Inorganic Chemistry
Article
Table 1. Antibacterial Data as MIC (μM) Values for Ligand (L), Pt-Acceptor (A), Cage (PMB1), and Methylated Ligand
(
[LMe ][4NO ]) in the Presence and Absence of Light
4
3
light
PMB1
dark
PMB1
L
A
[LMe4][4NO3]
L
A
[LMe4][4NO3]
MRSA
PA
>100
>100
>100
>100
30
30
>100
>100
>100
>100
>100
>100
40
30
>100
>100
1
an (DPBF) which has high reactivity toward O (Scheme
Scheme 3. Synthesis of Tetramethylated Analogue
2
2
5
3
The absorption properties of mixed solutions of PMB1 and
DPBF were examined with and without exposure of light. This
solution displays a strong absorption band at 418 nm due to
the presence of DPBF. However, before exposing to light there
was almost no effect on the absorption profile of the DPBF in
the presence of PMB1. Interestingly, a regular decrease in the
peak intensity was noticed in the absorption profile of DPBF in
the presence of the cage in aqueous medium as a function of
light exposure time (Figure 3c). No change in the absorption
cage even upon light exposure (Figure S13). The decrease in
the absorption intensity of DPBF in the presence of the cage
was due to the formation of a diketo derivative, namely, 1,2-
1
dibenzoylbenzene (DBB), through O -mediated ring opening.
2
Δ
Δ
S21), which is lesser when compared with the cage. The
enhanced ROS generation property of PMB1 can be attributed
to the heavy atom effect of platinum causing intersystem
crossing from a singlet to a triplet state. The charged species
(
1
the Pt(II)-cage is capable of generating O species in water in
2
the presence of light with a negligible effect on its own
absorption profile, which suggests high stability of the Pt(II)-
(
[LMe4][4NO ]) was found to kill bacteria at 50 times higher
3
cage as well. The relatively higher quantum yield and
S18). Such a high concentration can be toxic as well. This
result indirectly proves that, along with water solubility,
enhanced ROS generation, very high net positive charge, and
more surface area make the tubular cage unique and set it up as
a highly potent antibacterial agent.
1
photogeneration of O in aqueous media encouraged us to
2
explore the antimicrobial activity of the Pt(II)-cage.
Antibacterial Activity of the Cage (PMB1). The visible-
light induced antibacterial activity of the cage was evaluated by
using MRSA as a representative of Gram-positive bacteria and
PA as a representative of Gram-negative bacteria. The
minimum inhibitory concentration (MIC) was determined
with different concentrations of the cage in the presence and
absence of white light irradiation. The optical density (OD) at
The benzothiadiazole unit is known to improve the
antibacterial activity by damaging the bacterial membrane
through singlet oxygen (ROS) generation but suffers from
26
poor water solubility. The synthesized tubular cage (PMB1)
not only improved the solubility of the benzothiadiazole unit
but also possesses high positive charge. The positive charge
and hydrophobicity can help to depolarize the bacterial
membrane through electrostatic and hydrophobic interactions
6
00 nm of the bacterial solution was measured at intervals of 1
h to monitor the growth of bacteria. Similarly, control
experiments were performed using the cage and the
components (ligand and acceptor) of the cage separately in
the presence and absence of light. Among all, the cage showed
much better antibacterial activity at very low concentration in
both the presence and absence of light. In the absence of light,
PMB1 exhibited antibacterial activity against MRSA with
higher inhibitory concentration (Figure 4). However, the
building units (L and Pt-acceptor) did not show any inhibitory
action against MRSA as well as PA in our working
concentration range (up to 160 μM for ligand, up to 240
27
and thus can improve the antibacterial activity. It was found
that the minimum inhibitory concentration (MIC) of MRSA
in the presence of the cage in light is 30 μM, while in absence
of light irradiation it is 40 μM. This indicates that without
irradiation of light, the cage also inhibits the bacterial growth
due to its positive charge and hydrophobic interactions,
whereas in the presence of light higher inhibition occurs due to
additional singlet oxygen generation which causes the bacterial
membrane damage. The minimum bactericidal concentration
(MBC) is the concentration at which the viability of bacteria is
reduced to zero, which is generally determined by counting
colony forming units in the agar plate. At the MIC, the
bacterial growth is inhibited, but the microbial death may not
happen. We have observed 40 μM MBC concentration for
both of the cases. At that concentration no colony formation
was observed in the agar plate (Figure 4c,f). If the MBC is <4
times the MIC, then the antibacterial agents can be considered
Figure S17, Table 1). The main reason for ligand (L) showing
no antibacterial activity was its poor solubility in water and
aggregation caused quenching (ACQ) activity (Figure S11).
The detailed experimental procedure is mentioned in the
experimental section in the SI.
Now the question that arises is, if only water insolubility is
the reason for the ligand’s negligible antibacterial activity, can a
water-soluble analogue of L show better activity? To address
28
this issue, alkylation of the ligand was performed using CH I to
as bactericidal. More importantly, the cage is found to be a
more potent antibacterial agent with low concentration in
3
make a water-soluble species ([LMe4][4NO ]), and its
3
antibacterial activity was thoroughly examined (Scheme 3
comparison to the charged water-soluble ligand ([L
]
Me4 -
E
Inorg. Chem. XXXX, XXX, XXX−XXX