Abstract
Anthocyanins with various functions in nature are one of the most important sources of colours in plants. They are based on anthocyanidins or 3-deoxyanthocyanidins having in common a C15-skeleton and are unique in terms of how each anthocyanidin is involved in a network of equilibria between different forms exhibiting their own properties including colour. Sphagnorubin C (1) isolated from the cell wall of peat moss (Sphagnum sp.) was in fairly acidic and neutral dimethyl sulfoxide characterized by nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–vis) absorption techniques. At equilibrium, the network of 1 behaved as a two–component colour system involving the reddish flavylium cationic and the yellow trans–chalcone forms. The additional D- and E-rings connected to the common C15-skeleton extend the π-conjugation within the molecule and provide both bathochromic shifts in the absorption spectra of the various forms as well as a low isomerization barrier between the cis- and trans-chalcone forms. The hemiketal and cis-chalcone forms were thus not observed experimentally by NMR due to their short lives. The stable, reversible network of 1 with good colour contrast between its two components has previously not been reported for other natural anthocyanins and might thus have potential in future photochromic systems. This is the first full structural characterization of any naturally occurring anthocyanin chalcone form.
Highlights
Anthocyanins are ubiquitous in plants, and the total number of anthocyanin structures identified after isolation from plant extracts is about 800. These pigments are mainly divided into two groups, anthocyanins and 3-deoxyanthocyanins, with the latter group lacking an oxygen-function on the anthocyanidin 3-position on the C-ring [1]
The vast majority of the common anthocyanins are based on just 6 different aglycones varying with the substitutions on the B-ring, and range in colour from salmon red to dark purple and blue [2]
The vast majority of structure elucidations of anthocyanins have been undertaken in acidic environments on the flavylium cation form, which is considered to be the most stable form
Summary
Anthocyanins are ubiquitous in plants, and the total number of anthocyanin structures identified after isolation from plant extracts is about 800. These pigments are mainly divided into two groups, anthocyanins and 3-deoxyanthocyanins, with the latter group lacking an oxygen-function on the anthocyanidin 3-position on the C-ring [1]. The vast majority of the common anthocyanins are based on just 6 different aglycones (anthocyanidins) varying with the substitutions on the B-ring, and range in colour from salmon red to dark purple and blue [2]. The various pyranoanthocyanins have in common an extra pyranic ring formed by cyclic addition onto both carbon 4 and the hydroxyl group at carbon 5 of the anthocyanidin skeleton. In recent years it has been shown that synthetic flavylium compounds analogous to anthocyanidins possess the same general network of chemical reactions [10]
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