Poly(N-vinylimidazole/ethylene glycol dimethacrylate) for the purification and isolation of phenolic acids
Graphical abstract
Introduction
Phenolic compounds are well known phytochemicals with increasing interest for dietary supplements or phytopharmaceuticals relating to their potent antioxidative properties [1], [2]. The so-called “phenolics” comprise about 8000 naturally occurring compounds and include at least one phenol as the common structural characteristic [3]. Phenolic acids are a subclass of secondary plant metabolites containing two different structural frameworks, cinnamic acid and benzoic acid which in turn derive from chorismate, the final product of the shikimate pathway [4], [5], [6].
Phenolic compounds exist in nearly all parts of plants including leaves, stems, seeds or roots and serve there as protectors against pathogenic attacks of fungal, bacteria or high energy radiation such as permanent UV exposure [7], [8], [9], [10], [11]. Their antioxidative activities arise from the hydroxyl moiety of the phenol acting as a radical scavenger i.e., via hydrogen donation. Their stabilization by various substituents influence the impact of the scavenging ability [12], [13]. According to the positive antioxidative effect on plants, the impact of phenolic compounds on the maintenance of health and disease prevention is respectively high. Hydroxyl radicals and superoxides as byproducts of cellular metabolism can react with surrounding water to form hydrogen peroxide which in turn damages DNA and other critical cell components and structures [14], [15], [16]. Apart from endogenous peroxidases, phenolics can serve as radical scavengers and quenchers in order to decrease the cellular degradation of the organism [1], [17], [18], [19]. The literature provides a huge variety of information that correlates phenolics in fruits, vegetables and nutritional supplements with a positive effect on health protection and disease prevention.
Phenolic acids are a part of the large group of phenolics and represent a carboxylic functionality in the phenolic constitutive structure. Their isolation is highly desirable as they show positive effects on health maintenance and disease prevention similar to other phenolic compounds. Currently phenolic acids are isolated and characterized using different chromatographic methods including UHPLC and HPLC separations hyphenated to mass spectrometry, liquid–liquid extraction or solid-phase extraction [20], [21], [22], [23]. However, the characterization of plant metabolites can be a complex procedure due to the presence of various compounds (sugars, chlorophyll, waxes, oils, etc.) which are able to damage or clog the analytical columns and influence the determination of target molecules [24]. Therefore, solid phase extraction (SPE) has been established as a pre-purification method which can further be combined with chromatographic separation [25], [26], [27]. Typical SPE sorbents for the isolation of phenolic acids include reversed-phase materials, ion-exchangers or mixed-mode materials whereby their retention mechanism is commonly based on π–π interactions, electrostatic ion–ion interactions, dipol–dipol interactions or hydrogen bonding. Michalkiewicz et al. [28] reported an extraction procedure for the determination of phenolic acids and flavonols in honey using commonly applied sorbents such as Bond Elut octadecyl C18, Oasis HLB, Strata-X and Amberlite XAD-2. Cleanert-PEP SPE cartridges were used for the extraction of phenolic acids in root exudates of allelopathic rice showing reasonable and acceptable recoveries [29]. Apart from commonly available sorbents, molecularly imprinted anion-exchange polymers have been successfully used for the purification of phenolic acids and their derivatives in complex samples [30], [31], [32].
The aim of this study was the development of a new material for the purification and isolation of phenolic acids. Therefore, an imidazole-based resin was synthesized, optimized and applied in the normal-phase and ion-exchange mode. For characterization scanning electron microscope (SEM) images were recorded which show morphological differences in the porogen compositions. Moreover, single standards were isolated to study maximal loading capacities which were determined according to the Langmuir adsorption model. Apart from recovery studies, rosemary leaves were extracted by an accelerated solvent extractor and purified by the new polymeric resin.
Section snippets
Chemicals
1-Vinylimidazole was purchased from Sigma–Aldrich (St. Louis, MO) and distilled under vacuum. α,α-Azoisobutyronitrile (AIBN) was purchased from Fluka (Buchs, Switzerland) and recrystallized from methanol before use. Ethylene glycol dimethacrylate (≤98%), activated aluminum oxide (58 Å), ferulic acid (≤98 %), tannic acid (ACS grade), gallic acid (97.5–102.5%), trans-cinnaminic acid (≤99%), 3,4-dihydroxyhydrocinnamic acid (98%), 2,3-dihydroxybenzoic acid (98%), 2,5-dihyroxybenzoic acid (≤98%),
General
Due to the positive effects of phenolic acids on the health maintenance, their isolation is highly desirable and some enrichment methods have already been described. Solid-phase extraction procedures are nowadays quite common enrichment tools as their use is easy, fast and simply automatable.
Generally, pKa values of phenols range from 9 to 13 and are therefore lower than common aliphatic hydroxyl groups [33], [34]. The stronger acidity of phenols can be attributed to the stabilization of the
Conclusion
In this study a novel polymeric resin based on N-vinyl imidazole is reported for the purification and isolation of phenolic acids. Optimization of monomer to crosslinker ratio and porogen composition resulted in higher loading capacity by also allowing the purification of aqueous samples. Additionally, ATR-IR measurements prove the incorporation of both N-vinyl imidazole and ethylene glycol dimethacrylate into the polymeric framework. Single standards have been recovered at almost 100% and the
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