Rational design and synthesis of molecularly imprinted polymers (MIP) for purifying tylosin by seeded precipitation polymerization
Graphical abstract
Introduction
Tylosin [1] is a 16-membered macrolide antibiotic extracted from the fermentation broth of Streptomyces fradiae. It has a wide range of antibacterial spectrum including Mycoplasmas, Haemophilus, etc., widely used for livestock due to its efficacy. Tylosin can be modified to produce more effective derivatives with less toxicity [2,3], like tilmicosin [4] and tylvalosin [5]. During fermentation process, by-products such as desmycosin and relomysin are the main impurities that need to be moved, and today, higher industrial tylosin productivity requires a more efficient purification method. However, the traditional method of solvent extraction [6] used in the industry does not meet the strict requirements of environmental protection. Other methods for purifying antibiotics may include cation-exchange resin [7], HPLC on a polymeric stationary phase [8], and the trial application of aqueous two-phase system [9]. These methods are cost intensive or time-consuming; in particular, it is very difficult to separate an appropriate single component from a mixture. Researchers have therefore focused on molecular imprinting technology due to its efficient molecular recognition of the desired molecules [10].
Molecular imprinting technology [11,12] is based on “key” theory, which explains many biological living phenomena such as antigen-antibody binding [13], signal-receptor recognition, substrate-enzyme reaction, etc. There are two methods of MIP synthesis, namely covalent imprinting technique and non-covalent imprinting technique. The former requires the existence of specific functional groups in a template that can be covalently bound together with monomers to join copolymerization. In the case of non-covalent imprinting techniques, due to commercially available monomers, simple synthetic processes and faster desorption and adsorption rates, it has been widely accepted in the field of molecular imprinting technology. Major intermolecular forces that contribute to molecular recognition in non-covalent imprinting techniques include Van der Waals forces, electrostatic interaction [14], hydrophobic or hydrophilic force [15], metal-ion interaction [16] and steric hindrance effect [17], etc. To date, molecular imprinting has been applied into a variety of research fields, such as artificial antibody synthesis [18], catalysis [19], environmental decontamination [20], solid-phase extraction [[21], [22], [23]], trace detection [24,25] and biochemical sensors [26,27], etc.
Although molecular imprinting is significant in its application, it still has some problems. (i), It is very time-consuming and resource-consuming to synthesize MIP based on extensive experimental trails and errors. (ii) Traditional bulk polymerization can lead to the entrapping of template within MIP and a low mass transfer rate. These problems should be addressed in order to prepare a tylosin-selective polymer for the extraction of tylosin from its fermentation broth. First, scientists have adopted computational approach to monomer screening from potential collection [[28], [29], [30], [31]]. In this work, Gaussian software was used to calculate the binding energy and to visually adduce the stabilized template-monomers complexes in a given solvent. The novelty in this work is that, by means of a splitting method, only active groups of templates were split to bind to different monomers in the simulation system (Fig. 1). Some researchers have adopted covalent organic frameworks to address the second problem [32,33]. In this work, seeded precipitation polymerization, which was based on our previous method [34] with further modification, was adopted to prepare core-shell structured particles with high capacity and fast mass transfer rate. Although precipitation polymerization in water-miscible solvents, such as acetonitrile, normally results in nano-particles [34,35], macron-sized particles have been directly produced by seed precipitation polymerization, which is more suitable for column packing in large-scale separation engineering.
There are several parameters for synthesizing a practicable polymer [35]: functional monomer, crosslinking agent, the relative ratio of the components, and porogen (solvent). Dimethyl sulfoxide (DMSO) was previously selected as the main solvent for solubility and polarity. Molecular simulation under solvent model was used to select an appropriate monomer with sufficient binding energy, since the stability of the template monomer complex at the pre-polymerization stage can make a difference in the selectivity and capacity of the MIP obtained [36]. Ethylene glycol dimethylacrylate (EGDMA) was used as a common crosslinking agent. The potential monomers and the ratio of monomer-template were tested to confirm the synthesis formulation. MIP was prepared and characterized by FE-SEM, FT-IR, LSPSA and BET test. Its static and dynamic adsorption behavior were studied. Finally, MISPE and reusability experiments were conducted to testify to its practical application for purifying tylosin from fermentation broth. Comparison (Table S1) of MIPs for tylosin or other macrolide antibiotics prepared by this paper and other previous researches have been provided. The study by Piletsky et al. [37] used molecular simulation, considering DMF solvent, to select the top three monomers, and the formulation of MIP synthesis was theoretically decided through the Simulated Annealing process. By comparison, this work used splitting method combined with pre-experiments to determine the monomers and the relative ratio. MIP for tylosin prepared by seeded precipitation polymerization method has advantages of higher adsorption capacity and IF.
Section snippets
Chemicals
Tylosin tartrate and its fermentation broth samples was provided by Qilu Pharmaceutical (Inner mongolia) Co., Ltd. Ethyleneglycol dimethacrylate (EGDMA), allylamine (AA), acrylamide (AAM), methacrylic acid (MAA), methyl methacrylate (MMA), 4-cyano-4-(phenylcarbonothioylthio)pentanoic Acid (CPPA) were purchased from Aladdin Chemical Co., Ltd. Ammonium persulfate (APS), NaHCO3, acetone, methanol, acetic acid (HAc), dimethylsulfoxide (DMSO) were brought from Shanghai Lingfeng Chemical Reagent Co.,
Theoretical selection of monomers based on density functional theory
In this case, a non-covalent interaction was considered in order to propose a general protocol. At the beginning of the synthesis, the pre-assembling of templates and monomers plays a key role in contributing to the complementary structure of the template.
Several functional monomers were theoretically chosen as alternative monomers for simulation under two circumstantial parameters, i.e. vacuum phase and the solvent DMSO. Screening for potential functional monomers was necessarily accelerated
Conclusion
In this work, a specialized MIP using tylosin as template was rationally synthesized through seeded precipitation polymerization. The result shows that MIP has fast mass transfer rate and stable adsorption sites. Its specific adsorption sites were demonstrated by its Q (106.5 mg/g) at maximum IF (3.6), and α (3.3). Finally, MISPE was effectively employed for extracting and purifying tylosin from diluted fermentation broth. The simulation result is referable and rather recommendable to other
Declaration of Competing Interest
None.
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