Novel potential metabolic enzymes inhibitor, photosensitizer and antibacterial agents based on water-soluble phthalocyanine bearing imidazole derivative

Highlights

• Single crystal x-ray diffraction measurements of 4-(1-methyl-1H-imidazole-2-thiol) phthalonitrile.

• Synthesis of novel water-soluble phthalocyanine hydrochloride bearing imidazole derivative.

• Investigation of photo-physicochemical, anti-α-glygosidase, anti-cholinesterases properties.

• Antibacterial properties: gram-negative and gram-positive bacteria.

Abstract

In this work, we have reported on the synthesis and characterization of water-soluble hydrochloride forms (2a-4a) based on novel peripherally [M= metal-free (2), zinc (II) (3), gallium (III) chloride (4)] phthalocyanines bearing 1-methyl-1H-imidazole-2-thiol substituents. Characterization of all compounds used was supported by a series of spectroscopic techniques such as UV–Vis, FT-IR ¹H NMR, ¹³C NMR and MALDI-MS, etc. The confirmation of the molecular structure of 4-(1-methyl-1H-imidazole-2-thiol) phthalonitrile (1) by single crystal x-ray diffraction experiment was performed for the first time in this work. Besides, the intra/inter-molecular interactions inside the obtained crystal structure have been investigated. Afterward, the effects of the central metal atoms and solvents on the photophysicochemical properties of the phthalocyanines were analyzed to investigate their potential to use as a photosensitizer in photodynamic theraphy (PDT). The obtained results show that the phthalocyanines have therapeutic outcomes for cancer treatment. It has been seen that all compounds have a better ability to inhibit compared to existing tried inhibitors. Among these, the best inhibitors against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes are (4) (Ki 47.71±9.14 µM and IC₅₀ 68.22) and (3a) (Ki 20.12±3.75 µM and IC₅₀ 19.24), respectively. Also, against α-Glycosidase, (4) showed the highest effect (Ki 9.13±1.05 µM and IC₅₀ 11.22). Phthalocyanines were performed to gram-negative and gram-positive bacteria using minimum inhibition concentration (MIC) assay and indicated an antibacterial effect.

Introduction

Phthalocyanines are macrocyclic aromatic blue-green dyes that cannot be obtained naturally. Despite the structural similarity in nature, such as hemoglobin, cytochrome, chlorophyll, they are obtained by synthetic methods [1]. Phthalocyanines are classified in various ways according to the nature of the central metal atom and the position where the substituents are attached to the phthalocyanine ring [2,3]. Tetra-substituted phthalocyanines, the most common species are obtained by attaching various substituents to the peripheral or non-peripheral positions of the phthalocyanine skeleton. In this way, it can not only alter the chemical and physical properties of phthalocyanines, but also change their solubility [4,5]. In particular, scientists have focused on synthesizing water-soluble phthalocyanines because of the usability of phthalocyanines in pharmacological and biological applications such as antioxidant, antibacterial, DNA binding/cleavage, enzyme inhibition, cytotoxic/phototoxic anticancer and photodynamic activities, etc. [6], [7], [8], [9], [10], [11]. To confer water solubility to Pcs containing amino groups, the common methods are quaternizing the nitrogen atoms to obtain cationic derivatives or preparing hydrochloride derivatives. Compared to the preparing process, to prepare hydrochloride derivative is much easier than quaternized derivative [12,13]. By using both methods, the obtained Pcs not only have good solubility in water, but also have low aggregation, high anticancer and photodynamic activity [14,15].

Photodynamic therapy is a new method of controlling and eliminating the tumor and is in principle a different version of chemotherapy and radiotherapy. In photodynamic therapy, photosensitizers stimulated by light at specific wavelength produce singlet oxygen (¹O₂) in the environment, thereby destroying the diseased tissue. In photodynamic therapy, phthalocyanines are used as second generation photosensitizer agents. Phthalocyanines bind to the amine groups of the antibody selected in accordance with the cancerous cell. When photosensitizer-bound antibody is delivered to the body, it only accumulates in diseased tumor cells without spreading throughout the body. When one of the electrons of the oxygen molecule receives energy from the outside, it switches to a different orbital opposite to its direction of rotation and singlet oxygen (¹O₂) forms. The resulting singlet oxygen is short-lived. Singlet oxygen, which is more active and has a very high energy, can be effective at a distance of 10–20 nm, but not at more remote locations. Phthalocyanines are compounds that have the potential to be used in the treatment of cancer by photodynamic therapy due to their high wavelength absorption and the ability to form singlet oxygen effectively [16], [17]–18]. The studies about hydrochloride derivatives of phthalocyanines have been rarely reported.

The investigation of binding or cleavage of DNA with compounds has been important to develop new agents for cancer [19,20]. The interaction of compounds with DNA can prevent proliferation of cancerous cells by leading to cell death via apoptosis and necrosis [21,22]. It is known that anticancer drugs can bind to DNA covalently or non-covalently and then can promote DNA cleavage via hydrolytic and oxidative pathways [23,24]. In order for phthalocyanines to be used as diagnostic usage in various pharmacological and biological applications, it is preferred that it has some features such as water solubility and strong binding capacity to DNA with their positive charge, etc. [25,26].

Imidazoles are well known heterocyclic compounds having 5-membered planar ring. Drugs containing imidazole and its derivatives are common and have important usability in pharmacological and biological applications such as antibacterial activity, anticancer activity, antitubercular activity, antifungal, anti-diabetic, etc. Scientists in medicinal chemistry have concerned with the discovery, development, interpretation, and identification of the mechanism of action of imidazoles [27], [28], [29], [30], [31]–32].

The α-glycosidase enzyme, which hydrolyzes the α-glycosidic bonds in carbohydrate molecules, is an enzyme that belongs to the glycoside hydrolase family. This enzyme breaks down starch and disaccharides to glucose by catalyzing the hydrolysis of terminal, non-reducing (1→4)-linked alpha-glucose residues [33], [34], [35]–36]. In 1940, the two major forms of cholinesterases called as acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) was discovered by Allis and Hawes [37], [38], [39]–40]. Of these two enzymes, AChE is found in erythrocytes, neuromuscular junction in the muscle tissue, and cholinergic nerve synapses while BChE is one of the important metabolic enzymes of the human body, found in the liver, nervous system, pancreas and plasma. BChE and AChE belong to the group of serine hydrolases. Additionally, the compounds in amino acid residues of AChE and BChE determine the several specificities for substrates and inhibitors for these metabolic enzymes [41], [42], [43]–44].

In this work, 4-(1-methyl-1H-imidazole-2-thiol) phthalonitrile (1) was synthesized according to the procedure described in Ref. [45] with some modifications. For the first time in this work, the molecular structure of the compound (1) was confirmed by single crystal x-ray diffraction experiment (XRD). And, its crystallographic information and the inter/intra-molecular interactions have been investigated by the analysis of XRD data. Then, we have reported on the synthesis and characterization of peripherally tetra-substituted phthalocyanines [M= metal-free (2), zinc (II) (3), gallium (III) chloride (4)] bearing 1-methyl-1H-imidazole-2-thiol substituents and their water-soluble hydrochloride forms (2a-4a). Besides, the photochemical and photophysical properties of the phthalocyanines were studied to show the therapeutic effects as a photosensitizer in photodynamic therapy studies. Then, we have investigated some metabolic enzymes inhibition of peripherally tetra-substituted phthalocyanines [M= metal-free (2), zinc (II) (3), gallium (III) chloride (4)] bearing 1-methyl-1H-imidazole-2-thiol substituents and their water-soluble hydrochloride forms (2a-4a). The antibacterial property of phthalocyanines was estimated according to chemical structure and features of Pcs, their binding affinity to cell and their singlet oxygen production [46]. In current study, the antimicrobial effect of Pcs against two Gram-positive and two Gram-negative was investigated using micro dilution assay.