the [Pd(phen)(pipdtc)] NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex on K562 cell line was assessed using MTT assay as described in material and methods. The tumor cells were incubated with varying concentrations of the complexes for 24 h.

CT-DNA-binding studies

CT-DNA denaturation data evaluation of thermodynamic parameters: Electronic absorption spectroscopy is one of the most useful techniques for CT-DNAbinding studies of metal complexes.34,35 The result of UVVis spectrum analysis about the interaction of CT-DNA with Pd(II) complexes is shown in Fig. 2. The maximum unfolding of CT-DNA by interaction with the two palladium(II) complexes occurs when all binding sites are occupied. The profiles of denaturation of CT-DNA by [Pd(phen)(pip-dtc)]NO3 and [Pd(phen)(mor-dtc)]NO3 in Fig. 2 at two temperatures of 300 K and 310 K are shown that the concentration of metal complexes in the midpoint of transition, [L]1/2, for [Pd(phen)(pip-dtc)]NO3 complex at 300 K is 0.0204 mmol/L and at 310 K is 0.0164 mmol/L and for [Pd(phen)(mor-dtc)]NO3 complex at 300 K is 0.0173 and at 310 K is 0.0169 mmol/L. The important observation of this work is the low values of [L]1/2 for these complexes36-38 i.e. both the complexes can denature CTDNA at very low concentrations (~18 μL for [Pd(phen)(pipdtc)] NO3 complex and ~17 μL for [Pd(phen)(mor-dtc)]NO3 complex). Thus, if these complexes will be used as antitumor agents, low doses will be needed, which may have fewer side effects. These values are lower or comparable with [L]1/2 values of binding of analogous complexes [Pt/Pd(bpy)(Et-dtc)]NO339 and [Pt/Pd(bpy)(Bu-dtc)]NO340 with CT-DNA. It is noticeable that, absorbance of DNA bases (purines and pyrimidines) decrease as their ring systems become parallel and near to one another, more stacking. Thus decrease in the absorbance at 260 nm with increase of amount of Pd(II) complexes added to CT-DNA may be due to: (i) a possibility that interaction between CT-DNA and the metal complex causes the double helix of CT-DNA to become more straight leading to stacking. This stacking may cause conformational changes leading to a sort of denaturation, or (ii) each strand after denaturation get associated in a more stacked structure and (iii) metal complex slips into the helix and masks the hydrophobic bases leading to a decrease in absorbance. As will be seen in the later part of this paper, the palladium(II) complexes can bind CT-DNA taking the mode of intercalation. This mode of binding supports the above three hypothesis.

Fig. 2.The changes of absorbance of CT-DNA (~0.185 mM) at λmax=260 nm due to increasing the total concentration of [Pd(phen) (pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3, [L]t, at constant temperatures of 300 K and 310 K.

Furthermore, some thermodynamic parameters found in the process of CT-DNA denaturation are discussed here: Using the CT-DNA denaturation plots given in Fig. 2 and Pace method,32,39 the values of K, i.e. unfolding equilibrium constant and ΔGo, unfolding free energy of CT-DNA at two temperatures of 300 K and 310 K in the presence of [Pd(phen)(pip-dtc)]NO3 and [Pd(phen)(mor-dtc)]NO3 have been calculated. In this method, Pace had assumed twostate mechanism, nature and denature, and then calculated unfolding free energy of DNA i.e. (ΔGo) by using Eqs. 1 and 2:

Fig. 3.The molar Gibbs free energies of unfolding (ΔGo vs [L]t) of CT-DNA in the presence of [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex.

Where Aobs is absorbance readings in transition region, AN and AD are absorbance readings of nature and denatured conformation of DNA, respectively. A straight line is observed when the values of ΔGo are plotted against the concentrations of each metal complex in the transition region at 300 K and at 310 K. These plots are shown in Fig. 3 for [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex. The equation for these lines can be written as follow:41

Here the values of ΔGo(H2O) for each curve are measured from the intercept on ordinate of the plots and it is conformational stability of DNA in the absence of metal complex. m (the slope of each curve in the same plots) is a measure of the metal complex ability to denature CTDNA and are summarized in Table 1. The values of m for the above complexes are much higher than those of Pt(II) and Pd(II) complexes reported earlier,16,39,40 which indicate the higher ability of [Pd(phen)(pip-dtc)]NO3 and [Pd(phen) (mor-dtc)]NO3 complexes to denature CT-DNA. As we know, the higher the value of G, the larger the conformational stability of CT-DNA. However, the values of ΔGo (Table 1) are decreased by increasing the temperature for both the complexes. This is as expected because in general, the decrease in ΔGo(H2O) value is the main reason for the decrease in CTDNA stability.42 The values of m for [Pd(phen)(pip-dtc)]NO3 complex are higher than that of [Pd(phen)(mor-dtc)]NO3 complex which indicate the higher ability of [Pd(phen) (pip-dtc)]NO3 complex to denature CT-DNA. Another important thermodynamic parameter found is the molar enthalpy of CT-DNA denaturation in absence of metal complexes i.e. ΔGo(H2O). For this, we calculated the molar enthalpy of CT-DNA denaturation in the presence of metal complexes, ΔHoconformation or ΔHodenaturation, in the range of the two temperature using Gibbs-Helmholtz equation.43 On plotting the values of these enthalpies versus the concentrations of metal complexes, straight lines will be obtained which are shown in Fig. 4 for [Pd(phen)(pip-dtc)]NO3 complex and the inset of [Pd(phen)(mor-dtc)]NO3 complex. Intrapolation of these lines (intercept on ordinate i.e. absence of metal complex) give the values of ΔGo(H2O) (Table 1). These plots show that in the range of 300 K and 310 K the changes in the enthalpies in the presence of [Pd(phen)(mor-dtc)]NO3 complex is descending. These observations indicate that on increasing the concentration of this complex, the stability of CT-DNA is decreased. In addition, the entropy ( ΔSo(H2O)) of CT-DNA unfolding by Pd(II) complexes have been calculated using equation ΔGo=ΔHo-TΔSo and the data are given in Table 1. These data show that the metal -DNA complexes are more disordered than that of native CT-DNA, because the entropy changes are positive for [Pd(phen)(pipdtc)] NO3-DNA complex as well as [Pd(phen)(mor-dtc)] NO3-DNA complex in the interaction of CT-DNA (Table 1).

Table 1.aMeasure of the metal complex ability to denature CT-DNA. bConformational stability of CT-DNA in the absence of metal complex. cThe heat needed for CT-DNA denaturation in the absence of metal complex. dThe entropy of CT-DNA denaturation by metal complex.

Fig. 4.The molar enthalpies of CT-DNA denaturation in the interaction with [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex in the range of 300 K to 310 K.

Electronic spectral studies and determination of binding parameters

Absorption spectral titration experiment was performed by keeping the concentration of the drugs constant while varying the CT-DNA concentrations. In this experiment, change in absorbance, A, was calculated by subtracting the absorbance reading of mixed solutions of each metal complex with various concentrations of CT-DNA, from absorbance reading of free metal complex. The values of ΔAmax, change in absorbance when all binding sites on CT-DNA were occupied by metal complex, are given in Table 2 and Fig. 5. These values were used to calculate the concentration of metal complexes bound to CT-DNA, [L]b, and the concentration of free metal complexes, [L]f and v, ratio of the concentration of bound metal complexes to total [CT-DNA] in the next experiment, that is, titration of fixed amount of CT-DNA with varying amount of each metal complex. Using these data (v, [L]f), the Scatchard plots44 were constructed for the interaction of metal complexes at the two temperatures 300 K and 310 K. The Scatchard plots are shown in Fig. 6 for [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex. These plots are curvilinear concave downwards, suggesting cooperative binding.33 To obtain the binding parameters, the above experimental data (v and [L]f) were substituted in equation (4), i.e. Hill equation.45

Table 2.aChange in the absorbance when all the binding sites on CT-DNA were occupied by metal complex. bThe number of binding sites per 1000 nucleotides. cThe apparent binding constant. dThe Hill coefficient (as a criterion of cooperativity). eMaximum error between theoretical and experimental values of ν.

Fig. 5.The changes in the absorbance of fixed amount of metal complexes in the interaction with varying amount of CT-DNA at 300 K and 310 K. The linear plot of the reciprocal of DA versus the reciprocal of [DNA] for [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex.

Fig. 6.Scatchard plots for binding of [Pd(phen)(pip-dtc)]NO3 (125-258 μL of 0.05 mM stock at 300 K and 310 K) to CT-DNA (25 μL of 1.8 mM stock at 300 K and 310 K) and [Pd(phen)(mordtc)] NO3 (50-150 μL of 0.1 mM stock at 300 K and 310 K) to CT-DNA (25 μL of 2.9 mM stock at 300 K and 310 K).

Thus, we get a series of equation with unknown parameters n, K and g. Using Eureka software, the theoretical values of these parameters could be deduced (Table 2). The values of n for two complexes will be more than 1, which means that the systems are cooperative thus, the binding at one site increases the affinity of others.43 The apparent binding constants of two complexes obtained were 2.82×105 M-1 at 300 K and 2.72×105 M-1 at 310 K for [Pd(phen)(pip-dtc)]NO3 complex and 2.95×105 M-1 at 300 K and 3.04×105 M-1 at 310 K for [Pd(phen)(mor-dtc)]NO3 complex. The values are comparable to those of CT-DNA intercalators [Ru(bpy)2(dpoq)]2+ 6.05×105 M-1, [Ru(phen)2 (dpoq)]2+ 7.22×105 M-1,46 [Ru(tpy)(ptp)]2+ 1.62×105 M-1 47 and [(bpy)2Ru(μ-bipp)Ru(bpy)2](ClO4)4 2.6×105 M-1 and [Ru(phen)2(PPIP)]2 1.1×105 M-1.48 Obviously, these spectral characteristics suggest that the two complexes intercalatively bind to CT-DNA, involving a strong stacking interaction between the aromatic chromophore (1,10-phenanthroline present in the structure of complexes) and the base pairs of the DNA.49 The maximum errors between experimental and theoretical values of v are also shown in Table 2 that is quite low. Knowing the experimental (dots) and theoretical (lines) values of v in the Scatchard plots and super impossibility of them on each other, these values of v were plotted versus the values of ln[L]f. The results are sigmoidal curves and are shown in Fig. 7 for [Pd(phen) (pip-dtc)]NO3 complex and the inset of [Pd(phen)(mordtc)] NO3 complex at 300 K and 310 K. These plots indicate positive cooperative binding at both temperatures for the two complexes. Finding the area under the above plots of binding isotherms and using Wyman-Jons equation,50 we can calculate the Kapp and ΔGo(H2O) at 300 K and 310 K for each particular v and also ΔGo(H2O). Plots of the values of ΔGo(H2O) versus the values of [L]f are shown in Fig. 8 for [Pd(phen)(pip-dtc)]NO3 complex and the inset of [Pd(phen) (mor-dtc)]NO3 complex at 300 K. Deflections are observed in both plots which may be due to binding of metal complexes to macromolecule or macromolecule denaturation. Similar observations can be seen in the literature where Pd(II) and Pt(II) complexes have been interacted with CT-DNA. 39,40

Fig. 7.Binding isotherm plots for [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex in the interaction with CT-DNA.

Fig. 8.Molar enthalpies of binding in the interaction between CT-DNA and [Pd(phen)(pip-dtc)]NO3 complex and the inset for [Pd(phen)(mor-dtc)]NO3 complex versus free concentrations of complexes at pH 7.0 and 300 K.

Fluorescent spectral studies

No fluorescence was observed for the above palladium(II) complexes at room temperature in aqueous solution or in the presence of calf thymus DNA. So the binding of palladium(II) complexes and CT-DNA cannot be directly presented in the emission spectra. Hence, competitive ethidium bromide (EB) binding studies were undertaken to gain support for the extent of binding of palladium (II) complexes with CT-DNA. The molecular fluorophore, EB, emits intense fluorescence light in the presence of CTDNA due to its strong intercalation between the adjacent CT-DNA base pairs. The results of the emission titration for the two complexes with helix CT-DNA that are illustrated in the titration curves are shown in Fig. 9 for [Pd(phen)(pip-dtc)]NO3 complex and the inset of [Pd(phen) (mor-dtc)]NO3 complex. Fig. 9 showed the emission spectra of DNA-EB system with increasing amounts of the Pd(II) complexes. The observation reflected that the stronger enhancement for Pd(II) complexes may be largely due to the increase of the molecular planarity of the complex and the decrease of the collision frequency of the solvent molecules with the complex which caused by the planar aromatic group of the complex stacks between adjacent base pairs of CT-DNA. The increase in the molecular planarity and the decrease of the collision frequency between solvent molecules with the complexes usually lead to emission enhancement.51,52 The binding of Pd(II) complexes to CT-DNA leading to marked increase in emission intensity also agrees with those observed for other intercalators.53

Fig. 9.Gel chromatograms of [Pd(phen)(pip-dtc)]NO3-DNA and [Pd(phen)(mor-dtc)]NO3-DNA complexes, obtained on Sephadex G-25 column, equilibriated with 20 mmol/L Tris-HCl buffer of pH 7.0 in the presence of 20 mmol/L sodium chloride.

Further studies to characterize the mode of binding of [Pt(Oct-dtc)(bpy)]NO3 and [Pd(Oct-dtc)(bpy)]NO3 to DNA were carried out.54,55 The number of EB molecules intercalated to CT-DNA in presence of different concentrations of the Pd(II) complexes was calculated using Scatchard analysis.44,56 The fluorescence Scatchard plots obtained for binding of EB to CT-DNA in absence (◇) and presence (◆, ▲, ●) of various concentrations of Pd(II) complexes were shown in Fig. 10. Consequently, it might be concluded that the Pd(II) complexes inhibit competitively the EB binding to CT-DNA (type-A behavior),44 where number of intercalated molecules per 1000 nucleotide, (intercept on the abscissa) remain constant and the slope of the graphs that is Kapp (apparent association constant) decrease with increasing the concentration of the complexes. The Kapp values for the Pd(II) complexes are listed in Table 3. These data suggested that the interaction of the [Pd(phen)(pipdtc)] NO3 complex with CT-DNA was stronger than that of [Pd(phen)(mor-dtc)]NO3 complex, which were consistent with the above absorption spectral results. Compare their Kapp values with those of other known CT-DNAintercalative complexes which possess analogical structure, [Pd(Et-dtc)(phen)]NO3 1.28×105 M-1,30 [Pd(Bu -dtc)(phen)] NO3 1.6×105 M-1,31 the Pd(II) complexes in our paper have similar or stronger affinities with CT-DNA.

Fig. 10.Fluorescence emission spectra of interacted EB-DNA in the absence (1) and presence of different concentrations of [Pd(phen)(pip-dtc)]NO3 and the inset for [Pd(phen)(mor-dtc)] NO3: 1.78(2), 2.11(3) and 2.55(4) μM and 2.44(2), 3.33(3) and 4.67(4) μM, respectively, at 300 K.

Table 3.aformal ratio of metal complex to nucleotide concentration. bAssociation constant. cNumber of binding sites (n) per nucleotide

CT-DNA binding mode

The solution of each interacted CT-DNA−metal complex was passed through a Sephadex G-25 column equilibrated with Tris–HCl buffer. Elution was done with buffer and each fraction of the column was monitored spectrophotometrically at 300 nm and 260 nm for Pd(II)–CT-DNA systems. These results are given in Fig. 11 for [Pd(phen)(pipdtc)] NO3 complex and the inset of [Pd(phen)(mor-dtc)]NO3 complex. These plots show that the peak obtained for the two wavelengths are not resolved and suggests that CTDNA has not separated from the metal complexes. Thus, it implies that the binding between CT-DNA and the metal complexes are not reversible under such circumstances. If the interaction between CT-DNA and metal complexes were weak, the CT-DNA should have come out of the column separately.39,40

Fig. 11.Competition between [Pd(phen)(pip-dtc)]NO3 and [Pd(phen) (mor-dtc)]NO3 with ethidium bromide for the binding sites of DNA (Scatchard plot). In curve no. 1, Scatchard,s plot was obtained with calf thymus DNA alone. Its concentration was 60 μM. In curves nos. 2, 3 and 4 respectively, 1.78, 2.11 and 2.55 μM for [Pd(phen)(pip-dtc)]NO3 complex and 2.44, 3.33 and 4.67 for [Pd(phen)(mor-dtc)]NO3 complex, were added, corresponding to molar ratio [complex]/[DNA] of 0.03, 0.035 and 0.042 for [Pd(phen)(pip-dtc)]NO3 complex and 0.041, 0.055 and 0.078 for [Pd(phen)(mor-dtc)]NO3 complex. Solutions were in 20 mM NaCl, 20 mM Tris-HCl (pH 7.0). Experiments were done at room temperature.

 

CONCLUSION

In summary, the two new water soluble palladium(II) complexes have been synthesized and characterized. The interactions of the two palladium(II) complexes with calf thymus DNA (CT-DNA) are investigated by using absorption and emission spectra and gel filtration techniques. Cytotoxic studies show that the two complexes exhibit good cytotoxic activity against chronic myelogenous leukemia cell line, K562, tested. The results suggest that both the two complexes interact with CT-DNA cooperatively in the mode of intercalation and the binding ability of the [Pd(phen)(pip-dtc)]NO3 complex to CT-DNA is more than [Pd(phen)(mor-dtc)]NO3 complex. Several binding and thermodynamic parameters have been determined, which may be useful to understand the mechanism of interaction of this type of compounds with CT-DNA or their antitumor activities. Gel filtration results show that the two peaks obtained at two wavelengths were not clearly resolved which indicate that metal complexes have not separated from CT-DNA and their binding with CT-DNA is strong enough that not readily break. Results obtained from our present work would be useful to understand the mechanism of interactions of the small molecule compounds binding to CT-DNA and helpful in the development of their potential biological, pharmaceutical and physiological implications in the future.