Copper(II)-N-hydroxy-N,N′-diarylformamidine complexes: Synthesis, crystal structures, antibacterial and molecular docking studies

Article abstract of DOI:10.1016/j.jinorgbio.2021.111600

A series of Cu(II) complexes were synthesized by using N-hydroxy-N,N′-diarylformamidine ligands: N-hydroxy-N,N′-(phenyl)formamidine (L1), N-hydroxy-N′-(4-methylphenyl)formamidine (L2), N-hydroxy-N,N′-(2,6-dimethylphenyl)formamidine (L3), N-hydroxy-N,N′-(2

Full text of DOI:10.1016/j.jinorgbio.2021.111600

Journal of Inorganic Biochemistry 225 (2021) 111600
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Copper(II)-N-hydroxy-N,N′-diarylformamidine complexes: Synthesis,
crystal structures, antibacterial and molecular docking studies
,
Wisdom A. Munzeiwaa b, Segun D. Oladipoa, Collins U. Ibejic, Chunderika Mocktard,
,
*
Bernard Omondie
a School of Chemistry and Physics, Westville Campus, University of Kwazulu-Natal, Private Bag X54001, Durban 4000, South Africa
b Chemistry Department, Bindura University of Science Education, P Bag 1020, Bindura, Zimbabwe
c Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka, Nigeria
d Discipline of Pharmaceutical Sciences, School of Health Sciences, University of Kwazulu-Natal, Private Bag X54001, Durban 4000, South Africa
e School of Chemistry and Physics, Pietermaritzburg Campus, University of Kwazulu-Natal, Private Bag X01, Scottsville 3209, South Africa
A R T I C L E I N F O
A B S T R A C T
A series of Cu(II) complexes were synthesized by using N-hydroxy-N,N′-diarylformamidine ligands: N-hydroxy-N,
N′-(phenyl)formamidine (L1), N-hydroxy-N′-(4-methylphenyl)formamidine (L2), N-hydroxy-N,N′-(2,6-dime-
thylphenyl)formamidine (L3), N-hydroxy-N,N′-(2,6-diisopropylphenyl)formamidine (L4). Reaction of ligands
L1–L4 with hydrated copper acetate furnished mononuclear Cu(II) complexes 1–4 with general formula [Cu-
(L)2]. The molecular structures of complexes 3 and 4, as determined by single crystal X-ray diffraction, showed
both to have square planar geometry with a near C2 symmetry. The antimicrobial potency of all four complexes
was evaluated against three gram-(–) bacteria (S. typhimurium, P. aeruginosa, and E. coli) and two gram-(+)
bacteria (Methicillin-resistant S. aureus (MRSA) and S. aureus), with ciprofloxacin as the reference drug. All tested
complexes were inactive against gram-(+) bacteria strains except for complex 1, which displayed excellent ac-
tivity when compared to the reference. Molecular docking studies showed that hydrogen bonding, pi-sigma and
van der Waals interactions are prominent complex-protein connections, with complex 2 displaying good binding
affinities with the studied biological targets.
Keywords:
Cu(II) complexes
N-hydroxy-N,N′-diarylformamidine
Antimicrobial
Molecular docking
1. Introduction
antifungal [15,16] and anticancer agents [17,18]. The biological activ-
ities of N-hydroxyl-N′-diarylformamidines have been reported [19,20].
The continuous threats posed to human health by infectious diseases
and the rapid development of multi-drug resistant microbial pathogens
have in the last decade caused serious global health concerns [1,2]. By
the year 2050, it is anticipated that the formidable challenge posed by
increased antimicrobial resistance to known clinical drugs will have
caused a total of 10,000,000 deaths from infectious disease [2,3]. An
urgent global action plan has been flagged by the World Health Orga-
nisation (WHO), which calls for all countries to take measures against
drug-resistant microbes and work towards the discovery of safer and
efficacious new antimicrobial drugs [4,5]. Such drugs should have
different mechanisms of action from those of well-known antimicrobial
agents, to which relevant pathogens are resistant [6,7].
In this study they are used as ligands because we envisaged that their
biological activities would be enhanced upon chelation with Cu(II) ions.
It is known that the formation of metal chelates would increase their
lipophilic character, thereby easing the permeability of these complexes
through lipid layers of cell membranes [21]. Recently, our research
group have reported the biological activities of metal complexes derived
from both symmetrical and unsymmetrical N,N′-diarylformamidine de-
rivatives [22,23]. To advance these studies, we have introduced a hy-
droxyl group (-OH) to the formamidines so as to afford metal complexes
with nitrogen and oxygen binding atoms as bidentate systems, and we
investigate if this might enhance their biological activities. Our interest
stems from ligands with O, N or S binding atoms being frequently found
in molecules of biological interests [24].
Copper(II) metal complexes synthesized from different ligands have
in the past been tested as antimicrobial agents [8–11]. They have also
been tested as corrosion inhibitors [12,13], antioxidants [14],
We, therefore, report the synthesis, structural characterization, in
vitro antibacterial and computational studies of Cu(II)-N,N′-
* Corresponding author.
E-mail address: owaga@ukzn.ac.za (B. Omondi).
https://doi.org/10.1016/j.jinorgbio.2021.111600
Received 6 March 2021; Received in revised form 26 August 2021; Accepted 28 August 2021
Available online 3 September 2021
0162-0134/© 2021 Elsevier Inc. All rights reserved.

W.A. Munzeiwa et al.
Journal of Inorganic Biochemistry 225 (2021) 111600
Table 1
Summary of X-ray crystal data collection and structure refinement parameters
for complexes 3 and 4.
3
4
Empirical formula
C34H38CuN4O2
598.22
C50H70CuN4O2
822.64
Fig. 1. N-hydroxy-N,N′-diarylformamidine ligands employed in the synthesis of
Formula weight
complexes reported herein.
T(K)
173(2)
173(2)
λ(Å)
0.71073
Monoclinic
P21/c
0.71073
Crystal system
Triclinic
diarylformamidine complexes. The antibacterial potential of each of the
metal complexes was evaluated against gram-positive bacterial strains
Methicillin-resistant S. aureus (MRSA) and S. aureus and gram-negative
bacterial strains S. typhimurium, E. coli, and P. aeruginosa.
Space group
P-1
a (Å)
b (Å)
c (Å)
a,
8.6554(2)
8.4414(2)
20.6115(6)
90
10.5284(3)
10.8147(3)
11.7799(4)
109.5950(10)
105.149(2)
99.928(3)
1168.75(6)
1
β,
91.4330(10)
90
2. Experimental section
γ (◦)
V (Å3)
Z
1505.48(7)
2
2.1. Materials
ρ
calc (mg/m3)
1.320
1.169
μ
(mmꢀ 1
)
0.762
0.508
All experiments were carried out under argon, 5.0 technical grade,
(Airflex Industrial Gases, South Africa) using Schlenk techniques. All
solvents were obtained from Sigma-Aldrich. Reagent grade absolute
ethanol (98%) was distilled and dried from magnesium turnings;
dichloromethane (DCM) (99%) and hexane (98%) were dried from a
sodium benzophenone mixture. The reagents, Cu(OAc)2.H2O (98%), and
3-chloroperoxybenzoic acid (m-CPBA) (77%), were also obtained from
Sigma-Aldrich. Anhydrous MgSO4 (98%), NaOH (99%), anhydrous
NaHCO3 (97%) and anhydrous K2CO3 (99%) were obtained from Pro-
mark Chemicals, South Africa.
F(000)
630
443
Crystal size (mm)
0.320 × 0.230 ×
0.140
0.240 × 0.220 ×
0.130
θ range for data collection (◦)
1.977 to 27.514
ꢀ 11 ≤ h ≤ 9
ꢀ 10 ≤ k ≤ 10
ꢀ 22 ≤ l ≤ 26
19,352
1.954 to 27.492
ꢀ 13 ≤ h ≤ 13
ꢀ 13 ≤ k ≤ 14
ꢀ 15 ≤ l ≤ 12
17,575
Index ranges
Reflections collected
Independent reflections
3425 [R(int) =
0.0269]
5235 [R(int) =
0.0173]
Completeness to theta = 25.24◦
(%)
99.6
99.9
Data/restraints/parameters
3425/0/191
1.062
5235/0/267
1.063
Goodness-of-fit (GOF) on F2
2.2. Instrumentation
Final R indices [I ˃2
σ
(I)]
R1 = 0.0285
wR2 = 0.0760
R1 = 0.0331
wR2 = 0.0781
0.338 and ꢀ 0.428
R1 = 0.0284
wR2 = 0.0737
R1 = 0.0309
wR2 = 0.0753
0.365 and ꢀ 0.307
The melting point of the complexes was recorded using Electro-
thermal (9100) digital melting point apparatus. IR spectra were ob-
tained from a PerkinElmer Universal ATR spectrum 100 FT-IR
spectrometer. Mass spectra of the complexes were obtained from a
Water synapt GR electrospray positive spectrometer. Elemental analyses
were recorded on a Vario elemental EL cube CHNS analyzer.
R indices (all data)
Largest diff. peak and hole (e
Åꢀ 3
)
by slow evaporation of the solvent, the desired products were obtained
as solids.
3. General synthesis methods
3.2.1. [Cu-(L1)2] (1)
3.1. Synthesis of N-hydroxy-N,N′-diarylformamidine ligands
The reaction of ligand L1 (0.30 g, 0.619 mmol) and Cu(OAc)2.2H2O
(0.087 g, 0.495 mmol) in ethanol furnished complex 1 as a brown
Amidine (1.0 mmol) was dissolved in DCM and then solid sodium
hydrogen carbonate (1.0 mmol) was added and the mixture cooled to
0 ◦C. Thereafter, m-CPBA (1.2 mmol) in DCM was added dropwise and
the reaction mixture was allowed to warm to room temperature with
stirring for a further 1 h. The reaction mixture was then washed with a
solution of potassium carbonate (5%; 2 × 25 mL) and the combined
organic fractions were dried over anhydrous magnesium sulphate and
filtered. The solvent was then removed by evaporation to afford N-hy-
droxy-N,N′-(phenyl)formamidine (L1), N-hydroxy-N,N′-(4-methyl-
powder. Yield 79%. Melting point: decomposes above 195 ◦C. IR
υ
(cmꢀ 1): 3018 (w), 2922 (w), 1608 (s), 1586 (s), 1466 (m), 1362 (w),
1298 (w), 1205 (m), ESI-TOF MS: m/z (%) 508.0912 (100) [M + Na]+.
Elemental analysis for C26H22CuN4O2 (%): calculated C 64.25, H 4.56, N
11.53; found: C 64.33, H 4.85, N 11.49.
3.2.2. [Cu-(L2)2] (2)
The reaction of ligand L2 (0.30 g, 0.555 mmol) and Cu(OAc)2.2H2O
(0.084 g, 0.454 mmol) in ethanol furnished complex 2 as a brown
phenyl)formamidine
formamidine (L3),
(L2),
N-hydroxy-N,N′-bis(2,6-dimethyl)
powder. Yield 71%. Melting point: decomposes above 200 ◦C. IR
υ
N-hydroxy-N,N′-bis(2,6-diisopropylphenyl)for-
(cmꢀ 1): 3018 (w), 2990 (w), 1623 (s), 1614 (s), 1456 (m), 1390 (w),
1302 (w), 1206 (m), ESI-TOF MS: m/z (%) 541.1745 (100) [M + Na]+.
Elemental analysis for C30H30CuN4O2 (%): calculated: C 66.46, H 5.58,
N 10.33; found: C 66.69, H 5.83, N 10.38.
mamidine (L4) in the respective reactions. Ligand L1 was obtained as an
oil whilst L2, L3 and L4 were obtained as white solids (Fig. 1).
3.2. Synthesis of Cu(II) complexes
3.2.3. [Cu-(L3)2] (3)
Copper(II) acetate (1 mmol) was dissolved in water and the pH
adjusted to 8.0 using 1 M NaOH solution. Thereafter, a solution of the
ligand (2.0 mmol) in aqueous ethanol (90%) was added. In each case, a
precipitate was formed immediately, and the reaction mixture was
stirred at room temperature for 8 h. Deionized water (100 mL) was
added and then the temperature lowered to 4 ◦C with stirring for a
further 2 h. The resultant solids were collected by filtration, washed first
with hot water and then with aqueous ethanol (50%). The complexes
were then dissolved in DCM, dried with anhydrous MgSO4, filtered and
The reaction of ligand L3 (0.30 g, 0.788 mmol) and Cu(OAc)2.2H2O
(0.076 g, 0.394 mmol) in ethanol furnished complex 3 as a brown
powder. Yield 76%. Melting point: decompose above 205 ◦C. IR
υ
(cmꢀ 1): 3018 (w), 2918 (w), 1608 (s), 1583 (s), 1466 (m), 1390 (w),
1296 (w), 1205 (m), ESI-TOF MS: m/z (%) 621.32(100) [M + Na]+.
Elemental analysis for C34H38N4O2Cu (%): calculated: C 63.05, H 6.50,
N 8.40. found: C 62.79, H 6.83, N 8.82.
2

W.A. Munzeiwa et al.
Journal of Inorganic Biochemistry 225 (2021) 111600
3.2.4. [Cu-(L4)2] (4)
The reaction of ligand L4 (0.30 g, 0.919 mmol) and Cu(OAc)2.2H2O
(0.078 g, 0.394 mmol) in ethanol furnished complex 4 as a brown
powder. Yield 76%. Melting point: decomposes above 238 ◦C. IR v 3064
(w), 2960 (s), 2867 (w), 1664 (m), 1620 (s), 1461(m), 1326 (w), 1290
(w), 1254 (w). ESI-TOF MS: m/z (%) 844.6 (100) [M + Na]+. Elemental
analysis for C50H70CuN4O2: C (%): calculated: 73.00, H 8.58, N 6.81;
found: C 73.15, H 8.53, N 6.78.
3.3. Single-crystal X-ray diffraction
i
Scheme 1. Synthesis of Cu(II) N-hydroxy-N,N′-diarylformamidine complexes.
Crystal evaluation and data collection for all samples were done on a
Bruker Smart APEXII diffractometer with Mo Kα radiation (I = 0.71073
Å), equipped with an Oxford Cryostream low-temperature apparatus
operating at 100 K. Reflections were collected at different starting angles
and the APEXII program suite was used to index the reflections [25].
Data reduction was performed using the SAINT [26] software and the
scaling and absorption corrections were applied using the SADABS [27]
multi-scan technique. The structures were solved by the direct method
using the SHELXS program and refined using SHELXL program [28].
Graphics of the crystal structures were drawn using OLEX2 software
[29]. Non‑hydrogen atoms were first refined isotropically and then by
anisotropic refinement with the full-matrix least-squares method based
on F2 using SHELXL [28]. The crystallographic data and structure
refinement parameters for complexes 3 and 4 are given in Table 1.
Table 2
IR azomethine (C=N) symmetry stretch frequency for ligands and complexes,
respectively.
Complex
IR ʋ(C=N) cmꢀ 1
Ligand
Complex
Δv
1
2
3
4
1608
1623
1620
1612
1586
1614
1610
1608
22
9
10
4
investigate the inhibitory potentials via calculated binding energies. The
3D crystal structures of proteins were retrieved from the protein data
bank (PDB). The proteins of S. aureus, E. coli, P. aeruginosa were iden-
tified with their PDB codes 2DHN (2.2 Å resolution) [32], 1wxh (1.97 Å
resolution) [33], 2w7q (1.88 Å resolution) [34], respectively. The grid
box size was determined using AutoDock tools [35] 1.5.4 for the binding
site as derived from the corresponding reference complexes and the
dimension applied for docking was X = 24 Y = 24 Z = 24 with 1.00 Å as
the grid spacing [36]. Gasteiger charges were added using the AutoDock
Tools graphical-user-interface from MGL Tools [37]. The Lamarckian
genetic algorithm was applied in the search for the optimum binding site
for the ligands. The ligands were optimized before docking using
Gaussian 09 [38], to achieve the global minimum.
3.4. In vitro antimicrobial studies
The antimicrobial activity of the Cu(II) complexes 1–4 were evalu-
ated against three gram-negative bacteria, viz: S. typhimurium ATCC
14026, P. aeruginosa ATCC 27853, and E. coli ATCC 25922, and two
gram-positive bacteria, viz: Methicillin-resistant S. aureus (MRSA) ATCC
700699 and S. aureus ATCC 25923. Ciprofloxacin was used as a standard
antibiotic for comparison while dimethyl sulfoxide (DMSO) was used as
a negative control, in which it showed no antibacterial activity against
any of the bacterial strains used for this study at the different concen-
trations. The samples were prepared by dissolving 1000 μg of the test
sample in 1 mL of DMSO. The bacteria were inoculated onto nutrient
agar (NA) (Biolab, South Africa) plates using the streak plate technique
and incubated at 37 ◦C for 18 h [30]. A single colony was isolated and
inoculated into 10 mL sterile nutrient broth (NB) (Biolab, South Africa).
This was incubated at 37 ◦C for 18 h in a shaking incubator (100 rpm).
The concentration of each bacterial strain was adjusted with sterile
distilled water to achieve a final concentration equivalent to 0.5
McFarland Standard (i.e 1.5 × 108 cfu/mL) using a densitometer
(McFarland Latvia) [31]. Thereafter, the Mueller–Hinton agar (MHA)
plates were lawn inoculated with the diluted bacteria using a sterile
4. Result and discussion
4.1. Synthesis of N-hydroxy-N,N′-diarylformamidine ligands and their Cu
(II) complexes
The ligands L1 – L4 were synthesized via one-step oxidation of N,N′-
diarylformamidine precursors [39] using slight modifications of a
method in the literature [40]. Bis-ligated copper complexes [Cu-(L1)2]
(1), [Cu-(L2)2] (2), [Cu-(L3)2] (3), [Cu-(L4)2] (4) were obtained as
brown solids with excellent yield (75–84%) (Scheme 1) by reacting
copper acetate and the ligands in 2:1 metal:ligand ratio.
throat swab. After 5 μL of each sample had been spotted onto the MHA
plates, the plates were incubated at 37 ◦C for 18 h and then assessed for
antibacterial activity, which was denoted by a clear zone at the point of
spotting. Samples that showed antimicrobial potential during antibac-
terial screening were tested further to determine their minimum inhib-
itory concentration (MICs). In this determination, the samples were
serially diluted 10 times to achieve concentrations ranging from 1000
◦
◦
The complexes decomposed between 195 C and 245 C, with the
trend being influenced by substituents on the phenyl ring. The micro-
analytical data was consistent with the molecular structure, which
showed a metal:ligand ratio of 1:2. This was further complemented by
mass spectrometry data for the complexes with spectra exhibiting m/z
signals corresponding to the parent complex as sodium adducts
(Fig. S1a–d).
μ
g/mL to 0.2
0.2 g/mL, the solutions were further diluted serially 5 times to achieve
concentrations ranging from 0.100 g/mL to 0.00625 g/mL. Then 5 μL
μg/mL. For the samples where MICs had been lower than
μ
μ
μ
of each sample at the different concentrations was spotted onto the MHA
plates and the plates were incubated at 37 ◦C for 18 h and then assessed
for their MIC [32]. These tests were done in triplicate and the MIC was
determined as the lowest concentration of the complexes at which no
visible bacterial growth could be observed after incubation.
4.2. Spectroscopy studies
4.2.1. Fourier transform infrared (FT-IR) spectroscopy
The IR spectra of complexes 1–4 showed a general shift of the azo-
methine (C(H)=N) symmetric vibrations to lower frequencies compared
to ligands, which alludes to the participation of the imine nitrogen in
–
–
3.5. Molecular docking method
metal coordination. For example, the C N symmetric stretching vi-
brations in complex 1 appeared at 1612 cmꢀ 1 as compared to 1619 cmꢀ 1
in ligand L1. The characteristic positive shifts are a result of the
The molecular docking technique was carried out in this study to
3

W.A. Munzeiwa et al.
Journal of Inorganic Biochemistry 225 (2021) 111600
Fig. 4. X-ray crystal structure of complexes 3 and 4 with thermal ellipsoids
drawn at 50% probability level and hydrogen atoms having been omitted
for clarity.
Table 3
Selected bond lengths and angles for complexes 3 and 4.
3
4
Bond lengths [Å]
M—N
1.9138(10)–1.9385(12)
1.9138(10)
1.9350(10)–1.9351(10)
1.9185(9)
M—O
Fig. 2. Solid state EPR spectrum of complexes 3 and 4 (295 K, 9.786GHz).
C—N
1.9385(12)
1.3135(16)–1.3119(16)
Bond angles [◦]
O—M—N(cone)
O—M—O
83.59(5)–96.41(5)
180.0
95.96(4)–84.04(4)
180.0
N—M—N
180.0
180.0
Torsion angles [◦]
C—N—O—M
N—C—N—M
2,6-R-Ph-(NO)
ꢀ 1.58(15)
0.23(17)
ꢀ 0.40(12)
0.34(14)
60.21(17)
93.69(13)
Table 4
Minimum inhibitory concentration of the metal complexes (
μg/mL).
Fig. 3. Magnetization behaviour of complexes 3 and 4.
Complexes
Gram (ꢀ ) bacteria
Gram (+) bacteria
E. coli
S. typhimurium
P. aeruginosa
S. aureus
MRSA
migration of the imidine bridge
π
-electron density towards the metal
–
1
0.20
0.80
6.25
100
NA
NA
NA
NA
0.40
0.10
0.20
1000
1000
0.80
6.25
NA
NA
NA
25
12.50
NA
centre conferring partial bond character on the C N bond, leading to
–
2
vibration at lower frequencies [42]. The summarized data of shifts for
3
NA
other complexes are shown in Table 2.
4
NA
Ciprofloxacina
0.20
25
4.2.2. Electron paramagnetic resonance (EPR) studies of complexes 3 and
4
NA = No activity
a
Standard.
To further infer the solid-state electronic structure of the copper
complexes, X-band EPR spectra of selected complexes were acquired at
295 K (Fig. 2). The EPR spectra of complexes 3 and 4 are perfectly
isotropic with a single line (g = 2.1082). This infers that there is a
completely symmetric environment where the electrons in separate d-
orbitals interact in all directions (identical g-factors). The broad signals
and slight deviation of the g-factors from the free electron value
(2.0023) points to the d(x2-y2) orbital (B1g) ground state occupancy by
the unpaired Cu(II) electrons. The g-values are comparable to those of
other square-planar complexes reported in literature [41]. The EPR
spectra also showed a resolved hyperfine structure in the perpendicular
section due to the interaction of metal electrons with nitrogen atoms.
The spectra for all the complexes are devoid of ms = ±2 transitions a half
field signal ruling out any meaningful Cu⋅⋅⋅Cu interactions.
lies between 10.35 and 25.9 Ms. These low values are characteristic of a
soft magnet and strong short-range correlation.
4.4. X-ray structural analysis
Perspective views of complexes 3 and 4 are illustrated in Fig. 4 and
selected bond lengths and angles are recorded in Table 3. Complexes 3
and 4 are supported by symmetrical N,N-hydroxy formamidine ligands
and good quality crystals of both complexes were obtained by slow
diffusion of diethyl ether into their saturated dichloromethane solution.
The asymmetric units of both complexes contain half complex mole-
cules, with the other half being generated through inversion centres. The
complex molecule contains a metal ion that is bis-ligated via the imine
nitrogen and hydroxyl oxygen, and the acetate auxiliary ligands are
completely excluded from the coordination sphere. The donor atom
connectivity results in penta-metallacycles, which are similar to other N,
O bidentate ligands [42]. The Cu atoms also adopt a square planar ge-
ometry with bond angles of 83.59(5) – 96.41(5)◦. Steric repulsion be-
tween the bulky 2,6-iPr groups resulted in greater tilt angles in 4 as
4.3. Magnetic studies of complexes 3 and 4
The paramagnetic nature of the Cu(II) complexes was further
confirmed by magnetic studies. Fig. 3 shows almost linear hysteresis
loops and magnetization does not reach saturation, even at higher
applied magnetic field. This is because of the paramagnetic nature of Cu
(II) ions and their magnetic moments being aligned with the magnetic
field. The coercivity values range from 21.8–89.7 Hci and magnetization
–
–
compared to 3. The Cu O and Cu N bond distances are almost com-
parable in both complexes, and they lie between 1.9139(10) and 1.9384
(10) Å. Similar values have been reported in the literature for related
4

W.A. Munzeiwa et al.
Journal of Inorganic Biochemistry 225 (2021) 111600
Table 4 and Fig. 5. The in vitro antibacterial evaluation based on MIC
values showed that some of the complexes could inhibit the growth of all
the tested bacteria strains effectively, except S. typhimurium. All the
complexes were inactive against gram-(+) bacteria strains (S. aureus and
MRSA) except complex 1, which displayed excellent activity when
compared to ciprofloxacin (reference drug) with MIC values of 6.25
μg/
mL and 12.50 g/mL against S. aureus and MRSA (Fig. 5). This showed
μ
that complex 1 is 3-fold and 2-fold more potent against S. aureus and
MRSA, respectively, when compared to ciprofloxacin. We found
S. typhimurium was non-susceptible to all the complexes. The failure of
the complexes 2–4 to inhibit S. aureus and MRSA strains and all the
complexes to inhibit S. typhimurium could be as a result of the inability of
the compounds to penetrate through the bacterial cell wall or the
complexes might have been modified or rendered inactive as they
entered the cell wall [45]. Complexes 3 and 4 displayed the least activity
against P. aeruginosa, being effective at only the highest concentration
(1000 μg/mL), whilst complexes 1 and 2 displayed better activity rela-
Fig. 5. Minimum inhibitory concentration (MIC) of the metal complexes vs
tive to ciprofloxacin. Complexes 1 and 2 are 3-fold and 2-fold, repec-
tivley, more active than the reference drug. All the complexes were
active against E.coli, with 2, 3 and 4 showing moderate activity and
complex 1 having the same activity as the reference drug (ciprofloxa-
cin). Among all the complexes, we can speculate that complex 1 showed
a broad spectrum of effectiveness and the presence of alkyl substituents
did not increase the antimicrobial activity of the complexes.
bacteria. *The blank spaces represent no activity (NA) and 10 for MIC values of
≥12.5 μg/mL.
Table 5
Binding free energies (∆G) in kcal/mol of complexes with different targets ob-
tained using AutoDock 1.5.4.
Complex
S. aureus
E. coli
P. aeruginosa
4.6. Molecular docking
1
ꢀ 5.4
ꢀ 7.4
5.3
ꢀ 5.3
ꢀ 5.0
5.1
ꢀ 5.2
ꢀ 5.4
5.2
2
Ciprofloxacina
The docking studies simulation was carried out to give insight into
the inhibitory potential of metal complexes to target proteins via their
binding affinities. The binding energies of complexes 1 and 2 obtained
give insight into the thermodynamical feasibility of the different binding
interactions. As displayed in Table 5, complex 2 showed the highest
binding affinity with S. aureus compared to 1 and the reference drug.
Complexes 1 and 2 showed reasonable activity to E. coli and
P. aeruginosa compared to the reference drug. The docking simulation
obtained in this study displayed hydrogen bonding, pi-sigma, and van
structures [43,44].
4.5. Antimicrobial activities evaluation
Complexes 1–4 together with ciprofloxacin as a standard were
screened against five bacteria strains and the results are presented in
Fig. 6. Pictorial description of the 3-D complex 1 (A) in the binding pocket of E. coli and 2-D, (B) The interactions with amino acid residues inside the active pocket
of E. coli.
5

W.A. Munzeiwa et al.
Journal of Inorganic Biochemistry 225 (2021) 111600
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jinorgbio.2021.111600.
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Declaration of Competing Interest
There are no conflicts of interest.
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The authors acknowledge the College of Agriculture, Science and
Engineering, University of KwaZulu-Natal (UKZN), and the National
Research Foundation (NRF), South Africa for financial support and
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