Generation and structure elucidation of a red colorant formed by oxidative coupling of chlorogenic acid and tryptophan

Abstract

In view of the poor acceptance of synthetic food colorants by consumers, there is intense interest in novel natural compounds, preferably from plant-derived sources. We oxidized chlorogenic acid using NaIO₄ and reacted the resultant quinone with tryptophan (Trp) to obtain a red-colored product. The colorant was precipitated, freeze-dried, purified by size exclusion chromatography, and subsequently characterized using UHPLC-MS, high-resolution mass spectrometry, and NMR spectroscopy. Additional mass spectrometric studies were performed on the reaction product generated with Trp educts labeled with ¹⁵N and ¹³C. The data obtained from these studies allowed the identification of a complex compound consisting of two Trp and one caffeic acid moieties, and the proposition of a tentative pathway of its formation. Thus, the present investigation expands our knowledge about the formation of red colorants based on the reaction of plant phenols and amino acids.

Keywords: Chlorogenic acid; Mass spectrometry; NMR spectroscopy; Phenol oxidation; Red colorant; Tryptophan.