Zn(II)ferrocenylthiosemicarbazones: DNA binding and nuclease activity

Article abstract of DOI:10.1080/15533174.2012.680138

A series of Zn(II) ferrocenylthiosemicarbazones complexes derived from thiosemicarbazide and 4-methyl-, 4-ethyl-, and 4-phenyl-3-thiosemicarbazide were evaluated for their DNA binding propensity and chemical nuclease activity. The equilibrium binding constants, K_b, of the complexes for binding with calf thymus DNA (CT DNA) were in the range of 0.68 × 10³ to 2.8 × 10⁴ M^-1. The complexes do not intercalate into the nucleobases of CT DNA, as evident from viscosity measurements. They exhibit efficient nuclease activity in the absence of an activating agent and cleave supercoiled DNA into nicked and linear circular forms of DNA at very low concentrations. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2012.680138

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Zn(II)ferrocenylthiosemicarbazones: DNA Binding and
Nuclease Activity
Vikneswaran Rajamuthy a , Siang Guan Teoh a , Amin Malik Shah Abdul Majid b , Chin Sing
Yeap c , Hoong-Kun Fun c , Chew Hee Ng d & Seik Weng Ng e
a School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia
b School of Pharmaceutical Sciences, Universiti Sains Malaysia, Penang, Malaysia
c X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, Penang, Malaysia
d Faculty of Science, Universiti Tunku Abdul Rahman, Perak, Malaysia
e Department of Chemistry, University of Malaya, Kuala Lumpur, Malaysia
Accepted author version posted online: 09 Oct 2012.Published online: 28 Dec 2012.
To cite this article: Vikneswaran Rajamuthy , Siang Guan Teoh , Amin Malik Shah Abdul Majid , Chin Sing Yeap , Hoong-Kun
Fun , Chew Hee Ng & Seik Weng Ng (2013) Zn(II)ferrocenylthiosemicarbazones: DNA Binding and Nuclease Activity, Synthesis
and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:2, 149-156, DOI: 10.1080/15533174.2012.680138
To link to this article: http://dx.doi.org/10.1080/15533174.2012.680138
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:149–156, 2013
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.680138
Zn(II)ferrocenylthiosemicarbazones: DNA Binding and
Nuclease Activity
Vikneswaran Rajamuthy,1 Siang Guan Teoh,1 Amin Malik Shah Abdul Majid,2
Chin Sing Yeap,3 Hoong-Kun Fun,3 Chew Hee Ng,4 and Seik Weng Ng5
1School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia
2School of Pharmaceutical Sciences, Universiti Sains Malaysia, Penang, Malaysia
3X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, Penang, Malaysia
4Faculty of Science, Universiti Tunku Abdul Rahman, Perak, Malaysia
5Department of Chemistry, University of Malaya, Kuala Lumpur, Malaysia
They are capable of binding to DNA by a multitude of inter-
actions and cleaving DNA by virtue of their intrinsic chemical,
electrochemical, and photochemical reactivities.[4–7] Redox ac-
tive complexes are known to be useful for the oxidative cleavage
of DNA involving nucleobase oxidation and/or the degradation
of sugar by the abstraction of deoxyribose hydrogen atom(s)
while complexes containing strong Lewis acids are suitable for
the hydrolytic cleavage of DNA.[8] The oxidative method of
cleaving DNA has the disadvantage of affecting biomolecules
indiscriminately,[9] which is undesirable in treating diseases and
studying the interactions of DNA with other molecules. For ex-
ample, both carbon-centered and hydroxyl radicals are known to
modify histone proteins, causing either protein-DNA crosslink-
ing[10–12] or the dissociation of protein-DNA assemblies[13] in
addition to cleaving DNA. Furthermore, oxidative damage re-
sults in DNA strand termini which prevent subsequent enzy-
matic manipulation.[14] The hydrolytic cleavage of DNA in par-
ticular is challenging because the phosphoester linkages have
remarkable stabilities and are extraordinarily resistant to hy-
drolysis under uncatalyzed physiological conditions.[15] Among
the various metal ions that have been studied and are undergoing
studies with nucleic acids and nucleobases, Zn(II) is regarded as
one of the best suited metal ion for the development of artificial
metallonucleases. This is because Zn(II) is a strong Lewis acid
and exchange ligands very rapidly, it is of low toxicity and it
is not redox active, catalyzing only the hydrolytic cleavage of
DNA.[16,17] The selection of ligands in preparing Zn(II) complex
is very important as it should favor metal center-DNA interaction
while maintaining the stability of the complex. Ferrocene is suit-
able to be coupled with Zn(II) because it has excellent stability in
biological media and is composed of Fe(II), the only redox active
metal ion found in hydrolytic enzymes.[16] Recent studies show
that ferrocene moiety plays a significant role in the nuclease
activity of Cu(II) complexes.[18–20] Here in this study, we inves-
tigate the DNA binding and nuclease activity of zinc complexes
made with ferrocenylthiosemicarbazone ligands (Scheme 1).
A series of Zn(II) ferrocenylthiosemicarbazones complexes de-
rived from thiosemicarbazide and 4-methyl-, 4-ethyl-, and 4-
phenyl-3-thiosemicarbazide were evaluated for their DNA binding
propensity and chemical nuclease activity. The equilibrium bind-
ing constants, Kb, of the complexes for binding with calf thymus
DNA (CT DNA) were in the range of 0.68 × 103 to 2.8 × 104 M−1
.
The complexes do not intercalate into the nucleobases of CT DNA,
as evident from viscosity measurements. They exhibit efficient nu-
clease activity in the absence of an activating agent and cleave
supercoiled DNA into nicked and linear circular forms of DNA at
very low concentrations.
Keywords crystal structure, DNA binding, ferrocene, nuclease ac-
tivity, Zn(II) complex
INTRODUCTION
An important criteria for the development of metallodrugs
as chemotherapeutic agents are the ability of the metallodrug to
bring about DNA cleavage.[1] In general, anticancer agents that
are approved for clinical use are molecules which damage DNA,
block DNA synthesis indirectly through the inhibition of nucleic
acid precursor biosynthesis, or disrupt the hormonal stimulation
of cell growth.[2] The cleavage of DNA can be achieved by tar-
geting its basic constituents such as base and/or sugar by an
oxidative pathway or by the hydrolysis of phosphoester link-
ages.[3] Transition-metal complexes, which are characterized by
high stability, structural versatility, and unique spectroscopic
and redox properties, are exploited in many of these efforts.
Received 2 September 2011; accepted 28 December 2011.
The authors would like to thank Malaysian Ministry of Higher
Education for financial support.
Address correspondence to Siang Guan Teoh, School of Chemical
Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia. E-mail:
sgteoh@usm.my
149

150
V. RAJAMUTHY ET AL.
HFTSC
HFMTSC
HFETSC
HFPTSC
SCH. 1. The ferrocenylthiosemicarbazone ligands.
EXPERIMENTAL
and filtered. After several days, brown crystals were obtained
from the filtrate.
Materials and Measurements
Zn{(η5-C5H5)Fe(η5-C5H4)C(H) NN C(S)NH2}2 (1)
Yield: 79%. Anal. Calcd. for C24H24N6S2Fe2Zn (%): C, 45.20;
H, 3.79; N, 13.18. Found: C, 45.08; H, 3.48; N, 12.79. IR data
(KBr pellet, cm–1): 3453, 3344, ν(N–H); 1596, ν(C N); 834,
ν(C−S).
All the reagents and chemicals were obtained from com-
mercial sources (Acros Chemicals, Aldrich) and used as such.
Supercoiled (SC) pBR 322 DNA and loading dye were pur-
chased from Fermentas. Tris(hydroxylmethyl)aminomethane-
HCl (Tris-HCl) buffer solution was prepared using Barnstead
Nanopure ultrapure water (18.2 Mꢀ.cm). Calf thymus (CT)
DNA, agarose (molecular biology grade), and ethidium bromide
(EB) were from Sigma (St. Louis, MO, USA). The ferrocenylth-
iosemicarbazone ligands were prepared according to the liter-
ature procedure.[21,22] The elemental analysis was carried out
using Perkin-Elmer 2400 series-11 CHN/O analyzer (Waltham,
MA, USA). The infrared, electronic, and fluorescent spectral
were recorded on Perkin-Elmer 2000, Perkin Elmer-Lambda
35, and Jasco FP-750 spectrophotometers, respectively. Cyclic
voltammetric measurements were done at room temperature
on a BAS-Epsilon electrochemical system using a three elec-
trode setup (West Lafayette, IN, USA) comprising a platinum
disc working, platinum wire auxiliary and a Ag/AgCl reference
electrode. Ferrocene (E1/2 = 0.46 V) was used as a standard in
MeCN/0.1 M TBAPF6.
Zn{(η5-C5H5)Fe(η5-C5H4)C(H) NN C(S)NHCH3}2
CH3OH (2)
·
Yield: 83%. Anal. Calcd. for C27H32Fe2N6OS2Zn (%): C, 46.48;
H, 4.62; N, 12.04. Found: C, 46.10; H, 4.58; N, 12.36. IR data
(KBr pellet, cm–1): 3368, ν(N–H); 1601, ν(C N); 835, ν(C−S).
Zn{(η5-C5H5)Fe(η5-C5H4)C(H) NN C(S)NHC2H5}2 (3)
Yield: 81%. Anal. Calcd. for C28H32 N6 S2 Fe2 Zn (%): C,
48.47; H, 4.65; N, 12.11. Found: C, 48.12; H, 4.29; N, 11.71.
IR data (KBr pellet, cm–1): 3339, ν(N–H); 1601, ν(C N); 820,
ν(C−S).
Zn{(η5-C5H5)Fe(η5-C5H4)C(H) NN C(S)NHC6H5}2 · H2O
(4)
Yield: 77%. Anal .Calcd. for C36H34Fe2N6OS2Zn (%): C, 53.52;
H, 4.24; N, 10.40. Found: C, 53.51; H, 4.41; N, 10.01. IR data
(KBr pellet, cm–1): 3319, ν(N–H); 1601, ν(C N); 828, ν(C−S).
Preparation of Complexes
X-Ray Crystallographic Procedure
Complexes 1–4 were prepared by following a general syn-
The determination of the cell constant and data collection
thetic procedure in which Zn(CH3COO–)2 · 2H2O (1 mmol) were carried out at 100.0(1) K using the Oxford Cryosys-
dissolved in methanol was added drop wise at room temper- tem Cobra low-temperature attachment (Long Hanborough
ature to a mixture of appropriate ferrocenylthiosemicarbazone Oxford, United Kingdom) with Mo-Kα radiation (λ = 0.71073)
(1 mmol) and KOH (2 mmol) in absolute methanol (15 mL). on a Bruker SMART APEXII CCD area-detector diffrac-
The resulting orange suspension was stirred under reflux for 4 h tometer (Billerica, MA, USA) equipped with a graphite

ZN(II)FERROCENYLTHIOSEMICARBAZONES
151
TABLE 1
was restrained to 1.50 (1) Å. Table 1 summarizes the crystal
data, data collection, and refinement parameters for 3.
Crystal data and structure refinement of 3
Empirical formula
C28H32Fe2N6S2Zn
DNA Binding Method
The experiments were carried out in Tris-HCl buffer (5 mM
Tris-HCl, pH 7.1) using the complex solution in 10% DMF. The
CT DNA in the buffer medium gave a ratio of UV absorbance
at 260 and 280 nm of ca. 1.9:1 suggesting that the DNA was
apparently free from protein.[25] The concentration of the DNA
was estimated from its absorption intensity at 260 nm using its
known molar absorption coefficient value of 6600 M−1 cm.[26]
Absorption and emission titration experiments were carried out
by varying the concentration of the CT DNA while keeping
the complex concentration constant. The intrinsic equilibrium
binding constant (Kb) of the complexes to CT DNA were deter-
mined from the plot of [DNA]/(εa − εf) versus [DNA] where
[DNA] is the concentration of DNA in base pairs and the appar-
ent absorption coefficients, εa, εf, εb corresponded to Aobs/[Zn],
the extinction coefficient for the free zinc complex, and the
extinction coefficient of the zinc complex in the totally bound
form, respectively. The data were fitted to Eq. 1 with a slope that
equaled 1/(εb − εf) and the intercept equaled 1/[Kb(εb − εf)] and
Kb was obtained from the ratio of the slope to the intercept.[27]
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
693.79
100.0(1) K
0.71073 Å
Monoclinic
P21
a = 9.3383(1) Å
α = 90◦
b = 21.3792(3) Å
β = 90.158(1)◦
c = 15.3796(2) Å
γ = 90◦
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
3070.45(7) Å3
4
1.501g/cm3
1.871mm−1
1424
0.40 × 0.21 × 0.18 mm
1.32–25.00◦
−8 ≤ h ≤ 11, −25 ≤ k
≤ 23, −17 ≤ l ≤ 18
21524
[DNA]/(εa −εf ) = [DNA]/(εb −εf )+1/[Kb(εb −εf )] [1]
Reflections collected
Independent reflections
Completeness to theta = 25.00◦
Absorption correction
Max. and min. transmission
Refinement method
9416 [R (int) = 0.032]
Viscosity measurements were made using a Cannon Man-
ning Semi-Micro viscometer (State College, PA, USA). The
viscometer was thermostated at 30◦C in a constant temperature
bath. The concentration of CT DNA was 40 μM in NP and the
99.5%
Multiscan
0.7342 and 0.5186
Full-matrix least-squares
on F2
TABLE 2
Data/restraints/parameters
Goodness-of-fit onF2
3851/435/746
1.06
Selected bond distances (Å) and angles (◦)
Final R indices [I > 2sigma (I)]
R1 = 0.0412,
wR2 = 0.0837
R1 = 0.0649,
wR2 = 0.0977
0.527 and −0.245 e Å−3
Zn1A—N4A
Zn1A—N1A
Zn1A—S1A
Zn1A—S2A
Zn1B—N1B
Zn1B—N4B
Zn1B—S2B
Zn1B—S1B
N4A—Zn1A—N1A
N4A—Zn1A—S1A
N1A—Zn1A—S1A
N4A—Zn1A—S2A
N1A—Zn1A—S2A
S1A—Zn1A—S2A
N1B—Zn1B—N4B
N1B—Zn1B—S2B
N4B—Zn1B—S2B
N1B—Zn1B—S1B
N4B—Zn1B—S1B
S2B—Zn1B—S1B
2.060 (7)
2.068 (8)
2.287 (3)
2.294 (3)
2.028 (8)
2.067 (6)
2.271 (3)
2.293 (3)
R indices (all data)
Largest diff. peak and hole
monochromator.[23] The data were reduced using SAINT.[23]
A semiempirical absorption correction was applied to the data
using SADABS.[23] The structure was solved by direct meth-
ods and refined against F2 by full-matrix least-squares using
SHELXTL.[24] Non-hydrogen atoms were anisotropically re-
fined and all the hydrogen atoms were positioned geometrically
and refined using a riding model with isotropic temperature fac-
tors fixed at 1.2 times Ueq of the parent atoms (1.5 times for ethyl
groups). The crystal study is a pseudomerohedral twin with the
twin matrix (-1 0 0 0 -1 0 0 0 1) and the refined ratio of the
twin components was 0.371 (1)/0.729 (1). All the carbon and
nitrogen atoms were restrained so that their Uij components ap-
proximated isotropic behavior. The C27A–C28A bond distance
108.4 (3)
125.2 (2)
84.7 (2)
85.3 (2)
124.5 (2)
131.09 (12)
107.2 (3)
123.0 (2)
86.3 (3)
85.9 (2)
122.2 (2)
133.26 (12)

152
V. RAJAMUTHY ET AL.
FIG. 1. The asymmetric unit of 3 with 10% probability ellipsoids for non-H atoms. Open bonds show minor disorder component (color figure available online).
flow times were measured manually with a digital stopwatch.
The viscosity values were calculated from the observed flowing
time of DNA-containing solutions (t) corrected for that of the
solvent mixture used (t0), η = (t − t0)/t0.
DNA Cleavage Experiments
The cleavage of supercoiled pBR322 DNA (0.5 μg/μL) was
studied by agarose gel electrophoresis using metal complexes
in 10% DMF/5 mM Tris-HCl/50 mM NaCl buffer at pH 7.1. All
FIG. 2. The crystal packing of 3, viewed down the a-axis showing a 2-D plane parallel to bc plane. H atoms not involved in hydrogen-bonding (dashed lines)
have been omitted for clarity (color figure available online).

ZN(II)FERROCENYLTHIOSEMICARBAZONES
153
TABLE 3
Hydrogen-bonding geometry (Å, ◦)
Distance Distance Distance Angle
(Å)
D-H
(Å)
H···A
(Å)
D···A D-H···A
(◦)
D-H···A
C17—H17A···S2Aa
1.00
1.00
2.86 3.791(15) 155
2.61 3.436(12) 140
C7B—H7BA···N5A
C28A—H28B···N4Bb 0.98
2.51
3.48(3)
170
a2 − x, 1/2 + y, 1 − z.
bx, y, −1 + z.
the samples were incubated at 37◦C under argon atmosphere for
2 h in the dark. After incubation, the sample was added with a
loading dye and the solution was loaded on 1% agarose gel with
a running time of 1 h at a constant voltage of 50 V. After the
electrophoresis step, the resultant DNA bands were stained with
ethidium bromide before being photographed under UV light.
FIG. 3. Cyclic voltammogram of 2 (—), 3 (—), and 4 (—) (color figure
available online).
RESULTS AND DISCUSSION
The binary Zn(II) ferrocenylthiosemicarbazone complexes
(1–4) were isolated by the addition of a methanolic solution of
hydrated Zn(II) acetate to a mixture of KOH and Schiff base
in methanol. The crystal structure descriptions of 1, 2, and 4
have been reported elsewhere and that of 3 is determined in this
study.[28–30]
C26A—N6A—C27A—C28A = 80(2)◦, C12B—N3B—C13B
—C14B = 96.0(13)◦ and C26B—N6B—C27B—C28B =
86(2)◦. Each of the thiosemicarbazone ligands coordinate al-
most perpendicularly to the zinc atom with dihedral angles
between the mean plane of Zn1A—S1A—C12A—N2A—
N1A/Zn1A—S2A—C26A—N5A—N4A and Zn1B—S1B—
C12B—N2B—N1B/Zn1B—S2B—C26B—N5B—N4B, equa-
ling 84.8(3) and 86.2(3)◦, respectively. The intermolecu-
lar C17A—H17A···S2A hydrogen bonds (Table 3) link the
molecules into infinite one-dimensional chains along the
b-axis. The molecules are also linked into infinite one-
dimensional chains along the c-axis via the intermolecular
C7B—H7BA···N5A and C28A—H28B···N4B hydrogen bonds
(Table 3). All these hydrogen bonds consolidate the crystal struc-
ture into supramolecular two-dimensional planes that are paral-
lel to the bc plane (Figure 2).
Structure Description of Zn{(␩5-C5H5)Fe(␩5-C5H4)C(H)
NN C(S)NHC2H5}2 (3)
The asymmetric unit of complex 3 consists of two crys-
tallographic independent molecules with similar conforma-
tion (Figure 1) and the zinc coordinate environment is com-
parable to its related structure (Table 2).[30] The Cp (cy-
clopentadiene) rings of each ferrocene residue are parallel,
with dihedral angles of Cp1/Cp2 [C1A—C5A/C6A—C10A]
= 2.2(8)◦, Cp3/Cp4 [C15A—C19A/C20A—C24A] = 0.6(8)◦,
Cp5/Cp6 [C1B—C5B/C6B—C10B] = 3.5(7)◦ and Cp7/Cp8
[C15B—C19B/C20B—C24B] = 2.7(7)◦. The Cp rings adopt
closely to eclipse conformation [average torsion angles for
C—Cg—Cg—C being 15.05, 0.98, –6.64, and 3.52◦]. In
both of the ligands, the thiosemicarbazone chain and the
substituted ethyl group are practically perpendicular with
the torsion angles C12A—N3A—C13A—C14A = 81.7(14)◦,
Electrochemistry
The cyclic voltammetric behavior of 2, 3, and 4 were carried
out in MeCN-0.1 M TBAPF6 (Figure 3). All the complexes
showed redox activity (Table 4). The Fe(III)−Fe(II) couple of
the ferrocenyl moiety was observed as a reversible response. The
TABLE 5
Equilibrium binding constant (Kb)
TABLE 4
Electrochemical data (V)
Complex
Kb/M−1
Complex
E1/2 (ꢁEp)
1
2
3
4
2.3 × 103
0.68 × 103
3.6 × 103
2.8 × 104
2
3
4
0.551 (94)
0.572 (71)
0.580 (81)

154
V. RAJAMUTHY ET AL.
FIG. 4. Electronic spectra of 1 (10 μM) in the presence of increasing amounts of CT DNA. [DNA] = 0–90μM. Arrow shows the absorbance changes upon
increasing DNA concentration. Inset: plot [DNA]/(εa-εf) versus [DNA] (color figure available online).
conjugation of the {C(H)NN(H)C(S)NHR}Zn unit to ferrocenyl DNA Binding Properties
moiety caused a ca. 100 mV positive shift of the Fe(III)−Fe(II)
UV-visible absorption spectral measurements were per-
formed to determine the equilibrium binding constant (Kb) of
potential from 0.46 V of the ferrocene ([(η5-C5H5)2Fe]). The
shift toward more positive values suggests that the inductive
effect of zinc, when bound to the thiosemicarbazone chain, was
transferred to the iron atom thus reducing the proclivity of Fe(II)
to oxidize.
the complexes to CT DNA by monitoring the change in the ab-
sorption intensity of the ligand-centered band (Figure 4). The Kb
values of the complexes 1–4 are in the range of 0.68 × 103 to 2.8
× 104 M−1, giving an order of 4 > 3 > 1 > 2 (Table 5). The val-
ues are comparable with some reported nonintercalators[31,32]
and much lower than those observed for typical classical in-
tercalators (EthBr, Kb, 7.0 × 107 M−1 [33]
)
and partially inter-
calating metal complexes ([Ru(bipy)2(dppz)]2+ where dppz is
dipyrido-[3,2-d:2ꢀ,3ꢀ-f]-phenazine, Kb > 106 M–1) bound to CT
DNA.[34] Fluorescent titration showed emission enhancement
FIG. 5. Magnified view on the fluorescent spectra of 4 (10 μM) in the presence
of increasing amounts of CT DNA, [DNA] = 0–60μM. Arrow shows the emis-
sion enhancement upon increasing DNA concentration (color figure available
online).
FIG. 6. Effect of complexes [1(◦), 2 (ꢀ), 3(ꢁ), 4(ꢁ)] on the viscosity of CT
DNA. Relative specific viscosity versus [complex]/[DNA].

ZN(II)FERROCENYLTHIOSEMICARBAZONES
155
FIG. 7. Cleavage of pBR322 supercoiled plasmid DNA (0.5 μg/μL) by the zinc(II) complexes (100 μM) in 10% DMF/5 mM Tris-HCl/50 mM NaCl buffer at
pH 7.1 and 37◦C with an incubation time of 2 h under argon atmosphere in dark. Lane 1 = DNA; lane 2 = DNA + Zn(CH3COO−)2; lane 3 = DNA + ferrocene;
lane 4 = DNA + 1; lane 5 = DNA + HFTSC; lane 6 = DNA + 2; lane 7 = DNA + HFMTSC; lane 8 = DNA + 3; lane 9 = DNA + HFETSC; lane 10 = DNA
+ 4; lane 11 = DNA + HFPTSC. Forms I, II, and III are supercoiled and nicked and linear circular forms of DNA, respectively.
(Figure 5), which implies that the interaction between DNA Cambridge CB21EZ, UK; fax: +44-1223-336033; or de-
and the complexes occur due to the hydrophobicity of both the posit@ccdc.cam.ac.uk).
molecules.[35]
To understand the nature of the DNA binding of the com-
plexes, viscosity measurements were carried out on CT DNA
by varying the concentration of the added complexes. The val-
ues of the relative specific viscosity (η/η0), where η and η0
are the specific viscosities of DNA in the presence and ab-
sence of the complex are plotted against [complex]/[DNA] for
1–4 (Figure 6). The viscosity decreases with an increase in the
[complex]/[DNA] ratio indicating that these complexes do not
intercalate within the base pairs as expected because of their
nonplanar nature. So, surface binding has led to the formation
of kinks or bends in the DNA chain.[36]
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Chemical Nuclease Activity
To assess the DNA cleavage ability of the complexes, SC
pBR322 DNA (0.5 μg/μL) was incubated with 1–4 (100 μM)
in 10% DMF/5 mM Tris-HCl/50 mM NaCl buffer at pH 7.1
for 2 h under argon atmosphere without the addition of an ac-
tivator in the dark. Upon the gel electrophoresis of the reaction
mixture, the Zn(II) complexes displayed efficient cleavage ac-
tivities and cleaved the double-stranded SC DNA (form I) into
the nicked circular form (form II) and linear circular form (form
III; Figure 7). The precursor species, Zn(CH3COO–)2, ligands,
and ferrocene alone did not show any cleavage of DNA under
similar experimental conditions. At this stage, it was presumed
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DNA cleavage occurred under an argon atmosphere for all of
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CONCLUSION
We have presented here a series of the complexes designed
to have redox active ferrocene conjugated to strong Lewis acid
Zn(II) through bioactive thiosemicarbazone chelant showing ef-
ficient chemical nuclease activity and found to interact in a
nonintercalative manner with CT DNA.
SUPPLEMENTARY MATERIALS
CCDC-784760 contains the supplementary crystallographic
data for 3. This data can be obtained free of charge
via www. ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Center, 12, Union Road,

156
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