Novel and colourful flavonoids - characterisation, secondary structures and properties

Abstract

This dissertation focuses on isolation and structural elucidation of flavonoid pigments. The properties of these pigments have been explored, in terms of their chemical properties, especially regarding secondary structures in neutral solvents.

Liverworts (Marchantiophyta) are basal land plants considered the probable most recent common ancestor of land plants and algae. The liverwort Marchantia polymorpha produces red flavonoid pigmentation in response abiotic stress, bound to the cell wall, that is proposed to be early evolved anthocyanidin forms. Taking advantage of transgenic marchantia plants overexpressing flavonoid production, two red pigments were isolated and their structure elucidated to be 2,3,6,8-tetrahydroxybenzofuro[3,2-b]chromen-5-ium-6-O-(2-O-α-rhamnopyranosyl-β-glucopyranoside (1), and its aglycone (2). In this work we show that these pigments represent a previously unreported flavonoid class named auronidins, for their similarities in structure to anthocyanidins and to aurones. Chemically, auronidins are capable of expressing a wide range of colours depending on the pH similar to anthocyanins. They also express strong fluorescence under UV light, in contrast to the majority of known anthocyanins. The chemical properties of 1 have been compared to both an aurone (aureusidin 4-O-glucoside, 4) and an anthocyanin (cyanidin 3-O-glucoside, 3). We have also shown that auronidins comes from the flavonoid biosynthetic pathway, which is a part of the larger phenylpropanoid pathway, but from a path distinct from that leading to anthocyanins. A summary of their work is presented here.

Peat moss (Sphagnum) is another basal land plant that are more evolutionary advanced than liverwort. Several species of peat moss produce red pigmentation bound to the cell walls in response to abiotic stress. These pigments have been isolated from Sphagnum sp. and Sphagnorubin C (5) have been structurally elucidated. This anthocyanin lacks glycosidic substitution, have an extended aromatic skeleton, and lacks the characteristic hydroxyl group in 3-position (3-deoxyanthocyanidin) featured in the vast majority of anthocyanins. In addition to the flavylium cation form, the trans-chalcone form of 5 have been elucidated, which in pure DMSO is stable and is fully reversible to the flavylium cation form by the addition of acid. No signs of neither hemiketal forms nor the cis-chalcone form was observed. This was the first full structural elucidation of a trans-chalcone form of a naturally occurring anthocyanin.

In addition to the pigments isolated from basal land plants, several new sources of anthocyanins have been discovered. Isolation and structural elucidation of anthocyanins from the mauve flowers of Erlangea tomentosa provided this thesis with two new anthocyanins, based on the novel anthocyanidin erlangidin, the first reported natural anthocyanidin having C-ring methoxylation (erlangidin 5-O-(4-(E-caffeoyl)-6-(malonyl)-β-glucopyranoside)-3’-O-(6-(3-(β-glucopyranosyl)-E-caffeoyl)-β-glucopyranoside), 6, and erlangidin 5-O-(6-(malonyl)-β-glucopyranoside)-3’-O-(6-(3-(β-glucopyranosyl)-E-caffeoyl)-β-glucopyranoside), 7). Additionally, new sources of petanin (8) and negretein (9) were discovered, including ‘Sun black’ tomato (Solanum lycopersicum), a tomato cultivar bred to produce deep purple to black anthocyanin-rich skin, and several cultivars of blue potato (Solanum tuberosum).