A Dinuclear Ruthenium(II) Schiff Base Complex with Dissimilar Coordination: Synthesis, Characterization, and Biological Activity

Article abstract of DOI:10.1002/zaac.201600008

A dinuclear Schiff base Ru^II complex derived from 5-chlorosalicylaldehyde and 2-aminopyridine was synthesized. The structure of the compound was analyzed by mass spectrometry as well as IR, UV/Vis, and ¹H NMR spectroscopy, along with

Full text of DOI:10.1002/zaac.201600008

Journal of Inorganic and General Chemistry
www.zaac.wiley-vch.de
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201600008
A Dinuclear Ruthenium(II) Schiff Base Complex with Dissimilar
Coordination: Synthesis, Characterization, and Biological Activity
[a]
[a]
[a]
[b]
Emira Kahrovi c´ ,* Adnan Zahirovi c´ , Emir Turku sˇ i c´ , and Sabaheta Bekta sˇ
Keywords: Schiff bases; Ruthenium; Dinuclear complex; CT DNA; Antimicrobial activity
Abstract. A dinuclear Schiff base RuII complex derived from 5-
chlorosalicylaldehyde and 2-aminopyridine was synthesized. The
structure of the compound was analyzed by mass spectrometry as well
imine and Et
neutral complex species have different coordination environments,
RuN Cl and RuN Cl. Interaction with CT DNA showed moder-
NH = isodiethylamine]. The RuII atoms in the dinuclear
2
3
O
2
2 3
O
1
as IR, UV/Vis, and H NMR spectroscopy, along with chemical analy-
ate hydrophobic binding. The compound demonstrates strong activity
sis, as well as magnetic, cyclovoltammetric and conductivity measure-
ments. Two Ru atoms are octahedrally coordinated by azomethine tive Staphylococcus aureus, and especially Enterococcus faecalis.
against methicillin-resistant Staphylococcus aureus, methicillin-sensi-
II
and pyridine nitrogen atoms from two tridentate monobasic Schiff
bases and bridging phenol oxygen atoms. The formula of the complex
is [Ru L Cl (Et NH)(H O)] [L = N-(2-pyridyl)-5-chlorosalicylidene-
2 2 2 2 2
Microbiological tests showed significant inhibition of growth and
ability to kill pathogens, similar or even improved compared to refer-
ence antibiotics vancomycin.
Introduction
Experimental Section
Materials: Most chemicals were obtained from commercial sources
The design, synthesis, and applications of ruthenium com-
plexes have attracted special attention over recent decades for
and used without further purification. Tetraethylammonium perchlorate
[5]
was prepared from bromide salt as described previously. Fibrous calf
many reasons. From the synthetic point of view ruthenium is thymus DNA (A260/A280 = 1.3) was purified with phenol-chloroform-
intriguing due to its wide range of oxidation states and corre- isoamyl alcohol extraction up to A260/A280 = 1.8 ratio. DNA was pre-
sponding redox properties, which can be adjusted by various cipitated as sodium salt with ethanol in 0.1 m acetate buffer (pH 4.60).
Solid nucleic acid was suspended in 0.1 m Tris-HCl buffer, pH 7.40,
ligands. In typical oxidation states, +2 and +3, ruthenium as
and left one day for hydration at 4 °C before measurements. Gram-
soft and borderline-soft acid prefer nitrogen donor ligands.
positive and Gram-negative bacteria were collected from the Micro-
Many ruthenium complexes are widely studied due to their
biology Laboratory of the Institute of Public Health of Canton Sara-
jevo.
[1,2]
significant catalytic properties.
One of the main avenues of
research have focused on the anticancer activity of ruthenium
complexes, which exhibit improved selectivity and reduced Physical Measurements: Elemental analysis was performed with a
side effects in comparison to platinum, present in clinical prac- Perkin-Elmer 2400 Series CHNS/O Analyzer. Ruthenium content was
tice.[3,4]
determined by graphite furnace atomic absorption spectrometry with a
The development of new drugs is a permanent and
[
6]
Varian 240ZAA from DMSO solution. Chlorine was determined by
mercurimetric method. Mass spectrum was recorded with a matrix-
assisted laser desorption / ionization-time-of-flight MALDI-TOF/TOF
mass spectrometer (4800 Plus MALDI TOF / TOF Analyzer, Applied
Biosystems Inc., Foster City, CA, USA) equipped with Nd : YAG
laser, wavelength of 355 nm, with firing rate 200 Hz in positive ion
reflector mode. 1600 shots per spectrum were acquired in m/z range
growing global challenge thus integrating different fields of
science. It is important to have in mind that for few decades
not any substantially new antibiotic has been found.
As a part of the efforts to synthesize ruthenium complexes
with Schiff bases with potential biological activity, the synthe-
sis, characterization, and antimicrobial activity of the dinuclear
II
Ru complex, [Ru L Cl (Et NH)(H O)] [L = N-(2-pyridyl)-5- 10–1000 Da with delay time 300 ns. Thiamine mononitrate and az-
2
2
2
2
2
chlorosalicylideneimine] is reported herein. To the best of our ithromycin were used as internal calibrants. Proton NMR spectrum
II
knowledge, this is the first dinuclear Ru complex with bridg- was collected with a 300 MHz Bruker BioSpin GmbH instrument in
–
1.0 to 19 ppm range with 256 scans using [D
TMS as internal standard. Spectrum was analyzed using SpinWorks
.1.8.1. Infrared spectra were collected with a Perkin-Elmer BX FT-
6
]aceton as solvent and
ing Schiff bases through phenol oxygen.
3
–
1
IR spectrophotometer as KBr pallets in the region 4000–400 cm .
Electronic spectra were recorded with a Perkin-Elmer UV/Vis lambda
35 spectrophotometer in CH Cl solution in the range 200–700 nm.
2 2
Magnetic measurements were performed with a SQUID magnetometer
MPMS-XL5 (Quantum Design) at 300 K. Cyclic voltammograms were
recorded with a Autolab potentiostat / galvanostat (PGSTAT 12) elec-
trochemical workstation using three electrode system: glassy carbon
*
Prof. Dr. E. Kahrovic´
E-Mail: emira_kahrovic@yahoo.com
[
[
a] Department of Chemistry
Faculty of Science
University of Sarajevo
Sarajevo, 71000, Bosnia and Herzegovina
b] Institute for Public Health of Canton Sarajevo
Sarajevo, 71000, Bosnia and Herzegovina
Z. Anorg. Allg. Chem. 2016, 642, (6), 480–485
480
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ARTICLE
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electrode as working, Ag/AgCl as reference electrode and Pt wire as
counter electrode. Recordings were performed with scan rate 0.1 V·s
and step potential 0.025 V in the range of potentials –1.0 to 0.0 V for
nas aeruginosa. Turbidity of bacterial suspensions was set before test-
ing to be equivalent to 0.5 McFarland. Activity was tested by disc-
agar diffusion method. Pathogens were inoculated on Mueller-Hinton
–
1
dimethylformamide/sodium perchlorate solutions and –1.1 do –0.5 V agar and the holes were made by sterile Durham’s tubes. Volume of
–
1
for acetonitrile /tetraethylammonium perchlorate. The conductivity of
50 μL of Ru complex in DMSO solution containing 1.5 mg·mL was
inserted into drilled wells. Diameter (mm) zone of inhibitions of bacte-
rial growth were measured after 24 h of incubation at 37 °C. Vancomy-
cin and gentamicin were used as antibiotic controls. For extended anti-
microbial study minimum inhibitory concentration (MIC) and mini-
mum bactericidal concentration (MBC) values were determined.
–3
10 m solution of complex was measured in dimethylformamide solu-
tion on Conductivity meter Phywe.
Synthesis of Schiff Base: Schiff base, N-(2-pyridyl)-5-chlorosalicylid-
[
7]
eneimine, was prepared according to published procedure. Warm
solution of 2-aminopyridine (5 mmol, 471 mg) in absolute ethanol
II
Decreasing concentrations of the Ru compound were prepared in se-
rial two fold dilution from stock solution of complex in dimethylsulf-
(10 mL) was added to solution of 5-chlorosalicylaldehyde (5 mmol,
780 mg) in absolute ethanol (10 mL). The mixture was kept at 70 °C
–1
oxide (1.5 mg·mL ).
for 2 h. The resulting solution was slowly evaporated overnight, at
ambient temperature. Orange crystals were collected with yield of
Results and Discussion
60%.
Synthesis of Complex: Warm solution containing RuCl
3
·3H
2
O
Characterization of Complex Compound
(
0.5 mmol, 130 mg) in absolute ethanol (5 mL) was added to absolute
The ruthenium complex, hereinafter [Ru L Cl (Et NH)-
2
2
2
2
ethanol solution containing Schiff base (1 mmol, 232 mg) and triethyl-
amine (1 mmol, 0.14 mL). The reaction mixture was heated at 70 °C
for 4 h changing color from brown to dark green. The volume was
(
H O)]·H O, where L represents Schiff bases derived from 5-
2 2
chlorosalicyladehyde and 2-aminopyridine (Et NH = dieth-
2
reduced by distillation under reduced pressure. The solution was ylamine) was prepared according to Figure 1.
cooled in ice-salt bath. The solid was collected, washed with small
Deprotonation of the phenolic oxygen atom from Schiff base
portions of cold water, ethanol and ether and dried at 60 °C. was carried out using triethylamine. On the basis of chemical
Recrystallization was performed from ethanol : dichloromethane, 1/1
analysis, different spectroscopic techniques, and magnetic
v/v. Dark green solid is soluble in dimethylformamide (DMF), dimeth-
ylsulfoxide (DMSO), acetonitrile (MeCN), moderately soluble in
methanol and ethanol and insoluble in water and ether.
II
measurements, a dinuclear Ru structure with bridging Schiff
II
bases is proposed. In the dinuclear Ru complex species, each
ruthenium atom in octahedral environment is coordinated by
chloride and tridentate monobasic O,N,N Schiff base, whereas
Aqua-1κO-dichlorido-1κCl,2κCl-diethylamine-2κN-bis[N-(2-pyrid-
2
yl-κN)-5-chloro-2-(μ-oxy-1:2κ O)-benzylidenimine-κN(1-)]diruthe- the sixth positions are occupied by different ligand, water, or
III
nium(II,II): Dark green powder. Yield: 60%. C28
H31Cl
4
N
5
O
4
Ru
2
:
diethylamine. Reduction of starting Ru might be the result
calcd. C 39.77; H 3.70; N 8.29; Cl 16.77; Ru 23.91%; found: C 39.59; of oxidative decomposition of triethylamine to acetaldehyde
H 3.00; N 8.10; Cl 17.04; Ru 23.59%. MALDI-TOF MS: m/z:
and methylamine, similar to decomposition with hexacyanofer-
rate(III).
+
+
560.9712 ([C24
H16Cl
2
N
4
O
2
Ru] ), 737.8099 ([C24
H16Cl
4
N
4
O
2
Ru
2
] ).
[8]
IR (KBr): ν˜ max = 1602 vs νsym(C=N), 1289 m νsym(C–O), 1013 m
–
1
]: λMLCT/nm (log[ε /M–1 cm ]): 435
–1
δ
(
[
2
6
bend(C
2
1
N) cm . UV/Vis [CH
2
Cl
3.97). H NMR (300 MHz, [D ]acetone): δ = 10.05 [s, 2 H(4)], 7.81 Mass and H NMR Spectra
d, 2 H(3), J = 2.61 Hz], 7.53–7.62 [m, 12 H: 2 H(2), 2 H(1), 2 H(5),
2
1
6
4
3
The mass spectrum of the complex showed isotopic distribu-
H(6), 2 H(7), 2 H(8)], 5.61 [s, 1 H(9)], 3.45 [q, 4 H(10), J =
.44 Hz], 2.82 [s, H(12)], 1.11 [t, 6 H(11), J = 6.42 Hz] ppm.
3
tion characteristic for ruthenium species at m/z value 737.8099
1
confirming molecular formulation of [C H Cl N O Ru ]. H
24
16
4
4
2
2
Biological Studies: Spectroscopic titration of complex compound with
CT DNA was performed by successive addition of 10 μL-portions of
DNA into 2 mL of complex compound solution. Each addition of DNA
was compensated in the blank. Titration was carried out in 0.1 m Tris-
HCl buffer, pH 7.42 in the presence of 150 mM NaCl at 295 K. Anti-
bacterial in vitro activity of Ru complex was tested against methicillin-
resistant Staphylococcus aureus (MRSA), methicillin-sensitive Staphy-
NMR spectra prove binding of Schiff base as anionic tridentate
ligand. The absence of a singlet in high frequencies region
(10–12 ppm), which typically corresponds to phenolic
hydrogen, indicates that N-2-pyridyl-5-chlorosalicylideneimine
binds ruthenium by deprotonated phenolic oxygen. The singlet
that appears at δ = 10.05 ppm is ascribed to azomethine
lococcus aureus (MSSA), Enterococcus faecalis, Klebsiella pneumon- hydrogen, which is less shielded after coordination through
iae (wild type), Klebsiella pneumoniae (ESBL type), and Pseudomo- azomethine nitrogen. Typical doublet located at δ = 7.81 ppm
2 2 2 2 2
Figure 1. Synthesis of [Ru L Cl (Et NH)(H O)].
Z. Anorg. Allg. Chem. 2016, 480–485
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1
Figure 2. H NMR spectrum of [Ru
2
L
2
Cl
2
(Et
2
NH)(H
2
O)] in (CD
3
)
2
CO. Inset: Proposed structure and denotation of different types of hydrogen
atoms.
is ascribed to hydrogen atoms denoted as H(3) in Figure 2. The Stretching vibration of C–O(H) bond, found in free Schiff base
4
–1
–1
meta coupling constant JH(3)H(2), 2.61 Hz, confirms coupling at 1278 cm , was moved to 1289 cm in complex species as
of H(3) with H(2) atoms. The multiplet found in the spectrum a result of deprotonation and coordination to ruthenium. Schiff
of the complex in the 7.53–7.62 ppm region corresponds to base coordination through pyridine nitrogen affected bending
coupled hydrogen atoms of aromatic rings. A singlet at δ = deformation in plane, therefore the frequency in free ligands at
–1
5
.61 ppm was found and attributed to amine hydrogen H(9) 452 cm–1 was moved to a higher value and appears at 469 cm
II
confirming coordination of diethylamine to Ru . In free dieth- in the spectrum of the ruthenium complex. A new band in the
ylamine this singlet is located at lower values, around 2 ppm. spectrum of complex compound appears at 824 cm , which
A shift to higher frequencies for 3.5 ppm is a result of de- is ascribed to Ru–O–Ru stretching, and confirms O-bridging
–1
[
9]
shielding of amine hydrogen atoms after coordination of dieth- character of N-(2-pyridyl)-5-chlorosalicylideneimine.
ylamine to Ru atom by nitrogen. A quartet of methylene
II
The presence of diethylamine in the coordination sphere,
II
hydrogen is located at δ = 3.45 ppm in the complex compared attached to one of the two Ru atoms, was proven by C N
2
to 2.60 ppm in the free ligand. A methyl hydrogen triplet at δ frequency at 1013 cm–1 which occurs at 1037 cm in free di-
–1
=
1.11 ppm is not affected by coordination due its distance ethylamine. Well-resolved absorption in the complex around
1
–1
from nitrogen atom. H NMR spectroscopic data are in good 3200 cm is attributed to N–H stretching, whereas this fre-
agreement with other experimental results and strongly support quency occurs at 3288 cm–1 in free diethylamine. Triethyl-
the proposed structure of the complex. The H NMR spectrum amine, which was used in synthesis, has no absorption in the
1
of [Ru L Cl (Et NH)(H O)]·H O is presented in Figure 2.
region of N–H stretching. Strong and broad absorption around
3
2
2
2
2
2
2
435 cm–1 was attributed to water O–H stretching frequencies
(Table 1)
Infrared and Electronic Spectra
In the electronic spectrum of [Ru L Cl (Et NH)(H O)]·H O
2
2
2
2
2
2
The infrared spectrum of [Ru L Cl (Et NH)(H O)]·H O three ligand-centered transitions of coordinated Schiff base ap-
2
2
2
2
2
2
II
confirms coordination of Ru by tridentate anionic ON chelat- pear in the region 236–351 nm (bands I–III). These bands are
2
ing Schiff base, diethylamine, and water. The azomethine attributed to spin allowed πǞπ* and nǞπ* (charge transfer in
–
1
group appears in the complex at 1602 cm compared to whole ligand molecule) transitions. Bands II and III are shifted
–
1
1611 cm
in free N-(2-pyridyl)-5-chlorosalicylideneimine. toward higher energy after coordination therefore confirming
Table 1. IR frequencies and electronic spectrum transitions for [Ru
2
L
2
Cl
2
(Et
2
NH)(H
2
O)] and the ligand.
N)
IR spectrum
νsym(N–H)
ν
sym(C=N)
ν
sym(C–O)
δ
bening(C
2
δbending(Ru
2
O)
δin plane(py)
–
1
Complex (free ligand) /cm
3
200
1602
(1611)
1289
(1278)
1013
(1037)
824
(–)
469
(452)
(3180)
Electronic spectrum
Band I
Band II
Band III
MLCT
t
2g6Ǟπ*
λ /nm (log ε)
236 (4.44)
293 (4.20)
351 (3.82)
438 (3.97)
696 (0.33)
Z. Anorg. Allg. Chem. 2016, 480–485
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ARTICLE
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binding of ligand through O,N,N-donor atoms. Compared to
free ligand, two additional bands were found in spectrum of
complex. The absorption at 438 nm is attributed to MLCT
(metal ligand charge transfer), based on the position and ex-
tinction coefficient. A very poor but well defined absorption
band in the spectrum of complex at 696 nm is assigned to spin
1
1
6
II
allowed A Ǟ T (t2g Ǟπ*) transition of Ru in strong li-
1
g
1g
gand field. The electronic spectrum of [Ru L Cl (Et NH)-
2
2
2
2
(
H O)]·H O is shown on Figure 3.
2 2
Figure 4. Cyclic voltammograms of [Ru
2 2 2 2 2 2
L Cl (Et NH)(H O)]·H O in
Figure 3. Electronic spectrum of [Ru
2
L
2
Cl
2
(Et
2
NH)(H
2
O)]·H
2
O
DMF/NaClO and MeCN/Et NClO
4
4
4
.
4ϫ10– m) and
5
L
Cl .
2
=
N-(2-pyridyl)-5-chlorosalicylideneimine
(
(
2ϫ10–5 m) in CH
2
Table 2. Cyclic voltammetry for [Ru
2
L
2
Cl
2
(Et
2
NH)(H
2
2
O)]·H O.
Magnetic Susceptibility, Electrochemical Properties, and
Conductivity
Solvent
DMF
E
c
/V
E
a
/V
ΔE /V
E
1/2 /V
–0.875
–0.950
–0.349
–0.750
0.526
0.200
–0.612
–0.850
The magnetic susceptibility provides an insight into the im- MeCN
portant characteristics of complex compounds, especially elec-
tronic state of metal atoms. Also, susceptibility can suggest
the degree of metal-metal interactions in polynuclear complex
Quite negative values of half-wave potentials E1/2
=
species. Based on starting RuCl used in the synthesis, a posi-
3
5
–0.612 V in dimethylformamide and –0.850 V in acetonitrile
tive value of magnetic susceptibility arising from t ground
electronic state for Ru is expected. In contrast, experimental
2
g
III
II
III
are ascribed to Ru /Ru redox couple. The presence of oxy-
II
gen atoms in Ru complexes moves the redox potential toward
magnetic susceptibility (χ ) at 300 K for the ruthenium com-
ρ
II
plex has a negative value –1.79ϫ10– cm g strongly sup-
porting the presence of diamagnetic t2g Ru metal centers.
The conductivity of 1 mM solution of [Ru L Cl (Et NH)-
6
3
–1
more negative values as a result of modest affinity of Ru
toward oxygen atoms. In the case of mononuclear Ru com-
plex species with similar coordination core, e.g. RuCl N O
2 2
III
6
II
2
2
2
–
2
2
1
and RuN
negative values.
Ru L Cl (Et NH)(H O)]·H O stabilizes the complex toward
2 4
O , corresponding half wave-potentials have more
(
H O)]·H O in dimethylformamide is 20.4 μS cm , compared
2 2
[11,12]
–
1
The presence of O-bridging atoms in
to 223 μS cm for sodium perchlorate as reference electrolyte,
thus confirming non-electrolytic nature of neutral complex
species.
[
2
2
2
2
2
2
oxidation compared to corresponding mononuclear species.
Different ligands in coordination sphere can dramatically
change redox properties of central metal atoms, which are cru-
cial in numerous applicative fields like catalysis and especially
in design of metal-based drugs. Authentic examples are
Interaction with CT DNA
DNA is almost inevitably anticipated as a main target for
3
+
2+
[
Ru(NH ) N ] and [Ru(NH ) OH] having redox potentials metal-base drugs, along with enzymes and signaling pathways.
3 5 2 3 5
+
1.10 V and –0.08 V, respectively.[ Cyclic voltammograms Therefore, the study of metal complexes-DNA interactions
10]
of [Ru L Cl (Et NH)(H O)]·H O were recorded in dimethyl- might be the first indicator of biological activity of potential
2
2
2
2
2
2
formamide and acetonitrile (Figure 4).
drug. Metal complexes are able to bind DNA by covalent
Cathodic and anodic peaks are well defined in acetonitrile mode, or non-covalent intercalative and external groove bind-
due to poor solvation properties. Voltammograms show quasi- ing mode.[ Spectroscopic study of interaction of [Ru L Cl -
13]
2
2
2
III
II
reversible one-electron Ru /Ru process based on position, (Et NH)(H O)]·H O with CT DNA was performed by titration
2
2
2
ratio of cathodic and anodic peaks, and peak-to-peak separa- of fixed concentration of complex compound with increasing
tions. Electron transfer is more reversible in acetonitrile with concentrations of calf thymus DNA (Table 3). The values of
ΔE = 0.200 V compared to significantly higher peak-to-peak the binding constant, K were calculated on the basis of the
b
equation:[
14]
separation ΔE = 0.526 V in dimethylformamide (Table 2).
Z. Anorg. Allg. Chem. 2016, 480–485
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Table 3. Spectroscopic titration data of complex with CT DNA.
V
DNA /μL [Complex]
f a
[DNA]/[complex] [DNA]/(ε –ε )
–5
–8
2
/
10
M
/10 M cm
0
6.03
6.00
5.97
5.94
5.91
5.88
5.85
0.00
0.51
1.01
1.50
2.00
2.48
2.97
–
1
2
3
4
5
6
0
2.68
5.59
6.75
9.53
12.3
14.6
0
0
0
0
0
[
DNA] [DNA]
1
=
f a f b b f b
+
(ε – ε ) (ε – ε ) K (ε – ε )
b
Figure 6. Graphical determination of K ; regression is based on data
from Table 3.
where ε , ε , and ε correspond to extinction coefficients for
a
f
b
particular measurements (Aobs/[DNA]), free complex and com-
pletely bound form, respectively. The result of titrations in the Staphylococcus aureus (MSSA), Enterococcus faecalis, Kleb-
CT band region showed hypochromic effect suggesting non- siella pneumoniae (wild type), Klebsiella pneumoniae (ESBL
covalent binding to DNA with moderate binding constants K
type), and Pseudomonas aeruginosa.
However the complex showed activity only against Gram-
b
4
–1
=
3.82ϫ10 m (Figure 5). The binding constant was ob-
tained as the ratio of the slope and intercept from graphical positive bacteria. Zone of inhibition for MRSA and MSSA
dependence [DNA] / (ε – ε ) vs. [DNA] (Figure 6.) No shift strains of Staphylococcus aureus (24 and 20 mm, respectively)
f
a
of absorption maximum was observed; therefore the interac- are close to vancomycin as a reference antibiotic, while ac-
tion is attributed to groove binding.
tivity against Enterococcus faecalis exceeds vancomycin zone
inhibition. For further evaluation of antimicrobial activity,
MIC and MBC values were determined. MRSA and MSSA
were very sensitive to the tested complex with low MIC and
–1
MBC values (1.46 and 2.92 μg·mL , respectively). It is inter-
esting that [Ru L Cl (Et NH)(H O)]·H O has the same MIC
2
2
2
2
2
2
–1
value as vancomycin (0.73 μg·mL ) in the case of Enterococ-
cus faecalis, whereas the MBC for tested ruthenium drug
0.36 μg·mL ) demonstrate much stronger bactericidal ac-
–1
(
–1
tivity compared to vancomycin (0.73 μg·mL ).
The lack of the activity against Gram-negative bacteria sug-
gests that the activity of this complex is probably based on
inhibition of cell wall synthesis of tested pathogens. The differ-
ence in structure and composition of cell wall for Gram-posi-
tive bacteria, having thick murine layer of peptidoglycan, and
Gram-negative bacteria with thin peptidoglycan layer covered
by outer lipid membrane, seems to be responsible for different
activity of the tested Ru complex. In Gram-positive bacteria
the murine layers of peptidoglycans can hinder the passage of
hydrophobic compounds due to sugars and amino acids, or can
be chemically changed by drugs resulting in inhibition of cell
wall synthesis. Since tested Ru complex with Schiff bases con-
2 2 2 2 2
Figure 5. Spectroscopic titration of [Ru L Cl (Et NH)(H O)]
(
6.03ϫ10–5 m) with CT DNA (4.90ϫ10–3 m, A260/A280 = 1.81) in
0
5
.1 m Tris-HCl buffer, pH 7.42 and 150 mM NaCl; T = 295 K; t =
min.
Antimicrobial Activity
Many metal complexes have been studied in the light of tain (i) two chlorides, which are able to hydrolyze and release
their antimicrobial activity over last decades especially since two positions on ruthenium for further binding especially O-
pathogens are permanently developing resistance on existing and N-donor group, and (ii) moderate hydrophobic aromatic
drugs. Despite the remarkable increase in understanding of the part of Schiff bases, it is reasonable to assume that chemical
molecular mechanisms of antimicrobial resistance, an increase changes of unprotected peptidoglycan layer for S. aureus and
of antimicrobial-resistant bacterial species is evident. The spe- E. faecalis occur. The inhibitory effect of tested complex was
cial concern is ability of Enterococci and Staphylococci to found to be bactericidal, meaning that agent caused the death
adapt on different environments. Reports on S. aureus, having of pathogens (MBC/MIC ratio = 1–2).[ Results based on
full resistance to vancomycin (VRSA), have urgently opened disc diffusion method for studied complex and reference anti-
a new era in the development of bactericidal antibiotics. biotic vancomycin against Gram-positive bacterial strains, hos-
15]
[
Ru L Cl (Et NH)(H O)] was tested against methicillin-resist- pital acquired methicillin-resistant Staphylococcus aureus
2 2 2 2 2
ant Staphylococcus aureus (MRSA), methicillin-sensitive (HA-MRSA), methicillin-sensitive Staphylococcus aureus
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Table 4. Antibacterial activity of complexes against MSSA, MRSA, and Enterococcus faecalis.
Compound
Diameter of zone of inhibition /mm
MIC μg mL–1 / MBC μg mL–1
MSSA
MRSA
Enterococcus faecalis
MSSA
MRSA
Enterococcus faecalis
Complex
Vancomycin
20
28
24
28
29
28
2.92 / 2.92
0.73 / 0.73
1.46 / 1.46
0.730.73
0.73 / 0.36
0.73 / 0.73
(
MSSA), and Enterococcus faecalis ATCC 29212 are given in References
Table 4.
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Acknowledgements
The authors acknowledge Dr. Jovan Blanu sˇ a for his support with mag-
Received: January 5, 2016
netic susceptibility measurements.
Published Online: March 30, 2016
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© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim