Malvidin-3-O-glucoside-5-O-(6-acetylglucoside) and its colour manifestation in 'Johnson's Blue' and other 'blue' geraniums.

摘要

The physicochemical factors contributing to flower colour in 3 blue geraniums, Geranium cv. Johnson's Blue, G. pratense and G. sanguineum were studied. All 3 contained the same new anthocyanin, malvidin-3-O-beta-D-glucopyranoside-5-O-beta-D-[6-O-acetylglucopyranoside]. Its structure was established by 2D-NMR techniques. This anthocyanin was an isomer of malvidin-3-O-beta-D-[6-O-acetylglucopyranoside]-5-O-beta-D-glucopyranoside from G. sylvaticum, but it was indistinguishable by HPLC from the pigment sourced from a locally available G. sylvaticum. It was accompanied in Johnson's Blue and G. pratense by kaempferol and myricetin 3-O-glucosides and 3-O-sophorosides, but with only low levels of flavonols in G. sanguineum. The pH values of pressed juicefrom flowers of Johnson's Blue, G. pratense and G. sanguineum were 5.4, 5.4 and 4.6, respectively. In in vitro colour reconstitution experiments, the petal spectra of Johnson's Blue and G. pratense could only be produced from the anthocyanin at near vacuolar concentrations, in the presence of molar excesses of kaempferol-3-O-sophoroside (the major copigment) and at a pH of 6.6 to 6.8. It was concluded that the pH of epidermal cell vacuoles in these flowers was 1-1.4 units higher than that of the pressed juice. The colours and colour stabilities of all 3 studied flowers were adequately accounted for by pigment structure and concentration, the flavonol type and ratio, and the epidermal pH.