Red onion anthocyanins: a source of cyanidin-3-laminaribioside unique to plants

Abstract

The work analyzes anthocyanins of red onion, which contain a unique 3-glycoside of cyanidin–laminaribioside (Cy3Lami), synthesized in the plant from cyanidin-3-glucoside (Cy3Glu) by adding another glucose molecule at position 3. This anthocyanin is an isomer of the more commonly found naturally occurring cyanidin-3-sophoroside (Cy3Sopho), in which the second glucose molecule is attached at position 2. It was found that the retention of both isomers under reversed-phase HPLC conditions differs significantly: if Cy3Sopho is retained less strongly than Cy3Glu, then the retention times of Cy3Lami are close to the retention of Cy3Rut, and it is necessary to select the composition of the mobile phase to separate these components. It should be noted that the electronic absorption spectra of Cy3Glu, Cy3Sopho, and Cy3Lami are practically indistinguishable, but are distinguishable from the spectrum of Cy3Rut, which has a small bathochromic shift of the main absorption band by 1 nm. The only feature common to all three diglycosides was the slope of the trend lines on the separation maps, which is characteristic of all diglycosides under reversed-phase HPLC conditions. In addition to the two main glycosides (Cy3Glu and Cy3Lami), the chromatograms also revealed two products of their acylation with malonic acid, hydrolyzed quite quickly under the conditions of storage of anthocyanins in acidic solutions. Previously, when separating anthocyanins under hydrophilic chromatography conditions, it was found that both hexosides (glucoside and galactoside) have the same retention times, as do both pentosides (arabinoside and xyloside), which differ in lower retention compared to hexosides. This is typical for the distribution mechanism of anthocyanin retention in the water-enriched layer generated in situ on the surface of the sorbent. However, two cyanidin diglucosides (Cy3Sopho and Cy3Lami) showed different retention times when separated by hydrophilic interaction chromatography on a Kromasil 60-5DIOL stationary phase. The hope that the third possible (according to literary data) isomeric cyanidin diglucoside is contained in red rye seedlings was not justified: the diglycoside from the extract of this plant did not differ from Cy3Rut either in chromatographic behavior or in electronic absorption spectra.