Amphiphilic tricationic Zn(II)phthalocyanine provides effective photodynamic action to eradicate broad-spectrum microorganisms

Abstract

A novel tricationic Zn(II)phthalocyanine derivative, (NCH₃)₃ZnPc³⁺, was synthesized by ring expansion reaction of boron(III) [2,9(10),16(17)-trinitrosubphthalocyaninato]chloride. First, the reaction of this subphthalocyanine with 2,3-naphthalenedicarbonitrile and Zn(CH₃COO)₂ catalyzed by 8-diazabicyclo[5.4.0]undec-7-ene was used to obtain the A₃B-type nitrophthalocyanine. After reduction of nitro groups with Na₂S and exhaustive methylation of amino groups, (NCH₃)₃ZnPc³⁺ was formed in good yields. In addition, the tetracationic analog (NCH₃)₄ZnPc⁴⁺ was synthesized to compare their properties. The absorption and fluorescence spectra showed the Q-bands and the red emission, respectively, which are characteristic of the Zn(II)phthalocyanine derivatives in N,N-dimethylformamide. Furthermore, photodynamic activity sensitized by these compounds was studied in the presence of different molecular probes to sense the formation of reactive oxygen species. (NCH₃)₃ZnPc³⁺ efficiently produced singlet molecular oxygen and also it sensitized the formation of superoxide anion radical in the presence of NADH, while the photodynamic activity of (NCH₃)₄ZnPc⁴⁺ was very poor, possibly due to the partial formation of aggregates. Furthermore, the decomposition of L-tryptophan induced by (NCH₃)₃ZnPc³⁺ was mainly mediated by a type II mechanism. Antimicrobial photodynamic inactivation sensitized by these phthalocyanines was evaluated in Staphylococcus aureus, Escherichia coli, and Candida albicans, as representative microbial cells. In cell suspensions, (NCH₃)₃ZnPc³⁺ was rapidly bound to microbial cells, showing bioimages with red fluorescence emission. After 5 min of irradiation with visible light, (NCH₃)₃ZnPc³⁺ was able to completely eliminate S. aureus, E. coli and C. albicans, using 1.0, 2.5 and 5.0 μM phthalocyanine, respectively. In contrast, a low photoinactivation activity was found with (NCH₃)₄ZnPc⁴⁺ as a photosensitizer. Therefore, the amphiphilic tricationic phthalocyanine (NCH₃)₃ZnPc³⁺ is a promising photosensitizing structure for application as a broad-spectrum antimicrobial phototherapeutic agent.

Keywords: Microorganism; Photodynamic inactivation; Photoinactivation; Phthalocyanine; Reactive oxygen species.