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

Abstract

A study of the physicochemical factors contributing to flower colour in three ‘blue’ geraniums, ‘Johnson's Blue’, G. pratense and G. sanguineum, has led to an understanding of how such colour variants are produced in geraniums. All three contain the same new anthocyanin, malvidin-3-O-β- d-glucopyranoside -5-O-β- d -[6-O- acetylglucopyranoside] , the structure of which was established by 2D-NMR techniques. This anthocyanin is an isomer of the malvidin-3-O-β- d -[6-O- acetylglucopyranoside]-5-O-β- d-glucopyranoside reported recently 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 with 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 juice from these flowers 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 is concluded that the pH of epidermal cell vacuoles in these flowers is 1–1.4 units higher than that of the pressed juice. In vitro colour reconstitution of this type is proposed as a convenient method for determining the approximate pH of anthocyanin-containing epidermal cell vacuoles. The colours and colour stabilities of all three studied flowers can be adequately accounted for by the pigment structure and concentration, the flavonol type and ratio, and the epidermal pH.