Chelating behavior and biocidal efficiency of tryptophan based mixed-ligand complexes

Article abstract of DOI:10.1016/j.inoche.2012.03.016

A new family of tryptophan based mixed-ligand complexes has been synthesized and characterized. DNA (Calf thymus) binding properties of the complexes have been explored by UV-vis, viscosity measurements and cyclic voltammetry. All the experimental evidence suggest that the ancillary ligand 2,2′-bipyridine influences the intercalative binding of these complexes to CT DNA. The DNA cleavage efficiencies of these complexes with pBR322 DNA were investigated by gel electrophoresis. The complexes were found to promote the cleavage of pBR322 DNA from the supercoiled form I to the open circular form II in the presence of an oxidizing agent (H₂O₂). Microbial property of these complexes as antibacterial agents has been investigated against Gram-negative and Gram-positive bacteria.

Full text of DOI:10.1016/j.inoche.2012.03.016

Inorganic Chemistry Communications 20 (2012) 238–242
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Chelating behavior and biocidal efficiency of tryptophan based
mixed-ligand complexes
⁎
Muthusamy Selvaganapathy, Natarajan Raman
Research Department of Chemistry, VHNSN College, Virudhunagar-626 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of tryptophan based mixed-ligand complexes has been synthesized and characterized. DNA
(Calf thymus) binding properties of the complexes have been explored by UV–vis, viscosity measurements
and cyclic voltammetry. All the experimental evidence suggest that the ancillary ligand 2,2′-bipyridine
influences the intercalative binding of these complexes to CT DNA. The DNA cleavage efficiencies of these
complexes with pBR322 DNA were investigated by gel electrophoresis. The complexes were found to
promote the cleavage of pBR322 DNA from the supercoiled form I to the open circular form II in the presence
of an oxidizing agent (H2O2). Microbial property of these complexes as antibacterial agents has been inves-
tigated against Gram-negative and Gram-positive bacteria.
Received 13 February 2012
Accepted 8 March 2012
Available online 17 March 2012
Keywords:
Tryptophan-based mixed-ligand complexes
DNA binding
DNA cleavage
© 2012 Elsevier B.V. All rights reserved.
Microbial property
Many of chemotherapeutic agents currently used in cancer thera-
py are agents which inhibit tumor growth by inhibiting the replica-
tion and transcription of DNA. A number of metal chelates are of
current interest due to their important applications in nucleic acid
chemistry as DNA probes of DNA structure in solutions, reagents for
the mediation of strand scission of duplex DNA under physicochemi-
cal conditions, and as chemotherapeutic agents and in the genomic
research [1]. Complexes bearing amino acid functionality enhance
the selective effect of DNA binding as well as for other therapeutic
targets, providing a key recognition element of artificial nuclease
activity to the molecule [2].
[12–14]. As pathogenic bacteria continuously evolve resistance to
currently use antibacterial agents, the discovery of novel and potent
antibacterial agents is the best way to overcome bacterial resistance
and develop effective therapies [15].
As part of a research program aim at the design of chemical nucle-
ases, we describe herein the synthesis, structure, anti-microbial and
cleavage studies of tryptophan derived Schiff base and its mixed
ligand transition metal complexes. However, a survey of the literature
reveals that no work has been carried out on the above mentioned
mixed ligand complexes of tryptophan derivatives containing 2,2′-
bipyridine and various metal ions.
Amino acids play central roles both as building blocks of proteins
and as intermediates in metabolism. Tryptophan is an essential
amino acid for human nutrition, which is the precursor for the syn-
thesis of serotonin, an important neuromediator associated to
mood, stress response, sleep and appetite regulation. Various transi-
tion metal complexes derived from 2,2′-bipyridine [3,4] and amino
acid derivatives [5] have been reported as good candidates for DNA
structural probes, DNA cleavage studies, DNA molecular light
switches and new therapeutics.
At present, the interaction of transition metal complexes with
DNA has been a pet subject of researchers in the field of bioinorganic
chemistry [6–8]. Metal complexes binding with nucleic acid are cur-
rently investigated because of their utility as DNA structural probes,
DNA foot printing and sequence-specific cleavage agent and potential
anticancer drug [9–11]. Recent reports have also shown that amino
acid based metal(II) complexes show efficient DNA cleavage activity
The electron transfer mechanism of the mixed ligand metal com-
plexes is investigated by the aid of cyclic voltammetry. We hope that
our results should be of value in designing of new metallonucleases.
The Schiff base ligand and its mixed ligand complexes were pre-
pared by a typical procedure [16–22] (Fig. 1). We got the likely com-
position of complexes: [ML(bpy)2]Cl where bpy=2,2′-bipyridine
through elemental analyses, magnetic susceptibility, NMR, UV, IR,
Mass spectra and molar conductivity measurements since no single
crystals suitable for X-ray determination could be isolated (Fig. 2).
Absorption studies were performed to further ascertain the pre-
dicted binding trend of Schiff base complexes with CT DNA. The ab-
sorption spectra of [CuL(bpy)2]Cl in the absence and presence of
DNA are given in Fig. 3.
If the binding mode is intercalation, the orbital of intercalated li-
gand can couple with the orbital of the base pairs, reducing the
π–π* transition energy and resulting in bathochromism. If the cou-
pling orbital is partially filled by electrons, it results in decreasing
the transition probabilities and resulting in hypochromism [23].
Hypochromism means the metal complexes binding with calf thymus
DNA resulting in its breakage and perturbation. The extent of the
⁎
Corresponding author. Tel.: +91 092451 65958; fax: +91 4562 281338.
E-mail address: drn_raman@yahoo.co.in (N. Raman).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.03.016

M. Selvaganapathy, N. Raman / Inorganic Chemistry Communications 20 (2012) 238–242
239
Fig. 1. Synthesis of Schiff base.
hypochromism in the metal-to-ligand charge transfer (MLCT) band is
commonly consistent with the strength of intercalative interaction
[24]. The absorption spectrum of [CuL(bpy)2]Cl complex showed an
intensive absorption band at 383.6 nm, [CoL(bpy)2]Cl at 343 nm,
[NiL(bpy)2]Cl complex at 310.8 nm and [ZnL(bpy)2]Cl at 351.5 nm re-
spectively in 5 mM Tris–HCl and 50 mM NaCl buffer solution (pH 7.2).
These are due to intra ligand π–π* transitions. On increasing the con-
centration of CT DNA resulted in the slight bathochromic shift in the
range 3.1–1.3 nm and significant hypochromicity lying in the range
the DNA bases to the increase in the effective size in DNA which
could be the reason for the increase in the viscosity. Plot of (η/ηo)1/3
versus [complex]/[DNA] gives a measure of the viscosity changes
(Fig. 4).
A plodding increase in the relative viscosity was observed on addi-
tion of the metal complexes to DNA solution suggesting mainly inter-
calation mode of binding nature of the complexes.
The application of the cyclic voltammetric technique to study the
interaction between metal complexes and DNA provides a useful
complement to the previously used spectral studies. The cyclic vol-
tammogram of copper complex in buffer pH=7.2 at 25 °C is given
Fig. 5.
41–24%. Intrinsic binding constants of 3.1×106 M−1, 2.2×106 M−1
,
1.9×106 M−1 and 1.3×106 M−1 were determined for [CuL(bpy)2]
Cl, [CoL(bpy)2]Cl, [NiL(bpy)2]Cl and [ZnL(bpy)2]Cl respectively.
These values (Table 1) are comparable to the classical DNA intercala-
tors EB (ethidium bromide, 1.4×106 M−1 [25]). All the metal com-
plexes showed decrease in absorption intensity (hypochromism)
with a slight red shift which is due to the intercalative binding
between DNA and metal complexes.
The cyclic voltammogram of the [CuL(bpy)2]Cl in the absence of CT-
DNA featured three anodic peaks (EPa1 =−0.060 V, EPa2=0.008 V,
EPa3=0.277 V) and three cathodic peaks (EPc1 =−0.617 V, EPc2=
−0.415 V, EPc3=−0.175 V). The oxidation of peak EPa1 belongs to
Cu(0)/Cu(I) and reduction of Cu(I) occurred, upon scan reversal at
−0.617 V. The separation of the anodic and cathodic peak potential is
0.557 V, the ratio of anodic to cathodic peak currents, ipa/ipc=1.14, in-
dicating a quasi-reversible redox process. The oxidation of peak EPa2
belongs to Cu(I)/Cu(II) and reduction of Cu(II) occurred, upon scan re-
versal at −0.415 V. The separation of the anodic and cathodic peak po-
tential is 0.423 V, the ratio of anodic to cathodic peak currents, ipa/
ipc=1.41, indicating a quasi-reversible redox process. The oxidation
Viscosity measurement is sensitive to the changes in the length of
DNA molecule. It is regarded as the least ambiguous and the most
critical means studying the binding mode of metal complexes with
DNA in solution, and provides stronger arguments for intercalative-
binding mode [26,27]. The viscosity measurement was introduced
to furthermore support this interaction between the complex and
DNA. Intercalation was the effect of increasing DNA viscosity. The sig-
nificant increase in the viscosity of DNA on addition of complex re-
sults due to the intercalation which leads to the separation among
Fig. 3. Absorption spectral changes on addition of CT DNA to the solution of [NiL(bpy)2]
Cl in buffer pH=7.2 at 25 °C in the presence of increasing amount of DNA. Arrow indi-
cates the changes in absorbance upon increasing the DNA concentration.
Fig. 2. The proposed structure of the metal complexes, M_Cu(II), Ni(II), Co(II) and
Zn(II).

240
M. Selvaganapathy, N. Raman / Inorganic Chemistry Communications 20 (2012) 238–242
Table 1
Electronic absorption spectral properties of Cu(II), Co(II), Ni(II) and Zn(II) complexes.
Complex
λ max
Δλ
H%
Kb ×106
Free
Bound
(nm)
(M−1
)
[CuL(bpy)2]Cl
[CoL(bpy)2]Cl
[NiL(bpy)2]Cl
[ZnL(bpy)2]Cl
383.6
343.0
310.8
351.5
385.2
345.5
313.1
353.4
1.6
2.5
2.3
1.9
41
29
32
24
3.1
2.2
1.9
1.3
of peak EPa3 belongs to Cu(II)/Cu(III) and reduction of Cu(III) occurred,
upon scan reversal at −0.175 V. The separation of the anodic and ca-
thodic peak potential is 0.452 V, the ratio of anodic to cathodic peak
currents, ipa/ipc=1.85, indicating a quasi-reversible redox process.
The formal potential E1/2 taken as average of EPa and EPc is −0.338 V,
−0.204 V and 0.051 V. When CT-DNA is added to a solution of complex
both the anodic and cathodic peak current heights of the complex de-
creased in the same manner of increasing additions of DNA (Fig. 5).
Also during DNA addition the anodic peak potential (Epa), cathodic
peak potential (Epc), and E1/2 (calculated as the average of Epc and
Epa) all showed positive shifts. These positive shifts are considered as
evidence for intercalation of the complex into the DNA, because this
kind of interaction is due to hydrophobic interaction. From the other
point of view, if a molecule binds electrostatically to the negatively
charged deoxyribose-phosphate backbone of DNA, negative peak po-
tential shifts should be detected. Therefore, the positive shift in the
CV peak potentials of complex is indicative of intercalative binding
mode of the complex with DNA [28].
Fig. 5. Cyclic voltammogram of [CuL(bpy)2]Cl in the presence of increasing amount of
DNA in buffer pH=7.2 at 25 °C. Arrow indicates the changes in voltammetric currents
upon increasing the DNA concentration.
a positive shift in formal potential (E1/2) indicating that [CoL(bpy)2]
Cl has bonded favorably with DNA via intercalation. Finally Zn(II)
complex exhibits quasi-reversible transfer process with redox cou-
ple [Zn(II)→Zn(0)], cathodic peak appears at −0.091 V in the ab-
sence of DNA (Epa =0.412 V, Epc =−0.091 V, ΔEp=0.503 V and
E
1/2 =0.161 V). The ratio of ipa/ipc is 0.90 V. This also indicates the
The cyclic voltammogram of the [NiL(bpy)2]Cl in the absence of
CT-DNA featured two anodic peaks (EPa1 =0.008 V, EPa2 =0.274 V)
and two cathodic peaks (EPc1 =−0.418 V, EPc2 =−0.182 V). The ox-
idation of peak EPa1 belongs to Ni(I)/Ni(II) and reduction of Ni(II) oc-
curred, upon scan reversal at −0.418 V. The separation peak potential
is 0.426 V, the ratio of ipa/ipc =1.36, indicating a quasi-reversible
redox process. The oxidation peak EPa2 belongs to Ni(II)/Ni(III) and
reduction of Ni(II) occurred, upon scan reversal at −0.182 V. The sep-
aration peak potential is 0.456 V, the ratio of ipa/ipc =1.08, indicating
a quasi-reversible redox process. The formal potential E1/2 taken as
average of EPa and EPc is −0.205 V and 0.046 V.
For Co(III)→Co(II), the redox couple cathodic peak appears at
0.105 in the absence of CT DNA (Epa=0.357 V, Epc=0.118 V,
ΔEp=0.239 V and E1/2 =0.237 V). The ratio of ipc/ipa is approxi-
mately unity. This indicates that the reaction of the complex on
the glassy carbon electrode surface is quasi-reversible redox
process. The incremental addition of CT DNA to the complex causes
quasi-reversible redox process of the metal complex. The incremen-
tal addition of CT DNA to the complex also causes a positive shift in
formal potential (E1/2) indicating that [ZnL(bpy)2]Cl stabilizes the
duplex (GC pairs) by intercalation. The electrochemical parameters
of the Cu(II), Ni(II), Co(II) and Zn(II) complexes are shown in
Table 2. From these data, it is understood that all the synthesized
complexes interact with DNA through intercalating way.
There has been considerable interest in DNA cleavage reactions
that are activated by transition metal complexes [29,30]. Change in
the electrophoretic mobility of plasmid DNA on agarose gel is com-
monly taken as evidence for direct DNA–metal interactions. The
delivery of metal ion to the helix, in locally generating oxygen or
hydroxide radicals, yields an efficient DNA cleavage reaction. Fig. 6 il-
lustrates the gel electrophoretic separations showing the cleavage of
plasmid pBR322 DNA induced by the complexes under aerobic condi-
tions and in presence of H2O2, respectively.
When circular pBR322 DNA is subjected to electrophoresis, rela-
tively fast migration will be observed for the intact supercoiled form
(Form I). If scission occurs on one strand, the supercoiled form will
relax to generate a slower-moving nicked form (Form II). Control ex-
periments using DNA alone (Lane 1) and H2O2 +DNA (Lane 2) do not
show any apparent cleavage of SC DNA. All the complexes showed
pronounced nuclease activity in the presence of an oxidant H2O2
(Lanes 4–7), which may be due to the increased production of hy-
droxyl radicals. The production of hydroxyl free radical is due to the
Table 2
Electrochemical parameters for the interaction of DNA with Cu(II), Co(II), Ni(II) and
Zn(II) complexes.
Complex
Redox couple
E1/2 (V)
Free
ΔEp(V)
Ipa/Ipc
Bound
Free
Bound
[CuL(bpy)2]Cl Cu(III)→Cu(II)
Cu(II)→Cu(I)
0.051
0.056 0.452 0.468
1.85
1.41
1.14
1.08
1.36
1.18
0.90
−0.204 −0.217 0.423 0.398
−0.338 −0.332 0.557 0.554
0.046
−0.205 −0.191 0.426 0.402
0.237
0.161
Cu(I)→Cu(0)
[NiL(bpy)2]Cl
Ni(III)→Ni(II)
Ni(II)→Ni(I)
0.077 0.456 0.467
[CoL(bpy)2]Cl Co(III)→Co(II)
[ZnL(bpy)2]Cl Zn(II)→Zn(0)
0.257 0.239 0.245
0.178 0.503 0.524
Fig. 4. Effect of increasing amounts of [CuL(bpy)2]Cl
( )), [CoL(bpy)2]Cl ( ),
[NiL(bpy)2]Cl] ( ), [ZnL(bpy)2]Cl] ( ) on the viscosity of DNA.

M. Selvaganapathy, N. Raman / Inorganic Chemistry Communications 20 (2012) 238–242
241
1
2
3
4
5
6
7
Form II
Form I
Fig. 6. Changes in the agarose gel electrophoretic pattern of pBR322 DNA induced by H2O2 and Cu(II), Ni(II), Co(II) and Zn(II) complexes: Lane 1, DNA alone; Lane 2, DNA+H2O2;
Lane 3, DNA+[KL]+H2O2;Lane 4, DNA+[CuL(bpy)2]Cl complex+H2O2; Lane 5, DNA+[NiL(bpy)2]Cl complex+H2O2; Lane 6, DNA+[CoL(bpy)2]Cl complex+H2O2; and Lane 7,
DNA+[ZnL(bpy)2]Cl complex+H2O2.
reaction between the metal complex and oxidant. The OH. free radi-
cals participate in the oxidation of the deoxyribose moiety, followed
by hydrolytic cleavage of a sugar phosphate back bone.
with CT-DNA has been investigated by using absorption spectra, elec-
trochemical techniques and viscometry. All the experiments reveal
that the mixed ligand complexes interact with CT-DNA in the mode
of intercalation. The results of agarose gel electrophoresis indicate
that the complexes exhibit good cleavage potential of pBR322 DNA
in the presence of H2O2. Antibacterial activity study shows that the
complexes exhibit good biological activity against different pathogen-
ic species. These results should be valuable in understanding the
interaction with DNA as well as laying a foundation for the rational
design of powerful agents for probing and targeting nucleic acids.
Currently much attention has been focus to the synthesis of new
metal complexes and the evaluation of these agents for antibacterial
activity. In the present work, we wish to test the activity of the ligand
and its metal(II) complexes against bacteria. Ampicillin is used as a
standard drug for comparison. The antibacterial activity of the
synthesized azomethines was screened by the agar well diffusion
method [31,32] against Gram-negative and Gram-positive bacteria,
namely S. aureus and B. subtilis (as Gram positive bacteria) and P.
aeruginosa, E. coli and S. typhi (as Gram negative bacteria). The diffu-
sion agar technique was used to evaluate the antibacterial activity of
the synthesized mixed-ligand complexes. The biocidal activity data of
the investigated compounds are summarized in Table 3.
From this Table, it has been reported that the ligands having oxy-
gen and nitrogen donor atoms upon coordination with metal ion in-
hibit production of enzyme in the micro-organisms. All the metal
complexes have higher inhibitory effect than free ligand. Control
experiment using DMF alone does not show any antimicrobial ex-
pressions. This higher antimicrobial activity of the metal complexes
compared to Schiff bases may be due to the change in structure due
to coordination and chelating effect to make metal complexes to act
as more powerful antibacterial agents, thus killing the microbe or
by inhibiting multiplication of the microbe by blocking their active
sites [33]. The chelation reduces the polarity of the ligand due to
partial sharing of its negative charge with the metal ion, which
increases the lipophilic nature of the complex, favoring transporta-
tion of the complexes across the lipid layer of the cell membrane,
which enhances antibacterial activities.
Acknowledgments
The authors express their sincere thanks to the College Managing
Board, Principal and Head of the Department of Chemistry, VHNSN
College, Virudhunagar, India for providing necessary research facili-
ties. NR thanks University Grants Commission (UGC), New Delhi for
financial assistance.
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Complex
S. aureus
P. aeruginosa
E. coli
B. subtilis
S. typhi
[KL]
8
11
21
16
19
13
16
–
6
10
25
18
20
21
16
–
13
29
23
18
20
19
–
[CuL(bpy)2]Cl
[NiL(bpy)2]Cl
[CoL(bpy)2]Cl
[ZnL(bpy)2]Cl
Ampicillin
DMF
18
13
15
16
12
–
24
20
17
18
14
–
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∧
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itated was filtered off, washed thoroughly with ethanol and dried in vacuo. Yield
75%.
∧
m ×10−3 (Ω−1 mol−1 cm2) 57.91; λmax in DMSO 29,489, 33,109 cm−1; MS:
m/z 665.
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[28] Y. Ni, D. Lin, S. Kokat, Synchronous fluorescence, UV–visible spectrophotometric,
and voltammetric studies of the competitive interaction of bis(1,10-
phenanthroline)copper(II) complex and neutral red with DNA, Anal. Biochem.
352 (2006) 23.
[29] R.P. Hertzberg, P.B. Dervan, Cleavage of double helical DNA by methidium-
propyl-EDTA-iron(II), J. Am. Chem. Soc. 104 (1982) 313.
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tion complexes. superoxide formation in the oxidation of 1,10-phenanthroline-
cuprous complexes by oxygen — relevance to DNA-cleavage reaction, J. Am.
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95 (2000) 367.
[17] Anal.Calc. for [C18H14N2O2K]: C, 65.8; H, 4.5; N, 8.7; O, 9.9. Found: C, 65.6; H, 4.3;
N, 8.5; O, 9.7; FT-IR (KBr disc): 3,252 (NH), 1,667 (HC_N), 1,456 υasy(COO),
1,375 υsy(COO) cm−1 1H NMR (CDCl3) δ: 6.8–7.5 (phenyl multiplet), 8.9 (s,
;
1H, \CH_N), 2.4 (\CH2) ppm; 13C NMR (CDCl3): d=53.90 539 (CH2), 55.40
(CH), 105.62, 108.15, 111.82, 112.37, 117.95 (aromatic C), 118.74, 121.40,
125.11, 126.33, 128.53, 136.62 (indole C), 167.11 (CH_N), 171.56 (COO\)
ppm; λmax in DMSO 42,564 and 36,978 cm−1; MS: m/z 330.
[18] The complexes were prepared by mixing the appropriate molar quantity of the
above ligand and the metal salts using the following procedure. An ethanolic so-
lution of Schiff base (0.003 mol) was stirred with the ethanolic solution (5 mL) of
anhydrous metal(II) chlorides (0.003 mol) for ca.1 h. To the above mixture, a
methanolic solution (5 mL) of 2,2′-bipyridine (bpy) (0.006 mol) was added in a
1:1:2 molar ratio and the stirring was continued for ca.1 h. The solid product
obtained was filtered and washed with ethanol. Yield 73–85%. The ligand is solu-
ble in common organic solvents but the complexes are soluble only in CHCl3, DMF
and DMSO.
[19] Anal.Calc. for [C38H30N6O2Cu]Cl: C, 68.7; H, 4.8; N, 12.7; O, 4.9; Cu, 9.8%. Found: C,
68.5; H, 4.6; N, 12.6; O, 4.8; Cu, 9.5%; FT-IR (KBr disc): 3,252 (NH), 1,634 (HC_N),
1,444
υ
asy(COO), 1,317
υ
sy(COO); 513 (M_O), 478 (M_N)cm−1
;
∧
m ×10−3(Ω−1 mol−1 cm2) 45.32; μeff (BM) 1.73; λmax in DMSO 27,243,
35,109 and 13,780 cm−1; MS: m/z 663.
[20] Anal.Calc. for [C38H30N6O2Ni]Cl: C, 69.4; H, 4.8; N, 12.9; O, 5.1; Ni, 9.2%. Found: C,
69.0; H, 4.6; N, 12.7; O, 4.9; Ni, 8.9%; FT-IR (KBr disc): 3,252 (NH), 1,615 (HC_N),
[32] S. Stepanovic, N. Antic, I. Dakic, M. Svabic-Vlahovic, In vitro antimicrobial activity
of propilis and antimicrobial drugs, Microbiol. Res. 158 (2003) 353.
[33] R. Ramesh, S. Maheswaran, Synthesis, spectra, dioxygen affinity and antifungal
activity of Ru(III) Schiff base complexes, J. Inorg. Biochem. 96 (2003) 457.
1,431 υasy (COO), 1,315
m ×10−3(Ω−1 mol−1 cm2) 51.20; μeff (BM) 3.19; λmax in DMSO 14,746,
17,165 and 23,387 cm−1; MS: m/z 658.
υ ;
sy(COO); 516 (M_O), 473 (M_N)cm−1
∧