Characterization and spectroscopic study of enzymatic oligomerization of phenazopyridine hydrochloride
Introduction
The azo coupling reaction was developed in 1856 with the appearance of an orange powder [1]. Azo dye is composed of an azo chromophore (-NN-) in their structure, conjugated with mono- or polycyclic aromatic systems. Of all classes of dyestuffs, azo dyes are the most commonly utilized substances in various areas, such as pharmaceutical, cosmetics, and food industry [2], dyeing textile fibers and plastics, coloring the natural substances like cotton, paper, silk, leather, wool, in biological activities and advanced applications in organic synthesis [[3], [4], [5], [6], [7], [8], [9]]. Recently, holography, optical storage capacity, non-linear optical and optical switching properties of polymers with azo units bring along the applications for photoactive materials [10].
Azo dyes with bright colors, in particular oranges, red and yellow are also involved in photo-chromic applications [11]. Aromatic azo compound is a part of a very significant group of organic compounds owing to their electrochemical, optical, and medicinal relevance [12,13]. The reduction of NN- into NHNH and NH2 provides information about their optical, structural, electrochemical and thermodynamic properties [[14], [15], [16]].
Phenazopyridine hydrochloride (PHP) [2, 6-diamino-3-(phenylazo)pyridinehydrochloride], an azo dye, has been used as antiseptic and analgesic since 1920. PHP is an important aromatic compound related to antibacterial agents for the treatment of urinary-tract infections [[17], [18], [19]]. PHP exerts an analgesic effect on the mucosa of the urinary tract and induces symptomatic relief of pain in conditions such as cystitis and urethritis [[20], [21], [22], [23]].
Optical features of conjugated polymers make them good choices for the production of photoluminescence materials [24,25], such as the production of biosensors [26], light-emitting diodes [27], electrochromic displays [28], photovoltaic and optical devices [29]. Enzymatic polymerization of azo-functionalized anilines is preferred to traditional chemical methods, resulted in that the polymers have almost 100% dye content which is higher than those of chemically modified polymers carrying azo groups [30,31]. Enzymatic, peroxidase-catalyzed, polymerization has been explored as a promising alternative approach to the synthesis of electronically and optically active polymers. Enzymatic polymerization reactions are advantageous in that they can offer simple and environmentally friendly reaction conditions [32,33]. Peroxidase-catalyzed synthesis also involves a reaction mechanism that results in a direct ring-to-ring coupling of phenol and aniline monomers [34].
Health hazards of azo dyes have become a public concern. Therefore, it is necessary to discover environmentally benign and green synthesis method for the production of arylamine oligomer or polymer containing azo groups. Enzymatic oxidative polymerization is considered as a benign method compared to the chemical (or electrochemical) oxidative polymerization methods [35,36]. Therefore, the enzymatic polymerization can be regarded as an environmentally friendly synthetic process of polymeric materials, providing a good example to achieve green chemistry of conducting and redox-active oligomers and polymers [37,38]. Conducting polymers can also be efficiently produced by enzymatic polymerization [39,40] as a result of CC and NC couplings [41].
With a view to investigating potential oligomers for the aforementioned properties, the characteristic properties of the biologically derived an azo dye via enzymatic oligomerization of PHP were studied. Oligomer formation using HRP enzyme provided an alternative opportunity for the production of OPHP with no need to use toxic chemicals in the method. In parallel, enzymatically oligomerized OPHP exhibited balanced thermal and photoluminescence properties. Elucidation of the molecular structure of the oligomer synthesized was performed by FT-IR, 1H and 13C NMR techniques. Enzymatic oligomerization induced OPHP with an emission of blue and green light upon impulse of UV and visible excitation, respectively, in photoluminescence analysis.
- sbnd
NH2 group has the highest propensity for radical coupling in enzymatic oligomerization. Oligomer was composed of head-to-tail or ortho- and para-coupled units as a consequence of the oxidation of PHP using hydrogen peroxide as an oxidant. On the basis of prior explanations, the enzymatically synthesized oligomer was analyzed using UV–Vis, PL, TG-DTG-DTA, DSC, CV and GPC methods to elucidate optical, electrochemical and thermal properties of the material.
Section snippets
Chemicals
Phenazopyridine hydrochloride (PHP; 3-Phenylazo-2,6-diaminopyridine hydrochloride, 98%) was provided by Alfa Aesar (USA). Horseradish Peroxidase (HRP) with a specific activity of 259 purpurogallin units/mg and an absorbance ratio (Rz) of 3.0 was provided by Sigma Chemical Co. (USA). Merck Chemical Co. (Germany) supplied methanol, ethanol, tetrahydrofuran (THF), chloroform (CHCl3), N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), acetonitrile (CH3CN), hexane,
Characterization
Purple-colored phenazopyridinium chloride (PHP) was enzymatically oligomerized by H2O2 to form OPHP, as shown in Scheme 1. Brown colored powder of OPHP was synthesized with a yield of 59%. Even though PHP was soluble in polar solvents, such as DMSO, THF, DMF, DCM, NMP and acetonitrile, the synthesized oligomer (O-PHP) was completely insoluble in H2O and ethanol. In highly aprotic polar solvents, such as DMSO, THF, DMF, NMP and acetonitrile, OPHP was soluble. Both compounds were insoluble in
Conclusions
Phenazopyridine hydrochloride (PHP) was subjected to enzymatic oxidation and coupling at the ortho and para positions of the phenyl ring. A novel oligomer (O-PHP) was synthesized using HRP-catalyzed oligomerization. Structural characterization of OPHP by FT-IR define and NMR techniques further confirmed that peroxidase-catalyzed coupling reaction occurs at the ortho as well as para positions on the phenyl ring of the monomer. Biologically and enzymatically derived azo oligomer possessed a
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