Anthocyanidin

Anthocyanidin is a class of plant flavonoids that represent the aglycone forms of anthocyanins, the pigments responsible for red, purple, and blue colors in many fruits, flowers, and leaves. Structurally, anthocyanidins are flavylium cations that bear various patterns of hydroxyl and methoxyl substitutions. Because they are typically glycosylated in nature, the corresponding glycosides are known as anthocyanins; glycosylation increases water solubility and stabilizes the molecule.

Common natural anthocyanidins include cyanidin, delphinidin, pelargonidin, peonidin, malvidin, and petunidin. In acidic solutions they exhibit bright colors; at neutral or alkaline pH they may decolorize or shift to different forms. Anthocyanidins are usually present in plants as glycosides rather than as free aglycones.

Biosynthesis: In the flavonoid pathway, phenylalanine is converted through several steps to dihydroflavonols; anthocyanidin synthase (ANS) converts leucoanthocyanidins to anthocyanidins; subsequent glycosyltransferases attach sugars to form anthocyanins. The exact pigment produced depends on enzyme activity and substitutions on the rings.

Occurrence and roles: Anthocyanidins contribute to the coloration of fruits and flowers, aiding in attraction of pollinators and seed dispersers, while also providing protection from abiotic stress. They are used as natural food colorants when glycosylated as anthocyanins; in food labeling they are referred to as E163 in the European Union.

Stability and analysis: Anthocyanidins are highly pH-dependent and sensitive to light, heat, and oxygen. They are typically analyzed by high-performance liquid chromatography (HPLC) with UV-visible detection, often after conversion to their glycoside forms or with stabilized derivatives.