DNA interaction, enhanced DNA photocleavage, electrochemistry, thermal investigation and biopotencial properties of new mixed-ligand complexes of Cu(II)/VO(IV) based on Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2010.10.038

Using the Schiff base ligand, dbdppo (4-(3′,4′- dimethoxybenzaldehydene)2-3-dimethyl-1-phenyl-3-pyrazolin-5-one or hnbdppo (4-(3′-hydroxy-4′-nitrobenzaldehydene)2-3-dimethyl-1-phenyl-3- pyrazolin-5-one with polypyridyl ligand(s) as co-ligand(s), Cu(II) an

Full text of DOI:10.1016/j.molstruc.2010.10.038

Journal of Molecular Structure 985 (2011) 173–183
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
DNA interaction, Enhanced DNA photocleavage, electrochemistry, thermal
investigation and biopotencial properties of new mixed-ligand complexes
of Cu(II)/VO(IV) based on Schiff bases
⇑
Natarajan Raman , Abraham Selvan
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
Using the Schiff base ligand, dbdppo (4-(30,40-dimethoxybenzaldehydene)2-3-dimethyl-1-phenyl-3-pyr-
azolin-5-one or hnbdppo (4-(30-hydroxy-40-nitrobenzaldehydene)2-3-dimethyl-1-phenyl-3-pyrazolin-
5-one with polypyridyl ligand(s) as co-ligand(s), Cu(II) and VO(IV) complexes have been synthesized
and characterized by the physicochemical properties. The electrochemical properties, DNA binding affin-
ities, as well as photonuclease activities of the complexes, were examined in detail. The DNA binding
characteristics of the Cu(II) and VO(IV) complexes was investigated by spectroscopic, electrochemical
and viscosity measurements. The UV–Vis, and magnetic moment data revealed an octahedral geometry
around Cu(II) ion and square-pyramidal geometry around VO(IV) ion and conductivity data conveyed the
electrolytic nature of the complexes. The spectroscopic studies together with cyclic voltammetry and vis-
cosity experiments support that both of the complexes bound to CT DNA by partial intercalation into the
base pairs of DNA. Moreover, these complexes have been found to promote the photocleavage of plasmid
DNA pBR322 under irradiation at 365 nm. Further, the synthesized ligands, in comparison to their metal
complexes were screened for their antimicrobial activity against bacterial and fungal species. The activity
data show that the metal complexes have more antimicrobial potent than the parent Schiff base ligands.
Ó 2010 Elsevier B.V. All rights reserved.
Article history:
Received 9 September 2010
Received in revised form 20 October 2010
Accepted 21 October 2010
Available online 3 November 2010
Keywords:
Metal complex
Polypyridyl ligand
DNA interaction
DNA photocleavage
Antimicrobial activity
1. Introduction
exert their antitumor effects by binding to DNA thereby changing
the replication of DNA and inhibiting the growth of the tumor cells,
Antipyrine and its derivatives have been widely used as phar-
maceuticals due to broad bioactivities as antitumor [1], antimicro-
bial [2,3], antiviral [4], and analgesic [5], etc. Nowadays, antipyrine
derivatives (APDs) have been accepted as important biomodel
compounds in the biological and medical fields [6]. In recent years,
APDs have exhibited attractive multi-functional properties as coor-
dinate [7], antioxidant [8] antiputrefactive [9] and optical [10]
characteristics in chemical and material fields.
Schiff bases are characterized by the AN@CHA (imine) group
which is important in elucidating the mechanism of transamina-
tion and rasemination reaction in biological system [11]. Moreover,
the incorporation of transition metal into Schiff bases enhances the
biological activity of the ligand and decreases the cytotoxic effects
of both the metal ion and ligand on the host [12].
which is the basis of designing new and more efficient antitumor
drugs and their effectiveness depends on the mode and affinity
of the binding [15].
Despite a considerable amount of literature on metal complex–
DNA interaction, the knowledge of the nature of binding of the
complexes to DNA and their binding geometries has remained a
subject of intense debate. On the other hand, as compared with
the intercalative ligand, the influence of the ancillary ligands of
the complexes has received little attention. Since the octahedral
polypyridyl Cu(II) complexes bind to DNA in three dimensions,
the ancillary ligands can also play an important role in governing
DNA-binding of these complexes. At the same time, varying substi-
tutive group or substituent position in the ligands can also create
some interesting differences in the space configuration and the
electron density distribution of Cu(II) and VO(IV) polypyridyl com-
plexes, which will seed some differences in spectral properties and
the DNA-binding behaviors of the complexes, and will also helpful
in clearly understanding the binding mechanism of Cu(II) and
VO(IV) polypyridyl complexes to DNA.
Studies on the interaction of transition metal complexes with
DNA continue to attract the attention of researchers due to their
importance in design and development of synthetic restriction en-
zymes, chemotherapeutic drugs and DNA foot printing agents
[13,14]. DNA is an important cellular receptor, many chemicals
In this communication, we describe the chelation behavior of
Schiff base derived from the condensation of 4-aminoantipyrine with
3,4-disubstituted aromatic aldehydes towards some transition
⇑
Corresponding author. Mobile: +91 9245165958; fax: +91 4562 281338.
E-mail address: drn_raman@yahoo.co.in (N. Raman).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.10.038

174
N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
elements, which may help more in understanding the mode of chela-
tionofSchiff baseand mixedligandstowards metals. For this purpose
the mixed-ligand complexes of Cu(II) and VO(IV) ions with Schiff
bases as main ligands and polypyridyl ligands as co-ligands, are stud-
ied both in solution and solid state. Binding to DNA is usually accom-
panied by marked absorbance changes in the UV–Vis, due to
excitation of charge-transfer transitions [16]. DNA-interaction stud-
ies oftitledcomplexeswere investigated byelectronicspectrum, cyc-
lic voltammetry and gel electrophoresis so as to explore the mode
and effect. The biological activity of the parent Schiff base and its me-
tal complexes were also investigated. Information obtained from our
study would be helpful in understanding the mechanism of interac-
tions of the complexes with nucleic acid and might also be useful in
the development of potential probes of DNA structure and conforma-
tion, and new therapeutic reagents for some uncommon diseases.
sphere. The molar conductivity of the complexes in DMF solution
(10ꢀ3 M) was measured using the conductometer model 601/602.
Voltammetric experiments were performed on a CHI 620C elec-
trochemical analyzer in freshly distilled DMF solutions. 0.1 M
tetrabutylammonium perchlorate (TBAP) was used as the support-
ing electrolyte. The three-electrode cell comprised a reference Ag/
AgCl, auxiliary Pt and the working Glassy Carbon electrodes. All the
solutions examined by electrochemical techniques were purged
with nitrogen for 10 min prior to each set of experiments. All mea-
surements were carried out at room temperature (25 °C).
2.3. Synthesis of the Schiff base ligands (dbdppo/hnbdppo)
The Schiff bases under investigation were prepared by mixing
an ethanol solution (25 mL) of 4-amino-2-3-dimethyl-1-phenyl-
3-pyrazolin-5-one (2.032 g, 0.01 mol) with 3,4-dimethoxybenzal-
dehyde (1.66 g, 0.01 mol) or 3-hydroxy-4-nitrobenzaldehyde
(1.67 g, 0.01 mol) in the same volume of ethanol and was refluxed
for 3 h in water bath and the obtained appropriate precipitate was
collected and recrystallized from ethanol.
2. Experimental section
2.1. Reagents and standard solutions
4-(30,40-dimethoxybenzaldehydene)2-3-dimethyl-1-phenyl-3-
pyrazolin-5-one is a light yellow colored crystal with an yield of
CT DNA, gel loading buffer, Tris base, 4-amino-2-3-dimethyl-1-
phenyl-3-pyrazolin-5-one, 1,10-phenanthroline and 2,20-bipyridine
were purchased from Sigma–Aldrich. Ethidium bromide (EB), calf
thymus DNA (CT DNA) and pBR322 plasmid DNA were also pur-
chased from Sigma. All other chemicals used were of analytical
reagent grade and were used without any further purification.
All the experiments involved in the interaction of the ligand and
its metal complexes with CT DNA were carried out in doubly dis-
tilled water buffer containing 5 mM Tris [Tris(hydroxymethyl)–
aminomethane] and 50 mM NaCl and adjusted to pH 7.1 with
hydrochloric acid. Solution of CT DNA in Tris–HCl buffer gave ratio
of UV absorbance of about 1.8–1.9:1 at 260 and 280 nm, indicating
that the CT DNA was sufficiently free of protein [17]. The CT DNA
concentration per nucleotide was determined spectrophotometri-
cally by employing an extinction coefficient of 6600 Mꢀ1 cmꢀ1 at
260 nm [18].
89%, Anal. Calcd. (%)
C20H21N3O3; M.W: 351.40, C(68.36%),
H(6.02%), N(11.95%), found (%) C(68.25%), H(5.92%), N(11.48%);
1H NMR (DMSO-d6) d: 3.28(s, 3H, PZANACH3), 2.26(s, 3H, PZACA
CH3), 3.92(s, 3H, OACH3), 6.97–7.40(m, 3H, Ph), 6.90–7.37(m, 5H,
Ph), 8.01(s, 1H, N@CH); IR(KBr)
m
(cmꢀ1): 1634(s) (HC@N),
1308(s) (OACH3), 1281(s) (PzACACH3), 1181(s) (PzANACH3),
1714(s) (PzAC@O), 1471(s) (PhAC@C), 1545(m) (PhACAC),
3048(s) (PhACAH); UV–Vis (DMF) [nm(frequency, cmꢀ1)(transi-
tion)(geometry)]; 387(25,839) (ILCT), 352(28,409) (ILCT),
264(37,878) (ILCT).
4-(30-hydroxy-40-nitrobenzaldehydene)2-3-dimethyl-1-phenyl-
3-pyrazolin-5-one is a yellow colored powder with an yield of 90%,
Anal. Calcd. (%) C18H16N4O4; M.W: 352.35, C(61.35%), H(4.57%),
N(15.90%), found(%) C(68.25%), H(4.43%), N(15.48%); 1H NMR
(DMSO-d6) d: 3.11(s, 3H, PZANACH3), 2.58(s, 3H, PZACACH3),
10.28(s, 1H, OH), 6.77–7.15(m, 3H, Ph), 6.90–7.37(m, 5H, Ph),
2.2. Equipments
8.00(s, 1H, N@CH); IR (KBr)
m
(cmꢀ1): 1614(s) (HC@N), 1608(w)
(CANO2), 3442(b) (OH), 1293(s)(PzACACH3), 1145(s) (PzANA
CH3), 1710(s) (PzAC@O), 1423(s) (PhAC@C), 1530(m) (PhACAC),
3075(s) (PhACAH); UV–Vis, (DMF) [nm(frequency, cmꢀ1)(transi-
tion)(geometry)]; 399(25,062) (ILCT), 364(27,472) (ILCT),
270(37,037) (ILCT), 248(40,322) (ILCT).
UV–Vis, spectra were recorded on a Shimadzu Model 1601 UV–
Visible Spectrophotometer. IR spectra of the ligand and its metal
complexes were recorded on a Perkin-Elmer FTIR-1605 spectro-
photometer using KBr discs. The intensity of the reported IR signals
were defined as w = weak, m = medium and s = strong. 1H NMR
spectra were measured on a Varian XL-300 MHz spectrometer with
tetramethylsilane (TMS) as the internal standard at room temper-
ature. The complexes were analyzed for their metal contents, fol-
lowing standard procedures [19] after decomposition with a
mixture of conc. HNO3 and HCl, followed by conc. H2SO4. Microa-
nalyses (C, H, N) were carried out on a Perkin–Elmer 240 elemental
analyzer. Mass spectrometry experiments were performed on a
JEOL-AccuTOF JMS-T100LC mass spectrometer equipped with a
custom-made electrospray interface (ESI). The X-band EPR spectra
of the complexes were recorded at RT (300 K) and LNT (77 K) using
TCNE as the g-marker. Room temperature magnetic susceptibility
measurements were carried out on a modified Gouy-type magnetic
balance, Hertz SG8-5HJ. X-ray diffraction experiments were carried
out on XPERT-PRO diffractometer system. Copper Ka1 line, with
wavelength of 1.5406 Å generated with a setting of 30 mA and
40 kV with the electrodes, was used for diffraction. The slit width
setting was 91 mm. The diffracting angle (2h) was scanned from
10.0251 to 79.9251 continuously with a rate of 2° per minute.
The whole process took place at a temperature of 25°C. The ther-
mal analyses measurements (TGA and DTG) were carried out on
a Setaram Labsys TG-16 thermobalance under nitrogen atmo-
2.4. Synthesis of new Cu(II) and VO(IV) complexes
All the new Cu(II) and VO(IV) complexes were prepared by the
following general procedure as sketched in Scheme 1. To an etha-
nolic solution of the appropriate Schiff base (0.001 mol), metal
salts (0.001 mol) in ethanol (15 mL) were added and kept stirring
for 30 min. To the above stirring solution, about 0.001 mol of
2,20-bipyridine (bpy)/1,10-phenanthroline (phen) in the appropri-
ate ratio was added (Scheme 1) and refluxed for 2–3 h. The resul-
tant product was washed and recrystallized with ethanol.
[Cu(dbdppo)(bpy)2](Cl)2 (1): Green powder, yield 67%, M.F:
[CuC40H37N7O3]Cl2, M.Wt: 798.23, Anal. Calcd. (%) C(60.21%),
H(4.73%), N(12.32%), Cu(7.97%) found (%) C(60.15%), H(4.65%),
N(12.27%), Cu(7.94%); IR(KBr)
m
(cmꢀ1): 1566(strong, s) (HC@N),
1316(s) (OACH3), 1248(s) (PzACACH3), 1158(s) (PzANACH3),
1644(s) (PzAC@O), 1473(s) (PhAC@C), 1536(m) (PhACAC),
3052(s) (PhACAH), 479(m) (MAN), 578(m) (MAO); UV–Vis,
(DMF) [nm(frequency, cmꢀ1)(transition)(geometry)]; 965(10,362)
(2B1g ? 2B2g
)
[Octahetral(Oh)], 770(12,987) (2B1g ? 2A1g)(Oh),
386(25,906) (ILCT), 327(30,581) (ILCT), 243(41,152) (ILCT).

N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
175
Scheme 1. Synthetic route for metal complexes.
[Cu(dbdppo)(phen)2](Cl)2 (2): Green powder, yield 72%, M.F:
[CuC44H37N7O3]Cl2, M.Wt: 846.27, Anal. Calcd. (%) C(62.51%),
H(4.43%), N(11.59%), Cu(7.53%) found (%) C(62.43%), H(4.38%),
[VO(dbdppo)(bpy)](SO4) (5); Green powder, yield 68%, M.F:
[VOC30H29N5O3]SO4 M.Wt: 654.58, Anal. Calcd. (%) C(55.07%),
H(4.51%), N(10.72%), VO(7.81%) found (%) C(54.99%), H(4.43%),
N(11.57%), Cu(7.47%); IR(KBr)
m
(cmꢀ1): 1548(s) (HC@N), 1354(s)
N(10.69%), VO(7.78%); IR(KBr) m
(cmꢀ1): 1598 (s) (HC@N), 1332(s)
(OACH3), 1292(s) (PzACACH3), 1137(s) (PzANACH3), 1652(s)
(PzAC@O), 1452(s) (PhAC@C), 1537(m) (PhACAC), 3109(s)
(PhACAH),482(m) (MAN), 585(m) (MAO); UV–Vis, (DMF) [nm(fre-
(OACH3), 1292(s) (PzACACH3), 1181(s) (PzANACH3), 1659(s)
(PzAC@O), 960(s) (V@O), 1471(s) (PhAC@C), 1545(m) (PhACAC),
3052(s) (PhACAH), 465(m) (MAN), 582(m) (MAO); UV–Vis,
(DMF) [nm(frequency, cmꢀ1)(transition)(geometry)]; 979(10,214)
(2B2 ? 2E) (Square-Pyramidal (SP), 877(11,402) (2B2 ? 2B1)(SP),
386(25,906) (ILCT).
quency, cmꢀ1)(transition)(geometry)]; 975(10,256) (2B1g
?
2B2g
)
(Oh), 720(13,888)(2B1g ? 2A1g)(Oh), 386(25,906) (ILCT),
266(37,593) (ILCT).
[Cu(hnbdppo)(bpy)2](Cl)2 (3): Green powder, yield 68%, M.F:
[CuC38H32N8O4]Cl2, M.Wt: 799.17, Anal. Calcd. (%) C(57.21%),
H(4.12%), N(14.15%), Cu(7.97%) found (%) C(57.10%), H(4.01%),
[VO(dbdppo)(phen)](SO4) (6): Green powder, yield 72%, M.F:
[VOC32H29N5O3]SO4 M.Wt: 678.61, Anal. Calcd. (%) C(56.63%),
H(4.35%), N(10.34%), VO(7.56%) found (%) C(56.58%), H(4.27%),
N(14.03%), Cu(7.93%); IR (KBr)
m
(cmꢀ1): 1579(s) (HC@N), 1601(w)
N(10.31%), VO(7.50%); IR(KBr) m
(cmꢀ1): 1589(s) (HC@N), 1302(s)
(C-NO2), 3439(b) (OH), 1296(s)(PzACACH3), 1159(s) (PzANACH3),
1659(s) (PzAC@O), 1444(s) (PhAC@C), 1530(m) (PhACAC),
3052(s) (PhACAH), 504(m) (MAN), 596(m) (MAO); UV–Vis,
(DMF) [nm(frequency, cmꢀ1) (transition)(geometry)]; 1022(9784)
(OACH3), 1258(s) (PzACACH3), 1137(s) (PzANACH3), 1649(s)
(Pz-C@O), 984(s) (V@O), 1483(s) (PhAC@C), 1532(m) (PhACAC),
3071(s) (PhACAH), 507(s) (MAN), 587(m) (MAO); UV–Vis,
(DMF) [nm(frequency, cmꢀ1
) (transition) (geometry)]; 1022
(2B1g ? 2B2g
)
(Oh), 721(13,869)(2B1g ? 2A1g)(Oh), 443(22,573)
(9784) (2B2 ? 2E), (SP) 640(15,625) (2B2 ? 2B1)(SP), 377(26,525)
(ILCT), 302(33,112) (ILCT).
(ILCT), 215(46,511) (ILCT).
[Cu(hnbdppo)(phen)2](Cl)2 (4): Green powder, yield 75%, M.F:
[CuC42H32N8O4]Cl2, M.Wt: 847.21, Anal. Calcd. (%) C(59.61%),
H(3.84%), N(13.27%), Cu(7.53%) found (%) C(59.53%), H(3.76%),
[VO(hnbdppo)(bpy)](SO4) (7): Green powder, yield 63%, M.F:
[VOC28H24N6O4]SO4 M.Wt: 655.53, Anal. Calcd. (%) C(51.27%),
H(3.72%), N(12.87%), VO(7.81%) found (%) C(51.25%), H(3.66%),
N(13.18%), Cu(7.48%); IR (KBr)
m
(cmꢀ1): 1549(s) (HC@N),
N(12.81%), VO(7.77%); IR(KBr) m
(cmꢀ1): 1587(s) (HC@N), 1611(w)
1602(w) (CANO2), 3576(w) (OH), 1315(s)(PzACACH3), 1117(s)
(PzANACH3), 1651(s) (Pz-C@O), 1470(s) (PhAC@C), 1528(m)
(PhACAC), 3082(s) (PhACAH), 468(m) (MAN), 575(m) (MAO);
UV–Vis, (DMF) [nm(frequency, cmꢀ1) (transition) (geometry)];
(CANO2), 3433(b) (OH), 1292(s)(PzACACH3), 1163(s) (PzANACH3),
1651(s) (PzAC@O), 960(s) (V@O), 1441(s) (PhAC@C), 1523(m)
(PhACAC), 3077(s) (PhACAH), 503(m) (MAN), 552(m) (MAO);
UV–Vis, (DMF) [nm(frequency, cmꢀ1)(transition)(geometry)];
977(10,235) (2B2 ? 2E) (SP), 853(11,723) (2B2 ? 2B1)(SP), 395(25,
316) (ILCT), 284(35,211) (ILCT).
980(10,204) (2B1g ? 2B2g
)
(Oh), 702(14,245)(2B1g ? 2A1g)(Oh),
445(22,471) (ILCT), 302(33,112) (ILCT), 241(41,493) (ILCT).

176
N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
[VO(hnbdppo)(phen)](SO4) (8): Green powder, yield 65%, M.F:
that each of the two nitrogen atoms of phenanthroline ligands do-
nate a pair of electrons to the central metal forming a coordinate
covalent bond [24,25].
[VOC30H24N6O4]SO4 M.Wt: 679.55, Anal. Calcd. (%) C(53.01%),
H(3.62%), N(12.43%), VO(7.52%) found (%) C(52.97%), H(3.53%),
N(12.36%), VO(7.49%); IR (KBr)
m
(cmꢀ1): 1578(m) (HC@N),
Metal–ligand bond is further confirmed by the appearance of a
medium intensity band in the range 465–507 and 552–596 cmꢀ1 in
the spectra of the complexes assigned to stretching frequencies of
(M–N) bond [26,27] and metal–oxygen bond formation [28]
respectively and this can be explained by the donation of electrons
from nitrogen or oxygen to the empty d-orbitals of the metal atom.
In addition to other bands, the vanadyl complexes show a strong
band at 984–960 cmꢀ1, attributed to the stretching vibration of
the terminal V@O bond [29,30,21].
1626(w) (CANO2), 3451(b) (OH), 1304(s) (PzACACH3), 1162(s)
(PzANACH3), 1654(s) (PzAC@O), 975(s) (V@O), 1438(s) (PhAC@C),
1524(m) (PhACAC), 3084(s) (PhACAH), 479(m) (MAN), 567(m)
(MAO); UV–Vis, (DMF) [nm(frequency, cmꢀ1)(transition)(geome-
try)]; 984(10,162) (2B2 ? 2E) (SP), 858(11,655) (2B2 ? 2B1) (SP),
372(26,881) (ILCT), 267(37,453) (ILCT).
2.5. Pharmacology
3.2. Electronic spectra and magnetic measurement
In vitro antibacterial, antifungal assay and chemical nuclease
activity of the synthesized compounds are described in the Supple-
mentary file.
Visible spectral data along with magnetic susceptibility mea-
surements gave adequate support in establishing the geometry of
the metal complexes. These data along with the assignments are
presented in experimental. The electronic spectra of ligands and
complexes were recorded in DMF solution in the scan range
200–1100 nm. In the electronic spectra of the ligand and its mono-
nuclear metal complexes, the wide range bands were observed due
3. Results and discussion
The complexes were prepared by direct reaction of Schiff base
ligand(s) and bpy/phen with the appropriate molar ratios of Cu(II)
chloride and VO(IV) sulphate in ethanol. The yields were good to
moderate. The desired Cu(II) and VO(IV) complexes were separated
from the solution by suction filtration, purified by washing several
times with ethanol. The complexes were air-stable for extended
periods and soluble in DMSO and DMF; slightly soluble in ethanol
and methanol; insoluble in benzene and water. The molar conduc-
to either the
transfer transition arising from
p
?
p* and n ? p* of C@N chromophore or charge-
electron interactions between
p
the metal and ligand, which involves either a metal-to-ligand or li-
gand-to-metal electron transfer [31,32]. The absorption bands be-
tween 352 and 399 nm in free ligands change a bit in intensity for
metal complexes. The absorption shift and intensity change in the
spectra of the metal complexes most likely originate from the
metalation which increases the conjugation and delocalization of
the whole electronic system and results in the energy change of
tivities of the complexes were around 218.3–227.5
X
ꢀ1 cm2 molꢀ1
in DMF solution, showing that all complexes were electrolytes.
FTIR spectral data supported that the copper ion in all complexes
has N5O coordination sphere and oxovanadyl ion in all complexes
has N3O2 coordination sphere, bound by oxygen in pyrazole ring,
imine and pyridine type nitrogen atoms. The spectroscopic studies
and analytical data supported our proposed structure (Scheme 1).
p
?
p* and n ? p* transition of the conjugated chromophore
[33]. These results clearly indicate that the ligand coordinates to
Cu(II) and VO(IV) ions, which are in accordance with the results
of the other spectral data.
Furthermore, the electronic spectra of six coordinate copper(II)
complexes have either D4h or C4v symmetry, and the eg and t2g level
of the 2D free ion term will split into B1g, A1g, B2g and Eg levels,
respectively under the influence of the distortion, can be such as
to cause the two transitions, 2B1g ? 2B2g and 2B1g ? 2A1g which
are represented in experimental part. This supports the distorted
octahedral copper(II) complex which is usual in the d9 case
[34,35]. The magnetic moment of 1.78–1.97 BM falls within the
range normally observed for octahedral Cu(II) complexes [36].
The electronic spectra of the vanadyl complexes show two
absorption bands and their transitions corresponding to five-coor-
dinate square-pyramidal geometry [37] which are represented in
experimental part. This is also supported by the magnetic moment
value in the range of 1.68–1.76 BM observed for the five-coordi-
nate VO(IV) complexes. Suggested structures of the complexes
are given in Scheme 1.
3.1. IR spectra and coordination mode
IR spectra have proven to be the most suitable technique to give
enough information to elucidate the nature of bonding of the li-
gand to the metal ion. The IR spectra of the ligands and two repre-
sentative complexes are shown in Fig. S1 (Supplementary file). The
IR spectra of the free Schiff base ligands dbdppo and hnbdppo
show moderate intensity absorptions at 1634 and 1614 cmꢀ1
respectively attributable to the imine
m
(C@N) [20,21] and 1714
(C@O) of dbdppo
and 1710 cmꢀ1 bands are observed for free
m
and hnbdppo ligands respectively and no bands are observed for
free primary amine indicating that complete condensation has
occurred between 4-aminoantipyrine and 3,4-disubstituted benz-
aldehyde supporting the formation of Schiff base. The coordination
of the ligands with different metals through the nitrogen atom is
expected to reduce the electron density in the azomethine link
and lower the
m
(C@N) absorption frequency. This shift in absorp-
3.3. Mass spectra
tion of the (C@N) frequency in case of the complexes suggests
m
the coordination through the nitrogen of (>C@N) group [22]. A
strong to medium intensity band in ligands due to the presence
of m(C@O) groups, which is shifted towards lower frequency after
The ESI mass spectrum gives additional structural information
about the stereochemistry of the studied compounds. The mass
spectrum of the studied compound is characterized by moderate
to high relative intensity molecular ion peaks. It is obvious that
the molecular ion peaks are in good agreement with their sug-
gested empirical formula as indicated from elemental analyses.
The ESI mass spectrum of 3 shows a molecular ion (M+) peak at
m/z = 728, which suggests the monomeric nature of the complex
and confirms the proposed formula. The ESI mass spectrum of
hnbdppo Schiff base shows a molecular ion peak m/z at 352 which
is equivalent to its molecular weight and also exhibits two addi-
tional peaks m/z at 353 and 354, which are corresponding to
complexation, which proves the coordination with metal [21]. This
displacement can be attributed to the electronic donation of the
base to metal (N ? M), which increases the electron density on
the metal d-orbitals, and consequently the p ? d donation from
p
p
the oxygen atom to metal is expected to be reduced [23]. This
behavior indicates the fact that the carbonyl oxygen atom of the
antipyrine residue is coordinated. The IR values, m
(C–H) 862 cmꢀ1
and 736 cmꢀ1 observed for phenanthroline are red shifted to
854 cmꢀ1 and 725 cmꢀ1. These shifts can be explained by the fact

N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
177
(M+1) and (M+2) peaks respectively. The spectrum of hnbdppo
also shows a series of peaks corresponding to various fragments
which are shown in Scheme 2. Their intensity gives an idea of
the stability of the fragments and also about the geometrical
configurations.
nitrogen temperature (LNT) and at room temperature (RT). EPR
spectra of the copper(II) complexes are given in Figs. S2 and S3
(Supplementary file). g-Tensor values of copper(II) complexes can
be used to derive the ground state. In an elongated octahedron,
the 3d unpaired electrons for copper(II) ion lies in dx2ꢀy2 orbital
(2B1 as ground state). In addition, exchange coupling interaction
between two Cu(II) ions is explained by Hathaway expression
G = (g|| ꢀ 2)/(g? ꢀ 2). When the value G < 4.0, a considerable ex-
change coupling is present in solid complex. It has been reported
(Table 1) that g|| value of copper(II) complex can be used as a mea-
sure of the covalent character of the metal–ligand bond. If the
value is more than 2.3, the metal–ligand bond is essentially ionic
and the value less than 2.3 is indicative of covalent character
3.4. EPR spectra
The EPR spectra of copper(II) complexes provide information
about hyperfine and super hyperfine structures. It is very impor-
tant to understand the metal ion environment in the complexes,
i.e., the geometry, nature of the donating atoms from the ligands
and degree of covalency of the copper(II)–ligands bonds. The ESR
spectra of the Cu(II) complexes were recorded in DMSO at liquid
[38]. The covalency parameter (a
2) indicates considerable covalent
Scheme 2. Mass fragment pattern of hnbdppo ligand.

178
N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
Table 1
The spin Hamiltonian parameters of Cu(II) complexes in DMSO at 300 K and 77 K.
Complex
g-Tensor
Hyperfine constant ꢂ 10ꢀ4 cmꢀ1
g
g
giso
A
A
?
Aiso
k
k
?
1
2
3
4
2.1726
2.1766
2.1686
2.1807
2.0695
2.0732
2.0659
2.0768
2.1038
2.1076
2.1001
2.1114
240
250
250
240
30
40
20
30
100
110
96.6
100
character for the metal–ligand bond [39]. Also the trend g|| > g? > ge
(2.0027) observed for these complexes indicates that the unpaired
electron is most likely in the dx2ꢀy2 orbital of the Cu(II) ion in com-
plexes [40]. The ESR spectra only show signals that may be ac-
counted for the presence of free radicals that can be resulted
from the cleavage of any double bond and distribution of the
charge on the two neighbor atoms. The peaks are broad and have
the appearance of ill-resolved triplets. The breadth and triplet
appearance can be attributed to hyperfine splitting by the nitrogen
atom (I = 1) of the ligand. The triplet appearance is adduced as an
evidence for nitrogen coordination. The spin Hamiltonian parame-
ters suggest that the copper ion is strongly distorted from planar
geometry which is in good agreement with electronic absorption
spectral data.
Fig. 1. Powder X-ray diffraction pattern of (A) 1 and (B) 3 complexes.
out to understand the lattice dynamics of the complexes. The ob-
served interplanar spacing values (‘d’ in A°), 2 angles and the Miller
indices (h k l) values are listed in Table 3. The X-ray powder diffrac-
The EPR parameters g||, g , gav, A and A and the energies of the
?
||
?
d–d transitions were used to evaluate the bonding parameters
(Table 2)
of covalency of the in-plane
out of plane
-bonding respectively. The value of a2 and b2 were
a
2, b2 and c2 which may be regarded as measurement
tograms of 1 and 3 complexes were recorded using Cu K
a as the
r
-bonding, in-plane p-bonding and
source in the 2h range 10.02–79.92°. The X-ray powder diffraction
patterns throws light only on the fact that each solid represents a
definite compound of a definite structure which is not contami-
nated with starting materials. The X-ray data revealed that the
copper complexes had crystallized in the face-centered cubic sys-
tem [45]. The azomethine of 1 and 3 exhibited two main reflections
at 2 = 24.30° and 24.36°, respectively (d spacing 3.66 and 3.65 Å,
respectively).
p
estimated from the following expression [41].
a2 ¼ ꢀðAk=0:036Þ þ ðgk ꢀ 2:0023Þ þ ð3=7Þðg? ꢀ 2:0023Þ þ 0:04
ð1Þ
ð2Þ
b2 ¼ ðgk ꢀ 2:0023ÞE= ꢀ 8ka2
Finally, the above data indicate that the coordination occurs
through the nitrogen of the pby/phen ring, azo-methine nitrogen
atom and oxygen present in pyrazole ring to give the structures
as shown in Scheme 1.
According to Hathaway [42], K|| ꢁ K ꢁ 0.77 for pure
?
in-plane
out of plane
served that K < K which indicates the presence of significant of
r-bonding and K < K for in-plane p-bonding, while for
?
||
p
-bonding K|| > K . In all the Cu(II) complexes, it is ob-
?
?
||
in-plane
p-bonding than from out of plane p-bonding in metal–
3.6. Thermal properties of the ligand and its metal complexes
ligand -bonding. K is dimensionless quantity, which is a measure
p
of the contribution of s electrons to the hyperfine interaction and
is generally found to have a value of 0.30. The K values obtained for
all the complexes are in agreement with those estimated by Assour
[43] and Abragam and Pryce [44].
The thermal properties of the ligand and its metal complexes
investigated by thermal gravimetric analysis (TGA) and differential
thermogravimetry (DTG) are presented in the Supplementary file.
Hence, the EPR study of the copper(II) complexes has provided
supportive evidence to the conclusion obtained on the basis of
electronic spectrum and magnetic moment values. Thus, the re-
sults confirmed that, the Cu(II) complexes possess distorted octa-
hedral geometry.
3.7. Pharmacological results
The results of antibacterial, antifungal and chemical nuclease
activity of the compounds are given and discussed in the Supple-
mentary file.
3.5. Powder X-ray diffractogram
3.8. Electrochemical study of complexes
To obtain further evidence about the structure of the metal
complexes X-ray diffraction was performed. The powder X-ray dif-
fractogram study of the complexes 1 and 3 (Fig. 1) has been carried
Analysis of cyclic voltammograms with scan rates varying from
0.06 to 0.18 V sꢀ1 shows that the process is diffusion controlled
Table 2
The bonding parameters of Cu(II) complexes in DMSO solution.
Complex
a2
b2
c2
K
K
K ꢂ 10ꢀ4 (cmꢀ1
)
G
k
?
1
2
3
4
0.9051
0.9385
0.9273
0.9163
0.4066
0.3057
0.2849
0.3265
0.6394
0.4957
0.4342
0.5438
0.3331
0.2692
0.2450
0.2742
0.5617
0.5912
0.5292
0.6374
138.8
150.2
134.0
141.7
2.5434
2.4666
2.6250
2.4021

N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
179
Table 3
X-ray powder diffraction data of the 1 and 3 mixed ligand Schiff base complexes.
1
3
2h
Lattice spacing (d) (Å)
h k l
2h
Lattice spacing (d) (Å)
h k l
12.008
17.836
20.956
27.078
28.746
32.408
41.774
45.674
48.614
7.3644
4.9688
4.2355
3.2903
3.1030
2.7603
2.1605
1.9847
1.8713
2 0 0
2 2 0
3 1 1
3 3 1
4 2 0
5 1 1
5 3 3
7 1 1
6 4 2
12.124
17.913
20.480
21.020
24.548
27.214
32.526
39.819
45.805
7.2942
6.5381
4.3329
4.2228
3.6234
3.2741
2.7505
2.2620
1.9793
2 0 0
2 2 0
3 1 1
2 2 2
4 0 0
3 3 1
3 3 3
0 2 6
6 4 0
and coupled to relatively fast chemical complications. Electro-
chemical properties of the complexes were studied on a Pt disc
electrode in DMF containing 0.05 M n-Bu4NClO4 as the supporting
electrolyte. As a typical case, in the case of complex 2, the one-elec-
tron reduction peak, which is attributed to the Cu(II)/Cu(I) couple,
occurs at Epc = ꢀ0.785 V, with an associated reoxidation peak in the
reverse scan at Epa = ꢀ0.262 V as shown in Fig. S4 (Supplementary
file). The most interesting observation in the region appears to be a
well defined redox couple for which the anodic peak potential cor-
responds to the oxidation [46] of Cu(II)/Cu(III) and the cathodic
counter part is due to the reduction of Cu(III).
binding, the labile ligand of the complexes is replaced by a nitrogen
base of DNA such as guanine N7. On the other hand, the noncova-
lent DNA interactions include intercalative, electrostatic and
groove (surface) binding of metal complexes along outside the
DNA helix, along major or minor groove. It has been reported that
DNA can provide three distinctive binding sites for quinolone me-
tal complexes; namely, groove binding, electrostatic binding to
phosphate group and intercalation [50]. This behavior is of great
importance with regard to the relevant biological role of quinolone
antibiotics in the body [51].
The cathodic peak current of the complexes increased and the
peak potentials were shifted towards more negative direction with
an increase in scan rate. The difference in the cathodic and anodic
peaks suggests that the process involves transfer of one electron
and therefore corresponds to a Cu(II)/Cu(I) and Cu(III)/Cu(II) cou-
ples. The difference between forward and backward peak potential
scan provides a rough evaluation of the degree of the reversibility.
3.9.1. Electronic absorption titration with CT DNA
The binding of metal complexes to CT DNA have been studied
through the changes in absorbance and shift in wavelength. As
hypochromism and hyperchromism are both the spectral features
of DNA concerning its double helix structure, hypochromism
means the DNA binding mode of a complex via electrostatic effect
or intercalation whereas hyperchromism means the breakage of
the secondary structure of DNA on binding of the complex. The
absorption spectra of metal complexes in the absence and presence
of CT DNA are shown in Fig. 2A and B. With increasing concentra-
tion of CT DNA, the absorption band of the complex is affected,
resulting in the obvious tendency of hypochromism and blue shift.
It is clear from the spectra that, in the case of bipyridine complexes
where the bpy rings are not coplanar, only a moderate hypochro-
mism is observed compared to phen complexes. The intrinsic bind-
ing constants Kb of the two complexes with CT DNA were obtained
by monitoring the changes in the intraligand band with increasing
concentration of DNA using the following function equation;
The peak-to-peak potential separation (DEp) of the electrode cou-
ples increased with increase in the scan rate confirming the occur-
rence of a slow chemical reaction and a limited mass transfer
following the electrode process [47].
For Pt electrodes, when plotting cathodic peak current (Ipc
)
against the sweep rates, linear plots were observed as shown in
Fig. S4 (inset). This clearly demonstrates that the electron transfer
process within the film exhibits the characteristic features of a sur-
face bound species [48]. Similarly the cyclic voltammetric behav-
iors of vanadyl complexes have also been studied and the data
are presented in Table 4.
½DNAꢃ=ðea
ꢀ ef Þ ¼ ½DNAꢃ=ðeb ꢀ ef Þ þ 1=½Kbðeb ꢀ ef Þꢃ
3.9. Binding characteristics of complex with DNA
where [DNA] is the concentration of DNA in base pairs, the apparent
The transition metal complexes are known to bind with DNA via
absorption coefficient ea f and b correspond to Aobs/[complex], the
,
e
e
both covalent and/or noncovalent interactions [49]. In covalent
extinction coefficient for the free metal complexes and the extinc-
Table 4
Electrochemical behavior of Cu(II) and VO(IV) complexes in the presence of CT DNA.
Complex
Redox couple
Ipc(A) ꢂ 10ꢀ5
Epc (V)
E1/2 (V)
D
Ep (V)
Koxd/Kred
Free
Bound
Free
Bound
Free
Bound
Free
Bound
1
2
3
4
Cu(III)/Cu(II)
Cu(II)/Cu(I)
Cu(III)/Cu(II)
Cu(II)/Cu(I)
Cu(III)/Cu(II)
Cu(II)/Cu(I)
Cu(III)/Cu(II)
Cu(II)/Cu(I)
VO(IV)/VO(III)
VO(IV)/VO(III)
VO(IV)/VO(III)
VO(IV)/VO(III)
0.89
1.56
0.772
1.51
0.54
1.02
0.42
1.79
5.49
1.32
1.84
1.83
0.85
1.10
0.718
0.99
0.47
0.84
0.38
1.45
2.69
0.62
1.51
1.19
ꢀ0.079
ꢀ0.602
ꢀ0.111
ꢀ0.785
ꢀ0.066
ꢀ0.533
ꢀ0.028
ꢀ0.789
ꢀ0.637
ꢀ0.851
ꢀ0.811
ꢀ0.827
ꢀ0.057
ꢀ0.585
ꢀ0.103
ꢀ0.759
ꢀ0.042
ꢀ0.524
ꢀ0.055
ꢀ0.772
ꢀ0.604
ꢀ0.807
ꢀ0.764
ꢀ0.811
0.135
0.294
0.130
0.322
ꢀ0.356
0.125
0.428
0.616
0.482
0.523
0.372
0.381
0.770
0.593
0.136
0.244
0.042
0.080
0.436
0.458
0.457
0.477
0.315
0.349
0.154
0.546
ꢀ0.024
0.103
0.016
0.068
2.35
1.94
1.36
2.75
2.54
1.42
0.35
1.93
3.61
5.55
6.24
1.86
ꢀ0.523
ꢀ0.520
0.120
0.115
ꢀ0.342
0.010
ꢀ0.349
0.022
ꢀ0.492
ꢀ0.569
ꢀ0.729
ꢀ0.790
ꢀ0.787
ꢀ0.499
ꢀ0.616
ꢀ0.755
ꢀ0.756
ꢀ0.777
5
6
7
8

180
N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
resulting in higher hydrophobicity of phen than that of bpy. These
data indicate that the extent of hypochromism commonly parallels
the intercalative strength of the ligand and, moreover, a higher pla-
nar area, an extended
p system, hydrophobicity, and aromaticity
lead to deep penetration and hence more stacking within the base
pairs of DNA.
3.9.2. Electrochemical titration with CT DNA
CV has proved to be a very sensitive analytical technique to
determine changes in redox behavior of metallic species in the
presence of biologically important molecules [55,56].The electro-
chemical investigations of metal–DNA interactions can provide a
useful complement to spectroscopic methods, e.g., for nonabsorb-
ing species, and yield information about interactions with both
the reduced and oxidized form of the metal [57]. The quasi-revers-
ible redox couple for each complex in DMF solution that has been
studied upon addition of CT DNA and the shifts of the Epc, E1/2 and
D
Ep is given in Table 4.
Cyclic voltammetric technique has been applied to study the
interaction of the present redox active copper(II) complexes with
DNA with a view to further explore the DNA binding modes as-
sessed from the spectral and viscometric studies. The cyclic vol-
tammogram at Pt disk electrode for copper(II) complexes display
two one electron quasi-reversible peaks, Cu(II)/Cu(I) and Cu(II)/
Cu(III) redox couples (Fig. 3). It is known that the redox potential
of Cu(II)/Cu(I) process is shifted towards more negative potential
as the electron-donating ability of the substituents on the ligand
framework becomes higher [58]. The co-ordination chemistry of
high oxidation state metal complexes is very important because
of their biological significance as redox enzyme models. Only a
small number of ligands have an ability to stabilize the high oxida-
tion state of copper. dbdppo ligand, which possesses the strong
donating ability of methoxy groups which can stabilize the high
oxidation state of metal complexes.
The Cu(II)/Cu(I) and Cu(II)/Cu(III) redox processes are influ-
enced by the coordination number, stereochemistry and the
hard/soft character of the ligands, donor atoms However due to
inherent difficulties in relating coordination number and stereo-
chemistry of the species present in solution, redox processes are
generally described in terms of the nature of the ligands present
[59]. Patterson and Holm [60] have shown that softer ligands tend
to produce more positive E values, while hard acids give rise to
negative E values. The observed E values for the 1–4 complexes
indicate considerable ‘‘hard acid’’ character, as reported earlier
[61]. This is likely to be due to azo-methine nitrogen donor and
nitrogen atoms of the heterocyclic bases involved in coordination.
It can be observed (Table 4) that complexes 1–4 exhibit the
same electrochemical behavior upon addition of CT DNA. Upon
addition of CT DNA, both the cathodic and anodic peak currents de-
crease very remarkably. The drop of the voltammetric currents in
the presence of CT DNA can be attributed to diffusion of the metal
complex bound to the large, slowly diffusing DNA molecule [62].
The more pronounced decrease of the peak currents upon addition
of CT DNA, the stronger the binding affinity of the complex to DNA.
Partial intercalation mode, as deduced above, can account for this
[63]. The shift in E1/2 for Cu(II) complexes on binding to DNA sug-
gests that both Cu(II) and Cu(I) forms bind to DNA but to different
extents.
Fig. 2. Absorption spectra of complex (A) 3 and (B) 7 in the presence of DNA in
Tris–HCl buffer upon addition of CT DNA. [complex] = 25 lM. Arrow shows the
absorbance changing upon the increase of DNA concentration.
tion coefficient for the free metal complexes in the fully bound
form, respectively. In plots of [DNA]/(ea
ven by the ratio of slope to the intercept.
ꢀ
ef) versus [DNA], Kb is gi-
To compare quantitatively the affinity of the complexes toward
DNA, the intrinsic binding constants Kb were 2.59 ꢂ 106 Mꢀ1 with
hypochromicity 68.9% for 4, 4.61 ꢂ 105 Mꢀ1 with hypochromicity
16.75% for 3, 4.27 ꢂ 105 Mꢀ1 with hypochromicity 65.5% for 2,
3.49 ꢂ 105 Mꢀ1 with hypochromicity 47.91% for 1, 3.08 ꢂ 106 Mꢀ1
with hypochromicity 19.8% for 8, 4.59 ꢂ 105 Mꢀ1 with hypochrom-
icity 18.2% for 7, 4.19 ꢂ 105 Mꢀ1 with hypochromicity 23.0% for 6,
3.84 ꢂ 105 Mꢀ1 with hypochromicity 6.6% for 5 respectively which
are comparable with other Cu(II) and VO(IV) phen/bpy mixed-li-
gand complexes [52–54]. This may be due to the introduction of
two substituents reducing the planarity and the different shape
of the intercalating ligand. For complexes 3 and 4, the hnbdppo
ligand possessed appropriate planarity and possibly supported by
the formation of an intramolecular hydrogen bond between the
ortho phenolic group and the oxygen atom of nitro moiety and
overlap with the base pairs of DNA. While for complexes 1 and 2,
the two substituents of bulky methoxy group reduced the planarity
and steric hindrance may arise when the complex containing the
intercalative ligand dbdppo interacts with DNA. On the other
hand, the higher binding affinity of complex 4 to DNA than that
of complex 3 may be due to the greater planar area of phen, thus
The cyclic voltammogram (CV) studies for the oxovanadium
(IV) complexes 5–8 were realized in DMF solutions. The electro-
chemical data are summarized in Table 4. For all the complexes
one quasi-reversible wave were observed. Based on these observa-
tions, it is reasonable to suggest that the reduction process may in-
volve the stepwise redox processes depicted below:
VOðIVÞ ꢀ VOðIIIÞ
ð3Þ

N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
181
quasi-reversible peaks are assigned to reduction of the vanadyl(IV)
ion. As shown in Fig. 3C, for the CV curve of complex 6, the VO(IV)/
VO(V) redox processes are irreversible, in accord with the observed
behavior for other similar vanadium(IV) complexes [64,65]. It was
also observed that the ligands moiety has a significant effect on E1/2
for 5 and 6; electron-withdrawing groups stabilize the VO(IV) in
the complexes while the electron-donating groups favor oxidation
to VO(V). This is possibly because the electron-withdrawing anion
makes the complex more positively charged and it favors the
reduction of metal ion. Similarly the electron-donating groups
make the complexes less positively charged. Electropotential of
VO(IV)/VO(V) couple is showing sensitivity to the nature of atoms
in ligands moiety.
From the cyclic voltammetric studies it can be stated that the
complexes containing more aromatic rings get reduced at higher
negative potentials than the complexes containing less aromatic
rings. The higher reduction potential can be attributed due to the
greater planarity and electronic properties that are associated with
aromatic rings [66].
CV study shows that the complexes 5–8 exhibit the same elec-
trochemical behavior upon addition of CT DNA. For increasing
amounts of CT DNA, the cathodic potential Epc shows a positive
shift (
+47 mV for 7 and
Epa shift to negative values (
for 6,
the potentials show that complexes 5–8 can bind to DNA by both
intercalation and electrostatic interaction [67,68].
D
Epc = +33 mV for 5,
Epc = +16 mV for 8) while the anodic potentials
Epa = ꢀ127 mV for 5, Epa = ꢀ97 mV
DEpc = +44 mV for 6 (Fig. 3C), DEpc =
D
D
D
D
Epa = ꢀ21 mV for 7 and
D
Epa = ꢀ4 mV for 8). These shifts of
On comparing the cyclic voltammograms, we observe that the
variation in oxidation and reduction potential may be due to dis-
tortion in geometry that arises due to different metals in coordina-
tion and also the difference in E1/2, values which further support to
the involvement of number of co-ligands in coordination and nat-
ure of substituents on the ligands moiety.
Differential Pulse Voltammograms (DPV) of 2 and 6 (10ꢀ3 M) in
the absence and presence of varying addition of DNA are shown in
Fig. S5 (Supplementary file). It was observed that a significant
reduction in cathodic peak current of 2 and 6 decreased on the
addition of DNA is due to slow diffusion of an equilibrium mixture
of the free and DNA-bound complexes to the electrode surface and
its potential shifted after reacting with DNA, which was consistent
with the results of the cyclic voltammetry.
The decrease in the peak current of complex 2 and 6 resulted
from the addition of DNA into 2 and 6 solution in CV and DPV
can be employed to determine the binding constant (K). The bind-
ing behavior of 2 and 6 to DNA was measured by keeping the con-
centration of the 2 and 6 constant while varying the concentration
of the DNA by using the response of the one electron CV and DPV
step of the Cu(II)/Cu(I) and Cu(III)/Cu(II) for 2 and VO(IV)/VO(III)
for 6 during the addition of DNA solution. K+/K2+ value for the 2
suggesting that the preferential stabilization of Cu(II) form over
Cu(III) and Cu(I) forms on binding to DNA. DPV of the complexes
as a function of added DNA also indicates a large decrease in cur-
rent intensity with a small shift in formal potential due to the par-
tial intercalative interaction of complexes.
3.9.3. Viscosity measurements
Fig. 3. Cyclic voltammogram in DMF:buffer [mixture 50 mM Tris–HCl/NaCl buffer
A useful technique to prove intercalation is viscosity measure-
ments, which are sensitive to length change of DNA and regarded
as the least ambiguous and the most critical tests of binding mode
in solution in the absence of crystallographic structural data or
NMR spectra [69]. Under appropriate conditions, intercalation of
drugs like ethidium bromide (EB) causes a significant increase in
viscosity of DNA solution due to the increase in separation of base
pairs of intercalation sites and hence results in an increase in over-
all DNA contour length. On the other hand, drug molecules binding
(pH, 7.2)] (1:2) solution of (A) 1 (scan rate = 0.1 V sꢀ1), (B) 2 (scan rate = 0.08 V sꢀ1
)
and (C) 6 (scan rate = 0.12 V sꢀ1) with incremental addition of CT DNA. 0.05 M
n-Bu4NClO4 as supporting electrolyte and the arrow mark indicates the current
changes upon increasing DNA concentrations.
The redox potentials of the complexes are in the range (E1/2
)
ꢀ0.569 to ꢀ0.790 V, and are assigned to the vanadyl ion present
in the azomethine moiety. The more negative potentials of the

182
N. Raman, A. Selvan / Journal of Molecular Structure 985 (2011) 173–183
exclusively in the DNA grooves cause less pronounced or no
changes in DNA solution viscosity [70]. A classical intercalation
model demands that the DNA helix must lengthen as base pairs
are separated to accommodate the binding ligand, leading to the
increase of DNA viscosity. In contrast, a partial and/or non-classical
intercalation of ligand bend (or kink) the DNA helix, reduce its
length and, concomitantly, its viscosity [70].
Meanwhile, nature of the substituents present in the Schiff base li-
gands and central metal ions also affected the intercalative ability.
These results indicate that DNA might also serve as the primary
target of these compounds; in addition, they should have many po-
tential practical applications, just like the promising therapeutic
drug candidates. In the cyclic voltammograms of the complexes re-
corded in DMF, in all cases, quasi-reversible waves attributed to re-
dox couples, characteristic for each metal complex, have been
recorded at potentials expected for the central metal ions. The sub-
stituents at different positions on intercalative ligand can cause
some interesting differences in the properties of the resulting com-
plexes. On the basis of chelation theory, metal complexes have
more biological activity than the free ligands.
The effects of complexes 1–8, together with [Ru(bpy)3]2+ and EB
on the viscosity of rod-like DNA are shown in Fig. 4. EB increases
the relative specific viscosity for the lengthening of the DNA double
helix through the intercalation mode, while complex [Ru(bpy)3]2+
,
which has been known to bind with DNA in electrostatic mode, ex-
erts essentially no effect on DNA viscosity. Viscosity of CT DNA
(200
l
M) has been measured in the presence of varying amount
M), the relative viscosity of DNA in-
of complexes 1–8 (20–140
l
Acknowledgment
creased steadily, which is similar to the behavior of EB. In addition,
the relative viscosities of DNA increase with the order of
hnbdppo > dbdppo complexes. This order suggests the extent of
the unwinding and lengthening of DNA helix by compounds and
the affinities of compounds binding to DNA, which may be due
to the key roles of substituent effects and the larger coplanar struc-
ture of the complexes than those of ligands. Intercalation has been
traditionally associated with molecules containing bpy/phen ring
structures which can bind to DNA through intercalation. The re-
sults suggest that the above complexes intercalate the base pairs
of DNA, which is consistent with the result observed in absorption
titration curves.
The authors express their sincere thanks to the College Manag-
ing Board, Principal and Head of the Department of Chemistry,
VHNSN College for providing necessary research facilities and
financial support. Instrumental facilities provided by Sophisticated
Analytical Instrument Facility (SAIF) IIT Bombay and CDRI
Lucknow are gratefully acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.10.038.
4. Conclusions
References
Binary Schiff base and pby/phen complexes having essential
oxygen, azomethine and nitrogens were prepared and structurally
characterized. The correlation of the experimental data allows
assigning an octahedral geometry for Cu(II) and square-pyramidal
geometry for VO(IV) ions. The redox active Cu(II) and VO(IV) com-
plexes exhibit significant photo-induced DNA cleavage activity as
evidenced from their ability to convert SC pBR322 DNA to its linear
form upon excitation with UV-light irradiation at 365 nm. The DNA
cleavage reaction was found to follow a photo-redox mechanistic
pathway that generates reactive hydroxyl radical species. The bet-
ter binding properties of the complexes should be attributed to the
good coplanarity of the ligand after coordination with metal ions.
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