Enhanced anti-microbial effect through cationization of a mono-triazatricyclodecane substituted asymmetric phthalocyanine

doi:10.1016/j.jinorgbio.2018.10.001

Journal of Inorganic Biochemistry

Volume 189, December 2018, Pages 192-198

https://doi.org/10.1016/j.jinorgbio.2018.10.001 Get rights and content

Highlights

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A new asymmetric triazatricyclodecane substituted zinc phthalocyanine was prepared.
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The compound had strong antibiotic efficacy on S. aureus, but weak effect on E. coli.
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Further cationization led to potent anti-bacterial activity against both S. aureus and E. coli.
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The cationic compound eliminated >4 logs of bacteria at micromolar concentration.
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A new and rapid method using bioluminescent bacteria was adopted in anti-bacterial assay.

Abstract

Antimicrobial photodynamic therapy (aPDT) is an effective way to combat infectious diseases and antibiotic resistance. Photosensitizer is a key factor of aPDT and has triggered extensive research interest. In this study, a new asymmetric Zn(II) phthalocyanine mono-substituted with a triazatricyclodecane moiety (compound 3) and its cationic N-methylated derivative (compound 4) were synthesized. Their photodynamic antimicrobial activities were evaluated using bioluminescent bacterial strains. Compound 3 showed phototoxicity only toward the Gram-positive bacteria, whereas the cationic derivative compound 4 exhibited strong anti-bacterial activity against both Gram-positive and Gram-negative strains. These bacterial species were eradicated (>4.0 logs or 99.99% killing) at appropriate concentrations of compound 4 with 12.7 J/cm² of red light, demonstrating compound 4 as a potent aPDT agent.

Graphical abstract

A new asymmetric zinc phthalocyanine conjugated with triazatricyclodecane (3) was synthesized. Upon methylation to compound 4, the agent exhibited potent anti-bacterial activity with an IC₅₀ value at nano-molar range and >4 logs reduction of bacteria load.

Introduction

Phthalocyanines have many important industrial applications, including dyes and pigments in fabric, demonstrating that they are safe and environment-friendly materials. They also possess remarkable stability and unique photochemical and photophysical properties, broadening their uses in many high technology fields, including semiconductor materials [1,2], solar cells [3,4], optical data storage [5], chemical sensors [6,7], oxidation-reduction catalysts [[8], [9], [10], [11]] and photocatalysts [12], as well as photodynamic therapy (PDT) for antimicrobial [[13], [14], [15]] or antitumor applications [[16], [17], [18]]. The emergence of multi-resistant bacteria due to the over-use of antibiotics has become a global challenge [19,20]. Phthalocyanines have emerged as a new class of photosensitizer possessing potent antimicrobial effect, even toward drug-resistant bacterial strains separated from hospitals [[21], [22], [23], [24]].

In this study, we designed a new asymmetric Zn(II) phthalocyanine (ZnPc): triazatricyclo-substituted Zn(II) phthalocyanine (compound 3) by conjugating with a Nitrogen-rich compound (1,3,5-triazatricyclo [3.3.1.1(3, 7)] decane-7-amine, Scheme 1). The substituent is quite bulky in size and can reduce the aggregation of phthalocyanine. Aggregation of Pc typically leads to the quench of photodynamic effect. Moreover, the triazatricyclodecane is water soluble, and will be protonated at aqueous solution, potentially rendering the conjugate aqueous solubility and positive charges in a weakly acidic environment. The positive charge is a common property of antimicrobial agents, which allows the adsorption and binding to bacterial surface that carry large amount of negative charges [25,26]. We also wanted to avoid the positional isomer on ZnPc during conjugation because single compound is a key for approval by regulatory agent, should the compound proceeds to clinical trial stage.

It turned out that compound 3 was not water soluble, and did not show desirable antimicrobial effect toward bacterial strains. We suspect that the compound 3 was not protonated at aqueous solution. Quaternization of aliphatic or aromatic nitrogen atom at the end of the synthetic pathway of phthalocyanine is a common way to prepare cationic phthalocyanine [27]. Most of the published cationic phthalocyanines with quaternary amine groups exhibit excellent photodynamic antimicrobial effect [[28], [29], [30], [31], [32]]. Moreover, it was discovered recently by Hamblin group that aPDT can be greatly enhanced by the addition of simple inorganic salts especially iodine ion at micromolar concentration [33]. Thus, we carried out methylation on the tertiary amine group of compound 3 in a hope to form an cationic phthalocyanine compound 4. Photophysical and photochemical properties including UV–Vis spectra, photodynamic antibacterial and hemolysis activities of these compounds were characterized and evaluated. Encouragingly, the compound 4 exhibited very potent antimicrobial effect.

Section snippets

Synthesis of compound 3

The target compound (3) was synthesized using the scheme shown in Scheme 1. The intermediate compound (1) was prepared by statistic condensation method. Condensation of trimellitic anhydride or phthalic anhydride in the presence of urea and catalyst at high temperature can produce tetra-formamido-phthalocyanine or phthalocyanine, respectively. Using a mixture of trimellitic anhydride and phthalic anhydride at a ratio of 1:7 for condensation, we made a mixture of 2-formamidophthalocyanine zinc (1

Materials and instruments

All the reactions were carried out under the atmosphere of nitrogen. The beta-carboxy Zn(II) phthalocyanine compound 2 was synthesized in our lab according to our previously published method [42]. 1,3,5-Triazatricyclo [3. 3. 1. 1 (3, 7)] decan-7-amine, O-benzotriazole-N,N, N′,N′-tetramethyl-uronium-hexafluorophosphate (HBTU), N,N-diisopropylethylamine (DIPEA), dimethyl sulfoxide-d₆ and dry DMF (dried with molecular sieves) were purchased from the J & K Scientific LTD. The common solvents such

Conclusions

In conclusion, a new asymmetric mono-substituted Zn(II) phthalocyanine compound 3 and its cationic derivative compound 4 were synthesized and characterized. The photodynamic antimicrobial data showed that compound 3 only inactivated Gram-positive bacterial strains. However, the cationic compound 4 induced >4 logs antimicrobial effect against both of Gram-positive and Gram-negative strains under short incubation time and low light dose. In addition, the compound 4 exhibited negligible toxicity

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Our research work is financially supported by grants from National Key R&D Program of China (2017YFE0103200), and National Natural Science Foundation of China (31370737, 31400637, 31570745, 31670739).

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

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View full text

Enhanced anti-microbial effect through cationization of a mono-triazatricyclodecane substituted asymmetric phthalocyanine

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of compound 3

Materials and instruments

Conclusions

Conflicts of interest

Acknowledgements

Ethical approval

Nano Energy

Coord. Chem. Rev.

Coord. Chem. Rev.

J. Inorg. Biochem.

Curr. Opin. Microbiol.

Dyes Pigments

Coord. Chem. Rev.

J. Inorg. Biochem.

Dyes Pigments

Dyes Pigments

Eur. J. Med. Chem.

Photodiagn. Photodyn. Ther.

Biomaterials

Inorg. Chem. Commun.

Org. Electron.

Nano-Micro Lett.

J. Mater. Chem. A

Appl. Organomet. Chem.

K.M. Kadish

Inorg. Chem.

ACS Catal.

Appl. Organomet. Chem.