2.1 Synthesis and Photophysical Properties
The metalloporphyrins employed in this study were synthesized by following reported methods.39-44 The detail synthetic procedure, characterization data and photophysical properties of as synthesized compounds is shown in supporting information.
2.2 Antibacterial Activity
The as synthesized metalloporphyrins were tested for their in vitro antibacterial activity in the open condition under visible/white light and the results were compared with the ligand, metal salt and the commercially available drug, gentamycin,. The activity is 1st tested against two Gram-positive (Staphylococcus aureus (S. aureus) and streptococcus pyogenes (s. pyogenes) and two Gram-negative (Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae)) bacteria by using 31.25, 62.5, 125, 250 and 500 mg/L of each metalloporphyrins. All the tested metalloporphyrins were found to be active against all the tested pathogens and compared with the commercially available antibiotic drug (gentamycin). The result of antibacterial activities is reported as inhibition zone diameter (mm) for the concentration of 500 mg/L as shown in Table 1.
Table1. Antibacterial activiy (mean IZ diameter (mm) ± SD) of metalloporphyrins,
corresponding ligands, metal salts, and gentamycin with concentration 500 mg/L
Compounds
|
Antibacterial activiy (mean IZ diameter(mm) ± SD)
|
S. aureus
|
S. pyogenes
|
E. coli
|
K. pneumonia
|
CoTPPCOOH
|
16.5± 0.5
|
15±0.3
|
16± 0.5
|
16±0.6
|
CoTPPOMe
|
12±0.2
|
14 ± 0.6
|
10±0.3
|
12±02
|
CuTPPCOOMe
|
16± 0.2
|
15±0.5
|
13±0.6
|
13.5±075
|
CuTPPNH2
|
13±0.4
|
13.5±0.3
|
12±0.45
|
12.5± 0.6
|
H2TPPCOOH
|
8.5±0.2
|
9±0.3
|
7.5±0. 3
|
7.5± 0.4
|
H2TPPCOOMe
|
8.25±0.3
|
8±0.2
|
7.5± 0.03
|
7.5± 0.2
|
H2TPPOMe
|
7±0.3
|
7.25 ±0.3
|
7 ±0.3
|
7±0.2
|
H2TPPNH2
|
7.5± 0.2
|
7.75± 0.3
|
7± 0.01
|
7± 0.2
|
CuCl2. 2H2O
|
6.75± 0.2
|
7±0.3
|
6.25± 0.02
|
6.5± 0.4
|
Co acetate
|
6.5±0.1
|
7± 0.2
|
6.5± 0.1
|
6.5± 0.2
|
DMSO
|
0 ±0.00
|
0±0.00
|
0±0.00
|
0±0.00
|
10 µg gentamicin
|
25 ±0.6
|
27±0.75
|
26±0.75
|
25±0.5
|
Metalloporphyrins showed higher activities than free base porphyrins against all bacteria. This indicates the metal ion plays an important role in antibacterial activity. This increased activity of metal complex can be explained on the basis of the overtone concept and chelation theory. According to the overtone concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid-soluble materials in which lipo-solubility is an important factor that controls the antibacterial activity [45-48]. On chelation, the polarity of the metal ion will be reduced to a greater extent due to overlap of ligand orbital and partial sharing of the positive charge of the metal ion with donor groups. Furthermore, it increases the delocalization of π-electrons over the whole chelate ring and enhances the lipophilicity of complexes. It is likely that the increased liposolubility of the ligand upon metal complexation may contribute to its facile transport into the bacterial cell which blocks the metal binding sites in enzymes of microorganisms. These complexes also disturb the respiration process of the cell and thus block the synthesis of proteins, which restricts further growth of organism [49].
As can be seen from Table 2 and Figure 1 increasing the concentration of antibacterial agents increase the activity very slightly and metalooporphyrins under study are active and inhibit bacteria even at the lowest concentration (31.25 mg/L). Moreover, the bacteria growth inhibition activity of the complexes is not significantly different among different bacteria species. The 5, 10, 15, 20-tetrakis (p-carboxyphenyl)porphyrinato cobalt (II), exhibited the greater antimicrobial activities than other metalloporphyrins with inhibition zones 16.5 mm for S. aureus presumably attributing to its ability to strongly bind with cellular components.
Table 2. Antibacterial activiy (mean IZ diameter (mm) ± SD) of cobaltporphyrins,
at different concentrations.
Cobalt complexes at different concentration
|
Antibacterial activiy ( mean IZ diameter(mm) ± SD)
|
S. aureus
|
S. pyogenes
|
E. coli
|
K. pneumonia
|
500 mg/L
|
CoTPPCOOH
|
16.5± 0.5
|
15±0.3
|
16± 0.5
|
16±0.6
|
CoTPPOMe
|
12±0.2
|
14 ± 0.6
|
10±0.3
|
12±02
|
250 mg/L
|
CoTPPCOOH
|
12.5±0.45
|
13±0.3
|
13±0.4
|
13±0.3
|
CoTPPOMe
|
11.5±0.3
|
10 ±0.2
|
11.5±0.2
|
11± 0.0.45
|
125 mg/L
|
CoTPPCOOH
|
13± 0.3
|
11±0.55
|
11±0.3
|
11±0.75
|
CoTPPOMe
|
10.5±0.5
|
9±0.5
|
10± 0.5
|
9 ±0.3
|
62.5 mg/L
|
CoTPPCOOH
|
12±0.6
|
10.5±0.4
|
12±0.5
|
12±0.4
|
CoTPPOMe
|
8±0.2
|
8±0.3
|
9 ±0.4
|
9.5±0.2
|
31.25 mg/L
|
CoTPPCOOH
|
10.5±0.3
|
10± 0.2
|
9± 0.1
|
9.5±0.3
|
CoTPPOMe
|
8±0.4
|
8±0.2
|
8 ± 0.45
|
9±0.3
|
For copper complexes of 5, 10, 15, 20-tetrakis (p-X phenyl)porphyrin, as the concentration of the complexes increase, the antimicrobial activity also increase as shown in Table 3.
Table 3. Antibacterial activity (mean IZ diameter (mm) ± SD) of copperporphyrins,
at different concentrations.
Copper complexes at different concentration
|
Antibacterial activity ( mean IZ diameter(mm) ± SD)
|
S. aureus
|
S. pyogenes
|
E. coli
|
K. pneumonia
|
500 mg/L
|
CuTPPCOOMe
|
16± 0.2
|
15±0.5
|
13±0.6
|
13.5±075
|
CuTPPNH2
|
13±0.4
|
13.5±0.3
|
12±0.45
|
12.5± 0.6
|
250 mg/L
|
CuTPPCOOMe
|
12± 0.45
|
12.5±0.3
|
12±0.02
|
11.5± 0.3
|
CuTPPNH2
|
11±0.2
|
11± 0.6
|
10.5± 0.3
|
10±0.2
|
125 mg/L
|
CuTPPCOOMe
|
11.5±0.5
|
11± 0.45
|
9.5±0.02
|
10± 0.5
|
CuTPPNH2
|
10±0.75
|
11.5±0.4
|
9±0.3
|
9.5±0.4
|
62.5 mg/L
|
CuTPPCOOMe
|
10.5±0.2
|
10.25± 0.5
|
9± 0.03
|
9.5±0.2
|
CuTPPNH2
|
9±0.5
|
8.5± 0.5
|
9±0.2
|
8.5±0.45
|
31.25 mg/L
|
CuTPPCOOMe
|
9± 0.4
|
8.75±0.3
|
8.5±0.04
|
8±0.2
|
CuTPPNH2
|
8±0.2
|
8±0.1
|
7.5± 0.5
|
8±0.75
|
Though antimicrobial activity of porphyrin derivatives of natural origin with COOH groups at β-pyrrolic positions have been reported so far [50-57], metalloporpyhrins with p-COOH at meso position of phenyl ring is not reported. Moreover, consistent with the report by Ke G. Y. and coworkers, the electron withdrawing substituents enhance antibacterial activity attributing to increasing lipophilicity and polarity of the complex [15, 28]. Generally, the metal complexes containing electron withdrawal groups (with COOH and -COOMe showed better activities than the metal complex containing electron donating groups namely -NH2 and -OMe.