Evaluation of DNA interactions of metal complexes of ... · Hot ethanolic solution of metal...

Evaluation of DNA interactions of metal complexes of thenil

based Schiff bases: Synthesis, characterization and

validation.

S. Iyyam Pillai1*, M. Maria Dorathi anu1, K. VIjayaraghavan1 and S. Subramanian2

1. P.G and Research Department of Chemistry, Pachaiyappa’s College, Chennai, Tamilnadu, India

2. Department of Biochemistry, University of Madras, Guindy Campus, Chennai, Tamilnadu, India

*Corresponding author:

Dr. S. Iyyam Pillai,

Assistant Professor,

P.G and Research Department of Chemistry,

Pachaiyappa’s College,Chennai, Tamilnadu,India

Abstract:

A series of Cu(II), Ni(II) and Zn(II) complexes of Schiff base ligand (L) which has been

synthesized by condensing 1,2-Di(2-thienyl)-1,2-ethanedione and 1,8-Naphthalenediamine. The

synthesized ligand and their metal complexes were characterized by elemental analyses, ESI-MS,

IR, NMR and electronic spectral studies. The spectroscopic data of metal complexes indicated

that the metal ions are complexed with azomethine nitrogen’s. Further, DNA binding properties

of these metal complexes with CT-DNA in a (50 mM Tris-HCl buffer, pH 7.2) were investigated

using UV-Vis absorption spectroscopy, fluorescence spectroscopy, and Circular dichroic

spectroscopy and viscosity measurements. The experimental results from all techniques i.e.

binding constant (Kb), apparent binding constant (Kapp) were in good conformity and inferred

spontaneous DNA- complexes formation via intercalation.

Key words: Schiff base, Thenil, 1,8-Naphthalenediamine, Intercalation and Viscosity.

1. Introduction:

The Chemistry of coordination compounds is at present undergoing rapid development in

diverse disciplines like biological, biochemical, electrochemical studies. A significant rising

interest in the design of metal complexes as drugs and diagnostic agent is currently observed in

the area of scientific inquiry, appropriately termed medicinal chemistry1. DNA is the primary

target molecule of many anticancer agents, and the binding between DNA and metal complexes

were used in understanding the interaction between the drugs and DNA. In general, the tumor

Pramana Research Journal

Volume 9, Issue 2, 2019

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https://pramanaresearch.org/332

cells can be smashed by stopping the replication of the unnatural DNA. Using Schiff base

transition metal complex in particular, affected DNA may be dented by either following cleavage

or binding approach2. The interest in preparation of new metal complexes gained the tendency of

studying on the interaction of metal complexes with DNA for their applications in biotechnology

and medicine. Although cisplatin and carboplatin are in use, there are several side effects of

these chemotherapeutic drugs. Therefore, our main aim is to prepare the chemotherapeutic drugs

without side effects or fewer side effects. Many transition metal complexes are known to bind to

DNA via both covalent and non-covalent interactions. In covalent binding the labile ligand of the

complexes is replaced by a nitrogen base of DNA. On the other hand, the non-covalent DNA

interactions include intercalative, electrostatic and groove (surface) binding of cationic metal

complexes outside of the DNA helix, major or minor groove. DNA molecules are prone to be

damaged under various conditions like interactions with some molecules. This damage may

cause various pathological changes in living organisms, which is due to their possible application

as new therapeutic agents and their photochemical properties which make them potential probes

of DNA structure and conformation3.

Much of the impetus was purely directly toward the synthesis of structurally imine or

azomethine (Schiff base) ligands due to they are one of the most popular ligand systems in

inorganic chemistry. Due to their preparative accessibility and structural flexibility, researchers

have given unusual deliberation to design novel Schiff bases and metal chelates. Stupendous

chemical and biological significance of metal chelates in which the Schiff bases play a role as the

ligands arise from the presence of azomethine linkage (C = N), embedded in their skeletons4. In

view of the above considerations, we have synthesized three new Cu(II), Ni(II) and Zn(II)

complexes of Schiff base ligand (L) which has been synthesized by condensing 1,2-Di(2-

thienyl)-1,2-ethanedione and 1,8-Naphthalenediamine. The ligand and their respective metal

complexes have been characterized by elemental analyses, ESI-MS, IR, NMR and electronic

spectral studies. Furthermore, the interactions of these complexes with calf thymus DNA (CT-

DNA) have been investigated by using spectroscopies like UV-Vis absorption spectroscopy,

fluorescence spectroscopy, and Circular dichroic spectroscopy and viscosity measurements.

2. EXPERIMENTAL PROTOCOLS:

2.1 Reagents instruments

All the chemicals used in the current work viz, 1,2-Di(2-thienyl)-1,2-ethanedione and 1,8-

Naphthalenediamine and metal(II) chlorides were of analytical reagent grade (produced by were

obtained from Fluka and Sisco Chemicals. Commercial solvents were distilled and then used for

the preparation of ligands and their complexes. DNA was purchased from Bangalore Genei

(India). Tris(hydroxymethyl)aminomethane–HCl (Tris–HCl) buffer solution was prepared by

means of deionized and sonicated triple distilled water.

2.2 Instruments

Elemental analyses were performed using Carlo Erba 1108 analyzer and Coleman N

analyzer and were found within ±0.5%. Melting points of the compounds were determined with



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open glass capillary tubes on a Polmon instrument (Model No. MP-96). The IR spectral analysis

was carried out in the solid state as pressed KBr pellets by means of a Perkin-Elmer FT-IR

spectrophotometer in the range of 400-4000 cm-1. The electro spray mass spectra were

established on a Q-TOF micro mass spectrometer. The emission spectra were recorded on Perkin

Elmer LS-45 fluorescence spectrometer. The electronic spectra in diverse solvents were recorded

on using a Perkin Elmer Lambda 35 spectrophotometer working in the range of 200-900 nm with

quartz cells and ε are given in M-1cm-1. Circular dichoric spectra of CT-DNA were obtained

using a JASCO J-715 spectropolarimeter equipped with a Peltier temperature control device at

25 ± 0.1°C with 0.1 cm path length cuvette. Viscosity measurements were verified using a

Brookfield Programmable LV DVII+ viscometer. The entire glassware used was made of Pyrex

or corning glass.

2.3 Synthesis of Schiff base ligand L

The Schiff base ligand L used in current research was prepared by reacting hot ethanolic

solution (0.002 mol,) 1,2-Di(2-thienyl)-1,2-ethanedione and 1,8-Naphthalenediamine (0.002

mol) with methanolic solution (25 mL). The mixture was refluxed for 6-7 h on water bath and

allowed to cool at room temperature. The coloured compound formed was filtered, washed, dried

and recrystallized from ethanol.

Scheme 1. Synthesis of Schiff base ligand L.

2.3 Synthesis of Schiff base ligand L and their respective complexes:

All the complexes were prepared by the following general method. Hot ethanolic solution

of metal chloride salts (0.001 mol, 25 mL) were added to hot ethanolic solution of the ligand L

(0.001 mol, 25 mL) in 1:1 ratio. The reaction mixture was heated under reflux for about 6 hours.

The resulting solution was concentrated to half its volume and allowed to cool. On cooling the

complexes separated out which were filtered and washed several times with aqueous ethanol,

diethyl either and finally dried over in vacuo.



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Scheme 2. Synthesis of Schiff base metal complexes.

2.4 DNA binding and cleavage activity

2.4.1 Electronic absorption titration

All spectroscopic titrations were carried out in 5 mmol L–1 Tris–HCl buffer (pH 7.1)

containing 50 mmol L–1 NaCl at room temperature. The concentration of DNA was estimated

from its absorption intensity at 260 nm with a known molar absorption coefficient value of 6600

M–1 cm–1. Stock solutions were stored at 4 °C and used within 4 days5. Titration

experimentations were carried out varying the concentration of the CT-DNA from (0, 40, 80,

120, 160, 200, 300 and 400 µM) while keeping the metal complexes concentration constant (20

µM). While measuring the absorption spectra, equal quantity of CT-DNA was added to both the

complex solution and the reference solution to eliminate the absorbance of CT-DNA itself. The

intrinsic binding constant (Kb) for the interaction of the complexes with CT DNA was

determined from a plot of [DNA]/(εa - εf) versus [DNA] using absorption spectral titration data21

and the following equation,

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

where [DNA] is the concentration of DNA, the apparent absorption coefficients εa, εf and εb

correspond to Aobsd/[complex], the extinction coefficient for the free metal complex and the

extinction coefficient for the metal complex in the fully bound form, respectively. The Kb value

is given by the ratio of the slope to the intercept.

2.4.2 Fluorescence spectral study

Further support for the binding of complexes to DNA was given through the fluorescence

quenching study. A mixture of the CT-DNA (100 μM) and ethidium bromide (EB) (12.5 μM)

was subjected to competitive binding with increasing amount of complex (0, 40, 80, 120, 160,

200, 300 and 400 µM). On this basis molecular fluorophore Ethidium Bromide was used which

emits fluorescence in presence of CT-DNA due to its strong intercalation. Quenching of the

fluorescence of EthBr bound to DNA were measured with increasing amount of metal complexes



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as a second molecule and Stern–Volmer quenching constant Ksv was obtained from the following

equation6.

Io/I=1 + Ksvr. (2)

Where I0, is the ratio of fluorescence intensities of the complex alone, I is the ratio of

fluorescence intensities of the complex in the presence of CT-DNA. Ksv is a linear Stern –

Volmer quenching constant and r is the ratio of the total concentration of quencher to that of

DNA, [M] / [DNA]. A plot of I0 / I vs. [complex]/ [DNA], Ksv is given by the ratio of the slope to

the intercept. The apparent binding constant (Kapp) was calculated using the equation KEB[EB] /

Kapp[complex], where the complex concentration was the value at a 50% reduction of the

fluorescence intensity of EB and KEB = 1.0 x 107 M-1 ([EB] = 3.3 µM).

2.4.3 Viscosity measurements

Viscosity experiments were carried out by means of a capillary viscometer at an

unvarying temperature (25.0 ± 0.1 ⁰C). Flow times were calculated by means of a digital

stopwatch, and each sample was measured three times, and then a standard flow time was

calculated. To obtain the binding mode of the complexes towards CT-DNA, viscosity

proportions were carried out on CT-DNA (0.5 mM) by varying the concentration of the

complexes (0.01 mM, 0.02 mM, 0.03 mM, 0.04 mM, 0.05 mM). Data were accessible as (η/ηo)

versus binding ratio of concentration of complex to that of concentration of CT-DNA , where η

is the viscosity of DNA in the presence of complex and ηo is the viscosity of DNA alone.

2.4.4. CD spectral studies

The CD spectral analysis of CT-DNA in the presence or absence of complexes were

collected in Tris–HCl buffer (pH=7.2) containing 50 mM NaCl at room temperature. The spectra

were recorded in the region of 220–320 nm for 200 µM DNA in the presence of 100 µM of the

complexes. Each CD spectrum was collected after averaging over at least three accumulations

using a scan speed of 100 nm min-1 response time. Machine plus cuvette base lines, and CD

contribution by the CT-DNA and Tris buffer were subtracted and the resultant spectrum zeroed

50 nm outside the absorption bands. Circular dichroic spectra of CT DNA in the presence and

absence of metal complexes were obtained by using a JASCO J-715 spectropolarimeter equipped

with a Peltier temperature control device at 25 ± 0.1 °C with a 0.1 cm path length cuvette.

RESULTS AND DISCUSSION

3.1 Structural characterization of the Schiff base ligand (L) and their complexes.

3.1.1 Physical properties and elemental analysis

The physical properties and elemental analysis of the synthesized ligand and their metal

complexes are described in Table 1. Structures have been recommended according to these data



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collectively with obtained from spectral analysis. All the metal complexes have been found to

possess high melting points.

Sl. No Compound M.P °C Colour Yield

%

Found (Calculated %)

C H N S

1 Ligand L 119 Brown 84 69.74

(69.71)

3.51

(3.54)

8.13

(8.17)

18.62

(18.58)

2 Cu L 204 Deep blue 80 63.85

(63.81)

3.21

(3.26)

7.45

(7.49)

17.05

(17.09)

3 Ni L 212 Dull green 86 64.26

(64.21)

3.24

(3.19)

7.49

(7.44)

17.16

(17.11)

4 Zn L 226 Pale

brown

82 63.69

(63.64)

3.21

(3.17)

7.43

(7.38)

17.00

(16.95)

3.1.2 FT-IR spectral analysis.

The IR spectra of ligand L showed sharp peaks at 1639 cm−1, respectively (Figure 1),

which corresponds to azomethine (–C=N) group. These peaks are shifted to lower frequency by

18 to 20 cm−1 due to coordination of nitrogen of azomethine to the metal ion (Figure 2 and

Figure S1-S2)7,8. The formation of complexes is further confirmed by the new bands observed at

lower frequency region 445 – 475 cm−1, respectively due to M–N 9-10.

Figure 1. FT-IR spectra of Schiff base mixed ligand L.



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Figure 2. FT-IR spectra of Schiff base Cu (II) complex

3.1.3 Electronic spectra of the ligand L and its complexes.

The UV-Vis spectra of all the compounds were measured in DMSO at room temperature.

The ligands showed two absorption peaks between the range 270–280 and 340–350 nm and these

are assigned to π−π* and n-π* transition, respectively. The metal complexes showed d-d bands in

the range of 425 - 450 and 630 - 680 nm and CT bands at lower wavelengths. The Cu(II)

complexes showed a band due to 2B1g − 2Eg transition11. The nickel complexes are diamagnetic

with one band corresponding to 1A1g − 1B1g transition12, Based on the electronic spectral data as

represented in Figure 3 and Figure 4 a square planar geometry is assigned to all the complexes.

Figure 3. Electronic spectrum of the Schiff base macrocyclic ligand (L).



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Figure 4. Electronic spectrum of the Schiff base Cu(II) and Ni(II) complexes.

3.1.4 ESI - Mass spectra of the ligand L and its complexes.

The molecular ion peak [M+] at m/z = 688 confirms the molecular weight of the

macrocyclic Schiff base ligand C40H24N4S4. The peaks at m/z = 524, 360, 244, 186 and 104

corresponds to the various fragments C32H20N4S2, C24H16N4, C14H20N4 and C12H14N2

respectively as shown in Figure 5. This confirms the molecular structure of the ligand L.

Figure 5. ESI-Mass spectrum of Schiff base ligand (L).



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The molecular ion peak [M+] at m/z = 752 confirms the molecular weight of the

macrocyclic Schiff base Cu(II) complex C40H24N4S4Cu. The peaks at m/z = 670, 588, 506, 423,

297 and 171 corresponds to the various fragments C36H22N4S3Cu, C32H20N4S2Cu, C28H18N4SCu,

C24H16N4Cu, C14H10N4Cu and C4H4N4Cu respectively as shown in Figure 6 and Figure S3 – S4.

The molecular ion peak [M+] at m/z = 747 and 754 confirms the molecular weight of the

macrocyclic Schiff base Ni(II) and Zn(II) complex C40H24N4S4M [M = Zn and Ni]. The type of

fragmentation observed in Ni(II) and Zn(II) complex was similar with that of the Cu(II)

complex.

Figure 6. ESI mass spectrum of the Cu(II) complex.

3.2 DNA binding experiments

3.2.1 Absorption spectral studies

The absorption spectral titration method is one of the most important techniques which

have usually been utilized to monitor the interaction of small molecules with DNA, as any

interaction with the macromolecule like DNA will cause electronic perturbations in these

molecules13. Complexes bind to DNA through covalent bonding by substituting the

exchangeable ligand of the complexes with the nitrogen base of DNA14. or noncovalent

interaction, such as intercalation, electrostatic binding, or groove binding15. The binding of an

intercalative complex molecule to DNA has been well characterized by notable intensity

decrease (hypochromism) and red shift (bathochromism) of the electronic spectral bands due to a

strong stacking interaction between the aromatic chromophore of the ligand and DNA base pairs.

The degree of hypochromism and the size of the redshift depend on strength of the intercalative

interaction16-18. On the other hand, metal complexes which do not intercalate or interact

electrostatically with DNA may exhibit hyperchromism19.



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Fixed amounts (20 µM) of metal complexes were titrated with increasing amounts of CT-

DNA in the range from 0, 40, 80, 120, 160, 200, 300 to 400 µM. The electronic spectra of LMF

and its complexes in the absence and presence of CT-DNA are given in Figure 7 and S5-S6. The

sharp bands at about 251 nm are assigned to intraligand (IL) to π−π* transitions and the lowest

energy bands at about 362 nm are attributed to the metal-to-ligand charge transfer (MLCT)

transitions. The absorption spectral titrations exhibited evidently that the addition of DNA to

metal complexes succumbs a significant hypochromism and a slight red shift at MLCT band.

Distinctly, these spectral characteristics suggest that all compounds had some interaction with

DNA. In order to affirm quantitatively the affinity of the complex bound to DNA, the intrinsic

binding constants Kb of the complex with DNA was obtained by monitoring the changes in

absorbance at 354 nm for the title complex with increasing concentration of DNA as shown in

inset of Figure 7. The obtained Kb (Binding constant) values for the synthesized complexes are

as follows: for Cu(II) complex 9.96 x 104 M-1, for Ni(II) complex 7.82 x 104 M-1 and for Zn (II)

complex is 6.71 x 104 M-1.

Figure 5 The absorption spectra of complex Cu(II)(1 x 10-5 M ) in the absence and presence of

increasing amounts of CT-DNA (0-2.5 x 10-3 M ) at normal temperature in 50 mM Tris-

HCl / NaCl buffer (pH = 7.5). The Inset shows the plots of [DNA] / (єa-єf) versus [DNA]

for the titration of DNA with Cu(II), Ni(II) and Zn(II) complexes.

3.3.2. Fluorescence emission spectrometry

In order to further investigate the interaction mode between the compounds and DNA and

their relative DNA binding affinities, we have performed a competitive binding fluorescence

experiment using ethidium bromide (EB = 3,8-diamino-5-ethyl-6-phenyl-phenanthridinium

bromide) as a probe20-22. The fluorescence intensity of EB is very weak in an aqueous solution,

but due to the intercalation of the planar phenanthridinium ring of EB between the adjacent base

0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

[DN

A]/

(a-

f) x

10

-9

[DNA] x 10-6

13

14

15



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pairs of DNA, its fluorescence intensity increases 24-fold23,24. However, EB has an intense

fluorescence in the presence of DNA due to its strong stacking interaction (intercalation)

between the adjacent DNA base pairs, which reflects its efficient protection from water and

stabilization of its excited state25. It was previously reported that this enhanced fluorescence for

the EB-DNA system may be quenched, at least partly, by the addition of another DNA binding

molecule, therefore EB can be used in competitive fluorescence quenching experiments to probe

the interaction of metal complexes with DNA26. The presence of another DNA binding molecule

could quench the EBDNA emission by either displacing DNA-bound EB and/or by accepting the

excited state electron from EB27.

The emission spectra of EB bound to DNA in the absence and presence of the

synthesized metal complexes are prearranged in Figure 6, Figure S7 and S8. It can be renowned

that the intensity of the fluorescence intensity of the EB-DNA solutions reduces with the addition

of the complex evidently. The obtained results suggest that the complexes can restore the EB and

bind to the DNA molecule. The quenching plots (insets in Figure 6) illustrate that the

fluorescence quenching of EB bound to DNA by Cu(II), Ni(II) and Zn(II) complexes in linear

agreement with the Stern–Volmer equation, which corroborates that the complexes bound to

DNA. The Kapp values for Cu(II), Ni(II) and Zn(II) complexes are found to be 9.81 x 105 M-1,

7.92 x 105 M-1 and 7.26 x 105 M-1 respectively. The achieved results are in reliable with that of

absorption spectroscopic studies. Anyway, it may be concluded that the whole complexes bound

to DNA via the similar mode and the quenching constants of the synthesized complexes reveals

that the interaction of the compound with DNA should be intercalation28.

Figure 6. Emission spectrum of EB bound to DNA in the presence of Cu(II):([EB] =3.3

μM,[DNA] = 40 μM, [complex] = 0-25 μM, λex= 440 nm). Inset shows the plots of

emission intensity Io / I vs [DNA] / [complex] for the titration of DNA with Cu(II), Ni(II)

and Zn(II) complexes.

1.2 1.4 1.6 1.8 2.0 2.2 2.40.0

0.2

0.4

0.6

0.8

1.0

1.2

I 0/I

[Complex]/[DNA]

13

14

15



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3.3.3 Viscosity measurements

Viscosity measurement is a reliable method to study the interaction mode of a complex

with DNA, especially for the intercalative binding mode29. When a complex binds to DNA by a

classical intercalative mode, a significant increase in the viscosity of DNA solution is observed,

because the DNA base pairs are separated in order to accommodate the binding complex

resulting in the lengthening of the DNA helix and subsequently increased DNA viscosity. In

contrast, when a complex binds to DNA by a non-classical intercalative mode, the viscosity of

DNA solution is decreased. In addition, some complexes, which interact with DNA by an

electrostatic binding mode, have no change in the DNA viscosity30. The effects of complexes

together with the viscosity of rod-like DNA are shown in Figure 7. On increasing the amounts of

complexes, the relative viscosity of DNA increases steadily. The viscosity measurement shows

clearly that the synthesized metal complexes intercalates between the adjacent DNA base pairs,

origining a lengthening in the helix and thus increases the viscosity of DNA.

Figure 7 Viscosity measurements of the Cu(II), Ni(II) and Zn(II) complexes.

3.2.4 Circular dichoric spectral studies

Circular dichroic is a very sensitive, powerful analytical technique for diagnosing

changes in DNA morphology and drug-DNA interactions, as the positive band due to base

stacking (275 nm) and the negative band due to right-handed helicity (245 nm) are quite

sensitive to the mode of the DNA interaction with small molecules31. As shown in Figure 8, in

the presence of the complex, both the positive and negative peak intensities of the CD spectra of

DNA were decreased without any shift. This suggests that the DNA-binding of the complex

induces certain conformational changes, such as the conversion from a more B-like to a more Z-



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like structure within the DNA molecule. These observations clearly indicate that the binding

mode of the complexes should be intercalative, the stacking of the complex molecules between

the DNA base pairs leads to an enhancement in the positive band and the partial unwinding of

the helix decreases intensity of the negative band. So, the main interactions of the complexes

with DNA can be ascribed to the intercalative mode.

Figure 8 CD spectra recorded over the wavelength range 220-320 nm for solutions containing

2:1 ratio of CT-DNA (200 μM) and mononuclear Cu(II), Ni(II) and Zn(II) complexes (100 μM).

Conclusion:

The ligand (L) and their [CuL], [NiL] and [ZnL] complexes were synthesized and

characterized by elemental analysis, FT-IR, UV-Vis and Mass spectra. The binding properties of

the complexes [CuL], [NiL] and [ZnL] with CT - DNA were investigated by spectroscopic

titrations and viscosity measurements. The intrinsic binding constant Kb is 9.96 x 104 M-1, for

Ni(II) complex 7.82 x 104 M-1 and for Zn (II) complex is 6.71 x 104 M-1 complexes respectively

obtained from absorption spectroscopic titrations and in the emission spectroscopy the Kapp

values for Cu(II), Ni(II) and Zn(II) complexes are found to be 9.81 x 105 M-1, 7.92 x 105 M-1

and 7.26 x 105 M-1 respectively. Among the investigated complexes, the one containing copper

as the central metal ion showed better binding affinity than the other two complexes containing

zinc and nickel ions as metal counterparts respectively.



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Abbreviations

DNA - Deoxyribose nucleic acid

CT - Calf Thymus DNA

Cu - Copper

Ni - Nickel

Zn - Zinc

Tris-HCl - Tris(hydroxymethyl)aminomethane

EB - Ethidium Bromide

CD - Circular Dichroism

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Supplementary Figure captions.

Figure S1. FT-IR spectrum of Ni(II) complex.

Figure S2. FT-IR spectrum of Zn(II) complex.

Figure S3. Mass spectrum of Ni(II) complex.

Figure S4. Mass spectrum of Zn(II) complex.

Figure S5. Absorption spectra of complexes Ni(II), (1 x 10-5 M ) in the absence and presence of

increasing amounts of CT-DNA (0-2.5 x 10-3 M ) at room temperature in 50 mM Tris-HCl /

NaCl buffer (pH = 7.5).

Figure S6. Absorption spectra of complexes Zn(II), (1 x 10-5 M ) in the absence and presence of

increasing amounts of CT-DNA (0-2.5 x 10-3 M ) at room temperature in 50 mM Tris-HCl /

NaCl buffer (pH = 7.5).

Figure S7. Emission spectrum of EB bound to DNA in the presence of Ni(II); ([EB] = 3.3 μM,

[DNA] = 40 μM, [complex] = 0-25 μM, λex= 440 nm).

Figure S8. Emission spectrum of EB bound to DNA in the presence of Zn(II); ([EB] = 3.3 μM,

[DNA] = 40 μM, [complex] = 0-25 μM, λex= 440 nm).



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Figure S1: FT-IR spectrum of Ni(II) complex.



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Figure S2: FT-IR spectrum of Zn(II) complex.

Figure S3: Mass spectrum of Ni(II) complex.



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Figure S4: Mass spectrum of Zn(II) complex.

Figure S5. The absorption spectra of complex Ni(II)(1 x 10-5 M ) in the absence and presence of

increasing amounts of CT-DNA (0-2.5 x 10-3 M ) at normal temperature in 50 mM Tris-HCl / NaCl

buffer (pH = 7.5).

Figure S6 The absorption spectra of complex Zn(II)(1 x 10-5 M ) in the absence and presence of

increasing amounts of CT-DNA (0-2.5 x 103 M) at normal temperature in 50 mM Tris-

HCl / NaCl buffer (pH = 7.5).



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Figure S7 Emission spectrum of EB bound to DNA in the presence of Ni(II):([EB] =3.3 μM,[DNA]

= 40 μM, [complex] = 0-25 μM, λex= 440 nm).

Figure S8 Emission spectrum of EB bound to DNA in the presence of Zn(II):([EB] =3.3

μM,[DNA] = 40 μM, [complex] = 0-25 μM, λex= 440 nm).



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