Synthesis, Characterization and Photophysical Studies of Novel Porphyrin-based Compounds for Application in Photodynamic Antimicrobial Chemotherapy (Pact)

Abstract

There are a number of techniques employed to rid microbial contaminants from water. The inefficacy of some of these methods in removing microorganisms in contaminated water, has resulted to increased incidences of water-borne diseases and emergence of antimicrobial resistance by disease causing pathogens. Therefore, there is need to develop new, efficient and convenient methods for combating microbial water contaminants as well as addressing the current problem of antimicrobial resistance. Photodynamic antimicrobial chemotherapy (PACT) is one of the emerging green technologies for water treatment that uses the combined presence of light, oxygen and suitable photosensitizer to generate a reactive oxygen species that is effective against pathogenic microbial organisms. Porphyrins are by far one of the commonly used photosensitizers in PACT. They are stable, tend to absorb in the long wavelength region due to extended π conjugation, and have specific interactions with proteins in cell membrane of bacterial cells. Furthermore, upon irradiation with light of suitable wavelength, porphyrins can produce reactive oxygen species efficient in eradicating bacterial cells. In this study porphyrins were synthesized and their photophysical and antimicrobial activities were measured to determine their suitability in PACT. Free-base 5,10,15,20-tetra(4-bromophenyl) porphyrin (H2TBrPP) and 5,10,15,20-tetra(pyren-1-yl)porphyrin (H2TPyP) were synthesized using established methods. By inserting zinc, gallium and indium to the free-base of H2TPyP the corresponding metalloporphyrins were synthesized. The synthesized compounds were purified through column chromatography and characterized on the basis of their mass spectrometry, 1H NMR spectroscopy and elemental analysis. Metallation of the free base H2TPyP to form a metalloporphyrin afforded improved photophysical properties. There was a bathochromic shift in wavelength of absorption from the parent free base H2TPyP (λ = 431 nm) to metallated ZnTPyP (λ = 439 nm), GaClTPyP (λ = 440 nm) and InClTPyP (λ = 443 nm). The fluorescence quantum yield in H2TPyP was higher (ϕF = 0.131) than in ZnTPyP (ϕF = 0.039), GaClTPyP (ϕF = 0.041) and InClTPyP (ϕF = 0.017) due to efficient intersystem crossing to the triplet manifold. Upon illumination of the synthesized compounds: H2TBrPP, H2TPyP, ZnTPyP and InClTPyP, effective dose dependent antimicrobial activity against Staphylococcus aureus was exhibited with IC50 values of 49.57, 27.89, 12.90 and 16.67 μM, respectively.