Isolation and identification of colourless caffeoyl compounds in purple sweet potato by HPLC-DAD-ESI/MS and their antioxidant activities

More than 10 red anthocyanins and related glucosides have been isolated and identified from purple sweet potato (Ipomoea batatas, Ayamurasaki) in the recent decades. This paper reports the isolation of colourless caffeoyl compounds from purple sweet potato using AB-8 macroresin absorption and semi-preparative HPLC-DAD. The structures of the five isolated monomers were identified as: 5-caffeoylquinic acid (1), 6-O-caffeoyl-β-d-fructofuranosyl-(2-1)-α-d-glucopyranoside (2) and trans-4,5-dicaffeoylquinic acid (3), 3,5-dicaffeoylquinic acid (4), 4,5-dicaffeoylquinic acid (5), and by ESI/MS and NMR. Compounds 1, 4 and 5 were reported previously in combination with anthocyanins in purple sweet potato, whereas 2 and 3 were found for the first time. In vitro antioxidant assay showed trans-4,5-dicaffeoylquinic acid has significant antioxidant activities. These results should lay the groundwork for further work identifying purple sweet potato as a healthy food.