Cationic Au(I) complexes with aryl-benzothiazoles and their antibacterial activity

Article abstract of DOI:10.1016/j.jinorgbio.2018.05.003

Two cationic Au(I) complexes derived from aryl-benzothiazoles, namely [(PPh₃)Au(pbt)](OTf) (1) and [(PPh₃)Au(qbt)](OTf) (2) (where pbt = 2?(pyridyl)benzothiazole and qbt = (quinolyl)benzothiazole, and OTf^? = trifluorometha

Full text of DOI:10.1016/j.jinorgbio.2018.05.003

JournalofInorganicBiochemistry185(2018)80–85
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Cationic Au(I) complexes with aryl-benzothiazoles and their antibacterial
activity
Jenny Stenger-Smith, Indranil Chakraborty, Pradip K. Mascharak⁎
Department of Chemistry, University of California, Santa Cruz, CA 95064, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aryl-benzothiazols
Gold(I) complexes
Antibacterial activity
Gram-negative bacteria
Skin and soft tissue infection (SSTI)
Two cationic Au(I) complexes derived from aryl-benzothiazoles, namely [(PPh3)Au(pbt)](OTf) (1) and
[(PPh3)Au(qbt)](OTf) (2) (where pbt = 2‑(pyridyl)benzothiazole and qbt = (quinolyl)benzothiazole, and
OTf− = trifluoromethanesulfonate anion), have been synthesized and structurally characterized by X-ray
crystallography. Both complexes exhibit strong antibacterial effects against Gram-negative bacteria such as
Acinetobacter baumannii and Pseudomonas Aeruginosa. Results of examination of the reactions of 1 and 2 indicate
that these cationic Au(I) complexes rapidly cross the bacterial membrane and exert drug action by disrupting
cellular function(s) through binding of cytosolic thiol-containing peptides (such as glutathione) and proteins to
the highly reactive (PPh3)Au+ intermediate formed upon in situ dissociation of pbt or qbt.
1. Introduction
been synthesized and studied in vitro. The accumulation and in-depth
analysis of such data have led scientists to infer that the types of ligands
Rapid emergence of antibacterial resistance to common antibiotics
has raised alarm in hospitals around the globe. Infections and diseases
that were thought to be well controlled by antibiotics are reappearing
with resistance to traditional drug therapies [1]. In parallel, other mi-
croorganisms such as parasites, fungi, and viruses are also exhibiting
similar characteristics [2]. Many resistant strains have appeared in
hospitals where antibiotics are being used and administered as routine
procedures [3]. The once life-saving drugs like β-lactums that doctors
relied upon to keep many bacterial infections at bay are now com-
pletely ineffective [4]. Unfortunately, the pace at which new antibiotics
are emerging in the market is not nearly fast enough to combat such
resistance [5].
Although gold compounds have been used in medicine for centuries,
interest in such compounds was noticeably intensified following the
discovery by Robert Koch who showed that K[Au(CN)2] had activity
against Mycobacterium tuberculosis, the causative agent of tuberculosis
[6,7]. Shortly after this discovery, reports on gold compounds as an-
ticancer, antimicrobial, and antiarthritic agents started appearing in
scientific literature and such research led to several gold drugs for
commercial use [6–10]. Additional investigation into new gold com-
pounds for clinical use was however greatly diminished following the
discovery of antibiotics. Now that resistance is on the rise and the
discovery of new antibiotics is comparatively slow [5], reexamination
of the bioactivity of new gold compounds could be very relevant.
In recent years a relatively large number of Au(I) compounds have
in the gold compounds play a significant role in the effectiveness of the
compounds toward eradication of specific bacteria [6]. Interestingly,
compounds independently used as drugs, when used as ligands to Au(I),
lead to new gold compounds exhibiting improved antibacterial efficacy.
Positively-charged species are in general more effective in associating
with mostly electronegative bacterial cell walls [11]; close interactions
between the cell wall and drug molecules lead to interruptions in var-
ious cellular pathways that often result in microbial death.
For some time, we have been exploring the antibacterial properties
of heavy metal complexes derived from benzothiazoles, a class of an-
tibacterial and antifungal drugs [12,13]. Our work in such pursuit have
shown that Ag(I) complexes of benzothiazole-type ligands act as anti-
microbial agents [14]. In these complexes, 2‑(pyridyl)benzothiazole
(pbt) and 2‑(quinolyl)benzothiazole (qbt) are bound to the Ag(I) center
as bidentate N,N-coordinated fashion to give rise to tetrahedral geo-
metry (Scheme 1). The overall positive charge of these complexes
presumably leads to stronger interactions with the bacterial cell walls
and give rise to their antibacterial activity. These results prompted us to
explore the coordination characteristics of pbt and qbt to Au(I) centers
and the antibacterial activity of the Au(I) complexes derived from them.
In this account we report the synthesis, spectroscopy, and structural
characterization of two gold(I) compounds derived from pbt and qbt
namely, [(PPh3)Au(pbt)](OTf) (1) and [(PPh3)Au(qbt)](OTf) (2). As
shown in Figs. 2 and 3, both ligands bind as monodentate N-donors to
the Au(I) center. The other ligand in both complexes is
⁎
Corresponding author.
E-mail address: pradip@ucsc.edu (P.K. Mascharak).
https://doi.org/10.1016/j.jinorgbio.2018.05.003
Received 24 February 2018; Received in revised form 4 May 2018; Accepted 8 May 2018
Availableonline11May2018
0162-0134/PublishedbyElsevierInc.

J. Stenger-Smith et al.
JournalofInorganicBiochemistry185(2018)80–85
2.3. X-ray crystallography
Colorless and light yellow block-shaped crystals of complexes
1.0.5CH2Cl2 and 2 respectively were obtained by recrystallization
through diffusion of hexanes into their dichloromethane (CH2Cl2) so-
lutions. In case of 1, a suitable crystal was selected and mounted on a
Bruker D8 Quest diffractometer equipped with PHOTON II detector
operating at T = 298 K. Data were collected with ω shutterless scan
technique using graphite monochromated Mo-Kα radiation
(λ = 0.71073 Å) In case of 2, a suitable single crystal was selected and
mounted on a Bruker APEX-II CCD diffractometer with graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å). In this case the
crystal was also kept at T = 298 K during data collection and unit cell
determination. Data were measured using ω scan technique. The total
number of runs and images for both data collections was based on the
strategy calculation from the program APEX3 (Bruker) [17]. The max-
imum resolution achieved was θ = 28.4° for 1 and θ = 24.2° for 2. Cell
parameters were retrieved using the SAINT (Bruker) software [18] and
refined using SAINT (Bruker) on 9525 reflections for 1 and on 8496
reflections for 2. Data reduction was performed using the SAINT
(Bruker) software, which corrects for Lorentz polarization. The final
completeness is 99.6% out to 28.4° in θ for 1 and 98.8% out to 24.2° in
θ for 2. Multi-scan absorption corrections were performed with both
data sets using SADABS 2016/2 and SADABS 2014/5 respectively for 1
and 2 [19]. The absorption coefficient for 1 is 4.88 mm−1 and for 2 is
4.29 mm−1. Minimum and maximum transmissions for 1 are 0.499 and
0.746 and the corresponding values for 2 are 0.573 and 0.745. The
structures of 1 and 2 were solved in the space group C2/c (No. 15) and
Pbca (No. 61) respectively by intrinsic phasing using the ShelXT [20]
structure solution program and refined by full matrix least squares on
F2 using version 2016/6 of ShelXL [21]. All non‑hydrogen atoms were
refined anisotropically in both cases. Hydrogen atom positions were
calculated geometrically and refined using the riding model. In case of
1, there are two crystallographically independent molecules within the
asymmetric unit, while for 2 one full molecule is present in the asym-
metric unit. Calculations and molecular graphics were preformed using
SHELXTL 2014 and Olex2 [22] programs. Crystal data and structure
refinement parameters are included in Table 1 while the bond distances
and angles are listed in Table 2.
Scheme 1. Structures of the Ag complexes derived from pbt and qbt.
triphenylphosphine. The antibacterial properties of 1 and 2 against two
Gram-negative bacteria namely, Acinetobacter baumannii and Pseudo-
monas aeruginosa have been evaluated using a skin and soft tissue in-
fection (SSTI) model previously developed in our laboratory.
2. Experimental methods
2.1. Materials and methods
All reagents and solvents were of commercial grade and used
without further purification. (PPh3)AuCl and AgOTf were procured
from Sigma. The ligands pbt [15] and qbt [16] were synthesized ac-
cording to reported procedures. FTIR, UV–Vis, and emission spectra
were obtained using Perkin-Elmer Spectrum-One, Varian Cary 50, and
Agilent Cary Eclipse spectrophotometers respectively. The 1H-, 19F and
31P NMR spectra of the ligands and the complexes were recorded using
a Varian Unity Inova 500 MHz instrument at 298 K.
2.2. Synthesis of complexes
2.2.1. Synthesis of [(PPh3)Au(pbt)](OTf) (1)
To a solution of AgOTf (54.8 mg, 0.213 mmol) in 10 mL of methanol
was added a solution of (PPh3)AuCl (100.5 mg, 0.203 mmol) in 15 mL
of chloroform. After stirring for 30 min the white AgCl precipitate was
filtered through a bed of celite. To the filtrate was added a solution of
pbt (43.1 mg, 0.203 mmol) in 10 mL of chloroform and the mixture was
set to reflux for 18 h. The solution was again filtered through celite to
remove traces of black particles. The volume of the filtrate was then
reduced to approximately 4 mL and 15 mL of hexane was added. The
white solid thus formed was collected by filtration and dried in vacuo
(131.0 mg, 78.6% yield). Layering hexanes over a dichloromethane
(CH2Cl2) solution of this solid afforded colorless crystals of 1. Anal.
Calcd for C31H23AuN2O3PS2F3: C, 45.37; H, 2.83; N, 3.41; found: C,
45.48; H, 2.79; N, 3.37. IR (KBr, cm−1): 3468 (w), 3056 (w), 1459 (w)
1436 (m), 1267 (s), 1154 (m), 1032 (m), 763 (m), 695 (m), 546 (m). 1H
NMR (CDCl3, ppm): 8.62 (d, 1H), 8.40 (d, 1H), 8.25 (t, 1H), 8.12 (d,
1H), 8.01 (d, 1H), 7.83 (t, 1H), 7.63–7.55 (m, 17H). 31P NMR (CDCl3,
ppm from PPh3): 35.90.
Crystal data for complex 1 (CCDC 1824282) and 2 (CCDC 1824283)
have been included in the Supplementary data section. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request.cif.
2.4. Bacterial studies
The SSTI model previously developed in our lab [23] was employed
for antimicrobial studies. This uses a soft upper agar layer of evenly-dis-
persed bacterial “lawn” at the top of a nutrient rich bottom agar layer.
Such arrangement allows the bacteria to move slowly into the bottom
layer following the nutrient gradient, much like ditching of surface bac-
teria into the inner layers of skin and soft tissue. Different dilutions of A.
baumannii and P. aeruginosa were employed to grow ideal lawns in these
SSTI models. For A. baumannii, a frozen stock of bacteria was first streaked
on an LB plate and incubated for 18 h. A single colony of bacteria was
selected and grown in LB broth for another 18 h. The suspension was di-
luted with fresh LB until an A600 of 0.8 was reached. A batch of 100 mL of
0.8% (w/v) agar with 1% NaCl was prepared, autoclaved and cooled to
47 °C before addition of 80 μL of the diluted bacterial suspension. This
solution was gently vortexed and aliquots of 8 mL of it were spread evenly
over of the surface of six 100 × 15 mm2 plates prepared with 20 mL of
1.5% (w/v) TSB agar (hard nutrient-rich layer). The plates were then in-
cubated at 37 °C for 2 h to facilitate adhesion of the bacteria to the nu-
trient-rich bottom layer and cell-to-cell contact. For P. aeruginosa, the same
procedure was followed to prepare the SSTI model. Here, the bacterial
suspension in LB medium was diluted to an A600 of 0.5 and 120 μL of it
2.2.2. Synthesis of [(PPh3)Au(qbt)](OTf) (2)
The same procedure as above using qbt as the ligand. Complex 2
was isolated as a light yellow solid (60.1 mg, 75.0%). Layering hexanes
over a solution of 2 in dichloromethane afforded pale yellow crystals of
[(PPh3)Au(qbt)](OTf). Anal. Calcd for C35H25AuN2O3PS2F3: C, 48.28;
H, 2.89; N, 3.22; found: C, 48.02; H, 2.91; N, 3.12. IR (KBr, cm−1):
3436 (w), 3056 (w), 1436 (w), 1263 (s), 1156 (m), 1031 (m), 762 (m),
696 (m), 545 (m). 1H NMR (CDCl3, ppm): 8.76 (d, 1H), 8.52 (d, 1H)
8.23 (d, 1H), 8.19 (d, 1H), 8.06 (d, 1H) 7.69–7.53 (m, 19 H) 7.46 (t,
1H).
81

J. Stenger-Smith et al.
JournalofInorganicBiochemistry185(2018)80–85
Table 1
Crystal data and structure refinement parameters for 1 and 2.
1. 0.5 CH2Cl2
2
Formula
C31.5H24ClF3N2O3S2PAu
C35H25F3N2O3S2PAu
Dcalc./g cm−3
μ/mm−1
Formula weight
Color
1.777
4.875
863.03
Yellow
Block
1.601
4.285
870.62
Yellow
Block
Shape
T/K
298(2)
Monoclinic
C2/c
47.014(2)
8.7125(4)
36.5473(18)
90
120.4450(10)
90
12,905.9(11)
8
298(2)
Orthorhombic
Pbca
9.780(2)
22.679(5)
32.560(7)
90
90
90
7222(3)
8
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
Fig. 1. Emission spectra of 1 (λex 330 nm) and 2 (λex 350 nm) compared to free
ligands in dichloromethane (λex 310 and 335 nm for pbt and qbt respectively).
γ/°
V/Å3
Z
Wavelength/Å
Radiation type
2θmin/°
2θmax/°
Measured Refl.
Independent Refl.
Reflections used
Rint
0.71073
Mo-Kα
5.674
56.712
123,645
16,063
12,893
0.0386
802
0.71073
Mo-Kα
4.706
48.376
40,970
5736
4345
0.0487
424
3. Results and discussion
3.1. Synthesis and spectroscopic properties
The addition of 1.05 eq of AgOTf to (PPh3)AuCl afforded the highly
reactive ion-pair AuPPh3+.OTf− [24] which was then further reacted
with 1 eq of pbt or qbt under refluxing condition to obtain 1 and 2
respectively (Eq. (1)).
Parameters
aGooF
1.125
0.1120
0.0492
1.070
0.1008
0.0428
cwR2
bR1
a
GOF = [Σ[w(Fo2 − Fc2)2]/(No − Nv)]1/2 (No = number of observations,
Nv = number of variables).
b
(1)
R1 = Σ∣∣Fo∣ − ∣Fc∣∣/Σ∣Fo∣.
wR2 = [(Σw(Fo2 − Fc2)2/Σ∣Fo∣2)]1/2
.
c
The IR spectra of both 1 and 2 show a strong band around
1265 cm−1 corresponding to the presence of the OTf− counter ion.
Both complexes exhibit significant luminescence quenching in organic
solvents when compared to the free ligand at the same concentration
(Fig. 1). Similar partial quenching has been observed with [Ag(pbt)2]
BF4 and [Ag(qbt)2]BF4 [14]. However, in the Ag(I) complexes, both
ligands are bound in a bidentate fashion while in 1 and 2, the ligands
act as monodentate N-donors. The wavelength of emission for 1 and 2
are 380 nm and 400 nm respectively which are the wavelengths of
emission for the corresponding free ligands pbt and qbt. A close scru-
tiny of the literature reveals that both the singlet and triplate excited
states of Ag(I) and Au(I) complexes of similar coordination structures
essentially possess ligand-centered ππ* character with negligible metal-
to-ligand charge transfer (MLCT) contribution, and the excited triplet
state mostly decay through non-radiative channels [14,25]. These
characteristics presumably lead to significant luminescence quenching
observed with the pbt and qbt complexes.
Table 2
Selected bond distances (Å) and angles (°) for complex 1.CH2Cl2 and 2.
Complex 1
Molecule 1
Au(1)-P(1)
P(1)-C(13)
P(1)-Au(1)-N(2)
Au(1)-P(1)-C(19)
C(13)-P(1)-C(25)
Au(1)-N(2)-C(12)
2.227(13)
1.810(6)
166.50(15)
110.91(18)
107.5(3)
Au(1)-N(2)
N(2)-C(6)
Au(1)-P(1)-C(13)
Au(1)-P(1)-C(25)
Au(1)-N(2)-C(6)
C(6)-S(1)-C(7)
2.116(4)
1.292(7)
113.77(19)
112.66(19)
125.8(4)
90.4 (3)
122.3(4)
Molecule 2
Au(2)-P(2)
P(2)-C(29)
P(2)-Au(2)-N(4)
Au(2)-P(2)-C(42)
C(55)-P(2)-C(49)
Au(2)-N(4)-C(42)
2.221(14)
1.816(6)
163.26(19)
110.08(19)
107.9(3)
Au(2)-N(4)
N(4)-C(36)
Au(2)-P(2)-C(49)
Au(2)-P(2)-C(55)
Au(2)-N(4)-C(36)
C(37)-S(2)-C(36)
2.112(6)
1.319(9)
113.5(2)
113.7(2)
124.6(6)
90.9 (4)
123.0(5)
3.2. Crystal structure description
Complex 2
Single crystal analysis reveals that complexes 1 and 2 are iso-
structural with respect to coordination at the Au center; in both cases
the Au center resides in a linear N-Au-P coordination environment. For
complex 1, the asymmetric unit contains two crystallographically in-
dependent molecules. The perspective view with atom labeling scheme
for the two structures are shown in Figs. 2 and 3. The N(benzothiazo-
le)‑Au‑P angles in complexes 1 and 2 are 164.88(17) and 161.20(15)°
respectively. Steric bulk of the qbt ligand leads to lengthening of both
Au(1)-P(1)
P(1)-C(17)
P(1)-Au(1)-N(2)
Au(1)-P(1)-C(29)
C(17)-P(1)-C(23)
Au(1)-N(2)-C(11)
2.303(18)
1.881(7)
161.20(15)
110.2(2)
105.5(3)
125.5(4)
Au(1)-N(2)
N(2)-C(10)
Au(1)-P(1)-C(17)
Au(1)-P(1)-C(23)
Au(1)-N(2)- C(10)
C(10)-S(1)-C(16)
2.218(5)
1.359(8)
117.0(2)
111.6(2)
121.9(4)
91.1(3)
was added to the 100 mL of soft agar solution. The SSTI models were
prepared using 7 mL of the soft agar solution spread evenly over the sur-
face of the hard agar layer. After preparation the plates and initial in-
cubation of 2 h, the KBr pellets containing the Au(I) complexes were
placed on all SSTI models and incubated for 18 h to evaluate the anti-
bacterial activity of the Au(I) compounds.
the average Au-N(benzothiazole) distance (2.114(5)
Å for 1 vs.
2.218(5) Å) for 2) and the average Au-P distance (2.224(14) for 1 vs.
2.303(18) Å for 2) in complex 2. Although Cambridge structural da-
tabase revealed several crystal structures with N-Au-P coordination
mode, a representative structurally analogous complex namely, [(py)Au
82

J. Stenger-Smith et al.
JournalofInorganicBiochemistry185(2018)80–85
Fig. 4. Different modes of binding of pbt and aryl-benzothiazole.
3.3. Binding modes of 2‑(aryl)benzothiazoles
2‑aryl or 2‑pyridyl substituted benzothiazole type ligands (like pbt
and qbt) typically exhibit bidentate binding modes when coordinated to
d10 metal center. This can occur through the N-atom of the ben-
zothiazole moiety and either the N-atom of the 2‑pyridyl fragment [14]
or C-atom of the 2‑aryl portion [27] of the ligand (Fig. 4a and b re-
spectively). Similar N,N-binding of pbt and qbt has been noted in
complexes with low-spin d6 metal centers such and Mn(I) and Re(I)
[16,28–30]. In all cases, the ring S-atom does not coordinate.
When this type of ligands were employed for coordination to Au(I)
centers in this research, the ligands failed to coordinate as bidentate
ligands. Attempts to coordinate pbt and qbt to Au(I) center invariably
afforded linear 2-coordinated complexes 1 and 2. Despite strong affinity
of Au(I) to S-donors, both ligands are coordinated to the metal center
through the N-atom of the benzothiazole (N(2)) moiety. This is in
agreement with results from similar studies exploring the binding
modes of Au(I) to ligands with electronically similar N and S (or N)
donor atoms [31,32]. Mono-substituted binding mode of these types of
ligands (leading to linear complexes) also arises from steric hindrance
arising from the ligand itself [25]. Both the present complexes deviate
from linearity; the P-Au-N angle of 1 and 2 are 166.50° and 161.22°
respectively (Figs. 2 and 3, Table 2). Also, the P-Au-N angle of 2 with
sterically more encumbered qbt ligand deviates more from linearity
compared to 1.
Fig. 2. Perspective view of the cation of complex 1 with the atom-labeling
scheme. The thermal ellipsoids are shown at 50% probability level. Only one of
the two crystallographically independent molecules in the asymmetric unit is
shown.
It is important to note that in 1 and 2, the N atom of the pyridine/
quinolyl moiety (N(1)) is facing the metal center, but not bound.
Results of previous studies on the structures and ground state optimized
geometries of the free ligands [14] have demonstrated that although
the structure with the pyridyl‑N and benzothiazole‑N atom anti to each
other is lower in energy, in all cases both pbt and qbt bind metal ions as
the NeN syn structural isomer [14,16,28–30]. The pyridine fragment in
these cases rotates to present its N center to be available for co-
ordination in a NeN syn fashion (Scheme 1 and Fig. 4a). In 1 and 2, this
rotation of the aryl ring also occurs, but the pyridyl‑N or the quinolyl‑N
does not bind to the metal center. However, in both cases, significant
interactions between this N-atom and the Au(I) center is observed in 1
[Au(1)–N(1), 2.713 A] and 2 [Au(1)–N(10), 2.710A] (Fig. 4c) which is
in agreement with other gold(I) compounds of similar structure [33].
Fig. 3. Perspective view of the cation of complex 2 with the atom-labeling
scheme. The thermal ellipsoids are shown at 50% probability level.
(PPh3)]BF4 [26] (where py = pyridine), was chosen to compare its
crucial metric parameters with those of the present two complexes. In
[(py)Au(PPh3)]BF4, the N(py)-Au and AueP bond lengths are 2.073(3)
Å and 2.2364(8) Å respectively. These distances are quite comparable
to those noted for 1 and 2. Interestingly, the N-Au-P angle in [(py)Au
(PPh3)]BF4 is 178.09(8)°, more close to a linear geometry, unlike 1 and
2. In the present two structures, both pbt and qbt exhibit excellent
planarity with mean deviations of 0.024(3) and 0.090(4) Å respec-
tively. The packing patterns for both structures revealed no classical
hydrogen bonding interactions. However, several weak non-bonding
contacts consolidated the extended lattice of these two complexes
(Supplementary data, Figs. S1–S3).
3.4. Antibacterial studies
Antibacterial studies were done using the skin and soft tissue in-
fection (SSTI) model previously developed in this laboratory [23]. This
model mimics the gradual penetration of bacteria deeper into the skin
using a two-layer agar system which has a soft, evenly dispersed bac-
terial lawn on the top and a nutrient-rich bottom layer. The gradient
causes the bacteria to slowly migrate from the thin top layer to the
nutrient-rich bottom layer much like the way an infection of the skin
would proceed.
Both complexes and the corresponding ligands were tested in vitro
for their bactericidal activity against the Gram-negative bacterium A.
83

J. Stenger-Smith et al.
JournalofInorganicBiochemistry185(2018)80–85
Fig. 5. Bacterial lawns after 18 h incubation with KBr pellets of KBr, pbt and
qbt (top panel, left to right) and (PPh3)AuCl, 1, and 2 (bottom panel, left to
right).
Fig. 6. 19F NMR spectrum of the mixture of 1 and F3CC6H4SH (top trace),
PPh3Au–SC6H4CF3 synthesized independently (middle trace) and F3CC6H4SH
(bottom trace) in CDCl3. F3CC6H4S-SC6H4CF3 was present in the thiol as an
impurity.
baumannii. This bacterium has shown multidrug-resistance in hospitals
around the world and at present poses serious threat to human health.
In addition to resistance traits carried on mobile genetic elements, A.
baumannii also forms biofilms rapidly as defensive barriers. In addition,
A. baumannii infections are a primary concern to military personnel
injured by gun fire and improvized explosive devices (IEDs) in the
battlefields of Afghanistan and Iraq [34]. With a limited pipeline of new
antibiotics being developed for Gram-negative bacteria, new therapies
are in high demand, especially for this pathogen. In the present work, a
gold control (the neutral starting material PPh3AuCl) was also used to
show how the ligands and the cationic nature of 1 and 2 affect the
ability of the Au(I) complexes to interact with the bacterial colonies.
KBr pellets of similar weights containing 0.3 mol% of compounds were
placed on the top of the bacterial lawn and incubated at 37 °C for 18 h.
Circular zones of clearing were seen around the pellets containing 1 and
2, but not around the pellets containing KBr, pbt, qbt or (PPh3)AuCl
(Fig. 5). These results indicate that the two Au(I) complexes are better
able to migrate and interfere with a large area of infection while the
neutral gold compound exhibit no eradication of the bacteria on the
SSTI model.
The antimicrobial activity of various drugs is highly dependent on
their ability to permeate through the cell wall and interfere with cel-
lular pathways [9]. Most bacteria possess a slightly electronegative
surface potential which allows cationic complexes to associate more
readily with the bacterial membrane. If the drug molecules can easily
form bonds with biomolecules either on the membrane or in the cy-
tosol, the efficacy of the drug action is enhanced. For gold compounds
this implies that the nature of the ligands bound to the Au(I) center, not
just the amount of Au(I) present, plays an important role in their ac-
tivity [35]. The neutral and very stable compound (PPh3)AuCl therefore
exhibits no activity against A. baumannii while the more ligand ex-
changeable and cationic complexes 1 and 2 are very effective in era-
dicating bacterial loads throughout the entire “kill zone” of the 4 mm
thick SSTI disk.
glutathione) present in the cytosol of the bacteria, 1 was treated with
1.2 equiv. of p‑trifluoromethyl‑benzene thiol (F3CC6H4SH) and the re-
action was followed by both 19F and 1H NMR spectroscopy (in CDCl3).
Interestingly, addition of the thiol to the Au(I) complex generated a new
19F-peak at ~0.5 ppm downfield (with respect to F3CC6H4SH) in-
dicating binding of the thiol to the Ph3PAu+ unit (Fig. 6). The 1H NMR
spectrum of the reaction mixture clearly showed loss of pbt from the Au
(I) center upon addition of the thiol. In an independent experiment,
PPh3Au–SC6H4CF3 was synthesized by reacting (PPh3)AuCl with
CF3C6H4S− (prepared from CF3C6H4SH and Et3N). The 19F NMR spec-
trum of PPh3Au–SC6H4CF3 clearly confirmed its formation in the re-
action between 1 and F3CC6H4SH (Fig. 6). In order to confirm that the
PPh3 ligand is not deligated in such reactions of 1 with SH-containing
biomolecules, we allowed 1 to react with N‑acetyl‑L‑cysteine methyl
ester (HSC6H10NO4) in CDCl3. The 31P NMR spectrum of the reaction
mixture clearly showed that the cysteine ester replaced pbt to form
PPh3Au–SC6H10NO4 (Fig. 7). Complete absence of 31P resonance of free
PPh3 in such reaction mixture confirms that no PPh3 loss occurs upon
reaction of 1 with the S-containing amino acid.
Deligation of pbt or qbt from both complexes by SH-containing
It is important to note that although 1 and 2 exhibited high level of
antibacterial activity (Fig. 5), no reduction of bacterial load was ob-
served with (PPh3)AuCl. Because pbt and qbt by themselves showed
marginal antibacterial effect for A. baumannii, it is evident that effective
interaction between the cationic gold complexes and bacterial cell
membrane is the major cause of their bactericidal activity. Upon dele-
gation of pbt or qbt from the metal center, a step that delivers these
antibacterials to the cytosol, the highly reactive Ph3PAu+.OTf− species
could also exert strong antibiotic action through binding to membrane
and/or cytosolic molecules [36]. In order to check whether 1 and 2 can
bind to cellular SH-containing proteins and peptides (such as
Fig. 7. 31P NMR spectra of complex 1 and PPh3Au–SC6H10NO4 (formed upon
addition of HSC6H10NO4 to 1) in CDCl3 (the ppm values are w.r.t free PPh3 in
CDCl3).
84

J. Stenger-Smith et al.
JournalofInorganicBiochemistry185(2018)80–85
Ph3PAu+·OTf− species could exert synergistic drug effects in such ap-
plications leading to better outcome. The present two complexes 1 and
2 provide proof-of-the-concept examples of such drug design approach.
Abbreviations
pbt
2‑(pyridyl)benzothiazole
2‑(quinolyl)benzothiazole
trifluoromethanesulfonate anion
luria broth
qbt
OTf−
LB
TSB
SSTI
MLCT
tryptone soya broth
skin and soft tissue infection
metal-to-ligand charge transfer
Fig. 8. Dramatic rise in luminescence upon addition of CF3C6H4SH to 2 in
chloroform. Left: CHCl3 solution of 2; right: a mixture of 2 and CF3C6H4SH in
CHCl3.
Acknowledgements
Financial support from the NSF grant DMR-1409335 and a UCSC
COR-SRG grant is gratefully acknowledged.
Appendix A. Supplementary data
Crystal data for complex 1 and complex 2 (in CIF format) and the
packing diagrams (Figs. S1–S3). Supplementary data to this article can be
found online at doi: https://doi.org/10.1016/j.jinorgbio.2018.05.003.
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85