Possibilities and limitations in the analysis of covalent interactions between phenolic compounds and proteins

• Knowledge on interactions of phenolic compounds and proteins is still scarce.
• Covalent reaction products affect the properties of both reaction partners lastingly.
• Reaction products are quite complex including protein crosslinking.
• Analysis of phenol–protein-adducts is quite challenging.
• HPTLC might be a promising alternative.

Besides all comprehensive studies on botanical, analytical, and nutritional aspects of polyphenols, knowledge on interactions of these bioactive compounds with other constituents in the plant, food, or even in the human body is still scarce. From the several possibilities of interactions, covalent reaction products seem to be the most important ones, as they irreversibly affect the properties of both reaction partners. The phenolic compounds are highly reactive and many of them are able to be oxidized to their corresponding semiquinones and quinones which are further capable of undergoing covalent reactions with an enormous number of nucleophiles. When taking a look at nature, prominent nucleophiles are selected protein side chains such as lysine or cysteine. In the case of a reaction between phenolic compounds and protein side chains, properties of the single proteins and correspondingly the whole organism (plant or animal) might be severely affected. Even the smallest amounts of interactions can be of great impact. However, analysis of such reaction products is complex and therefore challenging. This is due to the fact that phenolic compounds remain reactive after the addition to the protein. This minireview shortly describes the chemical background(s) of the reaction and illustrates the main challenges in the analysis of protein–phenol-adducts. Moreover, questions that have to be considered when analyzing protein–phenol reaction products are raised and an alternative analytical idea is presented.