Abstract
In the last decade, electrochemical oxidation coupled with mass spectrometry has been successfully used for the analysis of metabolic studies. The application focused in this study was to investigate the redox potential of different phenolic compounds such as the very prominent chlorogenic acid. Further, EC/ESI-MS was used as preparation technique for analyzing adduct formation between electrochemically oxidized phenolic compounds and food proteins, e.g., alpha-lactalbumin or peptides derived from a tryptic digestion. In the first step of this approach, two reactant solutions are combined and mixed: one contains the solution of the digested protein, and the other contains the phenolic compound of interest, which was, prior to the mixing process, electrochemically transformed to several oxidation products using a boron-doped diamond working electrode. As a result, a Michael-type addition led to covalent binding of the activated phenolic compounds to reactive protein/peptide side chains. In a follow-up approach, the reaction mix was further separated chromatographically and finally detected using ESI-HRMS. Compound-specific, electrochemical oxidation of phenolic acids was performed successfully, and various oxidation and reaction products with proteins/peptides were observed. Further optimization of the reaction (conditions) is required, as well as structural elucidation concerning the final adducts, which can be phenolic compound oligomers, but even more interestingly, quite complex mixtures of proteins and oxidation products.
Highlights
Despite many years of research on protein-phenolic compound interactions, predictions of protein adduct formation between various food ingredients are still challenging and not yet satisfyingly solved [1]
In contrast to the prevalent oxidability of some phenolic representatives like caffeic acid, there are those phenolic compounds that are not suitable for a complete oxidation due to structural conditions and the redoxpotential necessary for a transformation. Those phenolic compounds comprise, for example, representatives that have structural elements such as methoxy or keto groups and only one hydroxyl group such as ferulic acid [15,16], in which the formation of semiquinones with different possibilities for follow-up reactions has to be considered, as well [5]. This present study focused on the comparison of the redox potential of various phenolic compounds and the characterization of oxidation products with regard to their ability to act as reaction partners for food proteins or peptides thereof using electrochemistry
Electrochemistry coupled with mass spectrometry was successfully used to investigate the redoxpotential of phenolic acids such as chlorogenic acid
Summary
Despite many years of research on protein-phenolic compound interactions, predictions of protein adduct formation between various food ingredients are still challenging and not yet satisfyingly solved [1]. Covalently bound phenolic compounds might significantly affect physicochemical protein properties such as the hydrophobicity/hydophilicity balance, correspondingly affecting the protein structure, as well the technofunctional properties (e.g., solubility). Studying the allergenicity of milk proteins is still of certain interest, as it might be influenced during processing and dairy product development [4]. In this regard, one can think of products in which milk or whey fractions are combined with ingredients derived from fruits
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