Symmetrical and asymmetrical meso-substituted porphyrins and zn-metalloporphyrins in gold colloid environment. Optical properties and evaluation of antibacterial activity

Abstract

Aiming to develop non-harmful and wide range efficient antibacterial agents, seven porphyrin derivatives, namely; 5,10,15,20-tetrapyridylphorphyrinato-Zn(II) (P1); 5, 10,15,20-tetra-(4-carboxy-phenyl)-porphyrin (P2); 5-(4-carboxyphenyl)-10,15,20-triphenyl-porphyrin (P3); 5, 10,15,20-tetra-(3-hydroxy-phenyl)-porphyrin (P4); 5-(3-hydroxy-phenyl)-10,15,20-tris-(3,4-dimethoxy-phenyl)-porphyrin (P5); 5-(4-carboxy-phenyl)-10,15,20-tris-(4-phenoxy-phenyl)-porphyrin (P6); 5,10,15,20-tetrakis(pentafluorophenyl)-porphyrinato-Zn(II) (P7) in gold colloid solution in comparison with gold nanoparticles were tested using the diffusion method. The porphyrins that presented an inhibition zone diameter larger than 15 mm, with proven antibacterial activity were selected and further tested by the dilution method to establish minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). Generating an inhibition zone diameter of only 7 mm, the gold colloid per se does not fulfil the criteria as antimicrobial agent but we take into account that the porphyrin-gold hybrid/mixture might offer an enhancement of the biological activity even in non-illuminated media. All the porphyrin-bases present antibacterial activity against the nosocomial Gram-positive Staphylococcus aureus. All the porphyrins meso-substituted to the phenyl ring with hydroxy-or carboxy-functional groups present a significant antimicrobial activity against Enterococcus faecalis. Zn-metalloporphyrins show a lower antibacterial efficiency compared to the porphyrin-bases. The best and wide antibacterial activity covering bacilli, cocci and fungi is provided by an A3B mixed asymmetrically substituted molecule (P5), acting even at very low concentrations of 0.06 mg/mL. Its significant bactericidal behaviour is probably due to its capacity to better penetrate the cell wall based on its amphiphilic character.