Photodynamic Studies and Photoinactivation of Escherichia coli Using meso‐Substituted Cationic Porphyrin Derivatives with Asymmetric Charge Distribution^¶

Abstract

The photodynamic activities of novel asymmetrically meso substituted cationic porphyrins, 5,10‐di(4‐methylphenyl)‐ 15,20‐di(4‐trimethylammoniumphenyl)porphyrin iodide 1 and 5‐(4‐trifluorophenyl)‐10,15,20‐tris(4‐trimethylammoniumphenyl)porphyrin iodide 2 and its metal complex with Pd(II) 3, have been investigated in both homogeneous medium bearing photooxidizable substrates and in vitro on a typical gram‐negative bacterium Escherichia coli. The amphiphilic character of porphyrin 2 was increased by the presence of a high‐lipophilic trifluoromethyl group and its photophysical properties changed by forming a complex with Pd(II). Absorption and fluorescence spectroscopic studies were compared in different media. Fluorescence quantum yields (øF) of 0.16 for 1 in tetrahydrofuran and 0.08 for 2 in N, N‐dimethylformamide (DMF) were calculated, whereas no significant emission was detected for Pd(II) porphyrin 3. The singlet molecular oxygen, O2(1Δ(g), production was evaluated using 9,10‐dimethylanthracene in DMF yielding relative values of 1, 0.55 and 0.47 for porphyrins 3, 2 and 1, respectively. A faster decomposition of L‐tryptophan was obtained using Pd(II) porphyrin 3 as sensitizer with respect to the free‐base porphyrins 1 and 2. In biological medium, the behavior of cationic porphyrins 1‐3 were compared with that of 5‐(4‐carboxyphenyl)‐10,15,20‐tris(4‐methylphenyl)porphyrin 4, which was used as a noncationic sensitizer. These porphyrins are rapidly bound to E. coli cells in 5 min and the amount of cell‐bound sensitizer is not appreciably changed incubating the cultures for longer times. The recovered porphyrin 2 after one washing step reaches a value of ∼2.9 nmol/106 cells and this amount remains high even after three washes, indicating that this sensitizer is tightly bound to cells. Photosensitized inactivation of E. coli was analyzed using cells without and with one washing step. In both cases, a higher photoinactivation of cells was found for tricationic porphyrin 2 and 3, causing a ‐5.5 log (99.999%) decrease of cell survival, when treated with 10 μM of sensitizer. Under these conditions, a lower effect was found for porphyrin 1 (‐4 log) whereas sensitizer 4 did not produce appreciable photodamage. The results were also confirmed by growth delay experiments. These studies show that the amphiphilic tricationic porphyrin 2 and 3 bearing a trifluoromethyl group can be a promising model for phototherapeutic agents with potential applications in inactivation of bacteria by photodynamic therapy.

Porphyrin derivatives as photosensitizers for the inactivation ofBacillus cereusendospores

“…Vegetative cells are, as expected, much more responsive to PDI than endospores. PDI of gram positive bacteria with cationic and neutral porphyrin derivatives has already been demonstrated (Hamblin and Hasan 2004;Lazzeri et al 2004;Caminos et al 2005;Demidova and Hamblin 2005b). In our experiments, Tri-Py + -Me-COOH caused the strongest inactivation (reduction of 6AE5 log units).…”

“…Cationic porphyrins are usually more efficient against bacteria than neutral molecules because of an improved interaction with their rather impermeable outer membrane and cell wall (Merchat et al 1996;Lazzeri et al 2004;Caminos et al 2005;Demidova and Hamblin 2005b). However, it is also recognized that the relationship with the charge of the molecule is not always linear (Carvalho et al 2007;Costa et al 2008).…”

“…The Tri-Py + -Me-Ph has a phenyl group with no substituents. The presence of these groups in Tri-Py + -Me-PF and Tri-Py + -Me-COOH can affect the interactions between the PS and the target structures, such as cell walls, nucleic acids, peptides and lipids (Lazzeri et al 2004) and consequently improve the efficiency of the PS. Our results confirm that small modifications in the substituent groups, in addition to charge, significantly modify the effectiveness of the porphyrin derivatives as PS for endospores photoinactivation.…”

Phthalocyanine Thio‐Pyridinium Derivatives as Antibacterial Photosensitizers†

“…The presence of one or more positively charged groups plays an essential role in driving the PS toward sites that are critical for the stability of cell organization and ⁄ or the cell functions (34,51,52). In fact, several studies demonstrate a high rate of bacterial inactivation with tri-and tetracationic porphyrinic PS compared with di-and monocationic molecules (31,52).…”

Effect of Organic Matter on the In Vitro Photoeradication of Pseudomonas aeruginosa by Means of a Cationic Tetraaryl‐porphyrin†

“…Photodynamic therapy (PDT) is an alternative approach to conventional antimicrobial therapies, which exploits the presence of a photosensitizer (PS), O 2 and visible light to produce reactive oxygen species (ROS), thus inducing lethal oxidative damages in the microbial pathogens (1). Differently structured PS shows a variable degree of phototoxic efficiency according to the cell wall organization of bacteria: the presence of the outer membrane makes Gram-negative bacteria more resistant than Gram-positive bacteria to photoeradication (2).…”