Semi-preparative isolation and physico-chemical characterization of 4-coumaroylquinic acid and phloretin-2′-xyloglucoside from laccase-oxidized apple juice

Two not commercially available apple polyphenols, 4-coumaroylquinic acid and phloretin-2′-xyloglucoside, were isolated from an apple juice previously oxidized with a laccase (polyphenol oxidase, EC 1.10.3.2). Juice oxidation in combination with ultrafiltration removed polyphenols with ortho-dihydroxyphenol structures, e.g. caffeic acid, flavanols and flavonols, which led to a reduced number of peaks in the chromatogram. Subsequently, a polyphenol extract from this juice was manufactured using an adsorber resin technique. The extract served as a basis for the proper isolation of the target substances by means of semi-preparative RP-HPLC. The structures of 4-coumaroylquinic acid and phloretin-2′-xyloglucoside were confirmed with LCMS- and NMR-data, respectively. The use of the isolated target substances as quantification references improved the quantitative HPLC of apple juices distinctly. Besides the reference compounds could be used for testing their biological activity.