Caffeoylquinic acid derivatives isolated from the aerial parts of Gynura divaricata and their yeast α-glucosidase and PTP1B inhibitory activity

The phytochemical investigation of natural products of Gynura divaricata led to the isolation of eleven caffeoylquinic acid derivatives. They were characterized by spectrometric methods as 5-O-caffeoylquinic acid (1), 5-O-p-coumaroylquinic acid (2), 5-O-feruloylquinic acid (3), methyl 5-O-caffeoylquinate (4), 3,4-dicaffeoylquinic acid (5), 3,5-dicaffeoylquinic acid (6), 4,5-dicaffeoylquinic acid (7), methyl 3,4-dicaffeoylquinate (8), methyl 3,5-dicaffeoylquinate (9), methyl 4,5-dicaffeoylquinate (10) and ethyl 4,5-dicaffeoylquinate (11). The individual compounds were screened for the inhibition of yeast α-glucosidase and Protein Tyrosine Phosphatase 1B (PTP1B) using in vitro assays. Among the isolated compounds, 3,4-dicaffeoylquinic acid (5), 4,5-dicaffeoylquinic acid (7), methyl 3,4-dicaffeoylquinate (8) and methyl 4,5-dicaffeoylquinate (10) exhibited significant inhibitory activities against α-glucosidase. In addition, 5-O-p-coumaroylquinic acid (2), 3,5-dicaffeoylquinic acid (6) and 4,5-dicaffeoylquinic acid (7) had considerable inhibitory effect against PTP1B. Based on these findings, the caffeoylquinic acid derivatives were deduced to be potentially responsible for the anti-diabetic activity of G. divaricata. The preliminary structure-activity relationship study suggests that the number and positioning of caffeoyl groups in the quinic acid derivatives are important for both α-glucosidase and PTP1B inhibitory potency. Moreover, the corresponding methyl esters of some dicaffeoylquinic acids have enhanced inhibitory activity against yeast α-glucosidase.