Chemical composition, in vitro color reconstruction, and bee vision modeling regarding anthocyanins and other phenolics from the flowers of Petrea macrostachya and Petrea volubilis

Delphinidin 3-O-[6-(4-O-(6-(4-O-β-glucopyranosylvanilloyl)-β-glucopyranosyl)-vanilloyl)-β-glucopyranoside]-5-O-β-glucopyranoside (petreadelphin), a novel anthocyanin, and β-(3,4-dihydroxyphenyl)ethyl 4-E-caffeoyl-3,6-di-β-xylopyranosylglucopyranose (petreathanoside), a novel phenylethanoid, were isolated from the violet petals and sepals of Petrea macrostachya (Verbenaceae), respectively. In addition, nine known phenolic compounds were also isolated from the flowers of P. macrostachya. The pH values of P. macrostachya petals and sepals were 5.7 and 5.6, respectively. In vitro color reconstruction experiments using the two major anthocyanins from each organ, namely petreadelphin from the petals and delphinidin 3-O-(6-caffeoylglucoside)-5-O-glucoside from the sepals, showed that their absorption spectra closely match those of the corresponding floral tissues. These results indicate that each anthocyanin is primarily responsible for the coloration of its respective organ. Stability tests showed that the sepal anthocyanin faded rapidly after 15 min in the buffer solution, whereas petreadelphin retained its color for at least 24 h. This difference is likely due to the greater structural stability of petreadelphin conferred by its polyacylation. In addition, bee color vision modeling using the reflectance spectra indicated that the petals have significantly higher color intensity than sepals. These findings suggest that the type and number of acyl groups in anthocyanin may influence color intensity and potentially floral detectability by bees.