Anthocyanins

From Stanford's Dahlia Project

Introduction

Anthocyanidins are a class of pigment molecules found in almost all plants. They are responsible for a complicated and diverse spectrum of plant color, ranging from salmon-pink, scarlet, violet, and deep blue. The variation in color produced by anthocyanidins is due to:

i) the co-existence of several anthocyanin pigments ii) variation in cellular concentrations of anthocyanins iii) the pH of the cell iv) copigmentation with other, individually colorless molecules v) the colloidal nature of the cell cytosol vi) the formation of metal cation-pigment complexes

Chemistry

Anthocyanidins rarely occur unmodified in nature. They are most commonly found glycosylated - referred to as anthocyanin. This glycosylation increases their solubility and stability in water. Solubility is augmented because the carbohydrates increase the number of hydroxyl groups per carbon atom, while stability is improved primarily through π-π stacking. This π-π stacking is generally due to acyl substituents on the carbohydrates. By arranging themselves such that hydrophobic moieties are stacked one on top of the other, the aromatic groups prevent water from attacking the oxonium cation or the ester bond to the carbohydrate.2 The importance of these sugar moieties has been demonstrated by comparing deterioration times of differentially substituted anthocyanins. Anthocyanins with slowly hydrolyzable carbohydrate groups had a more stable color than those with quickly hydrolyzable carbohydrate groups, and aromatic acyl groups provided greater stability than aliphatic acyl groups. Anthocyanin color varies significantly with pH; it is red in acidic conditions, colorless in mildly acidic conditions, and violet in base. This is due to structural modifications at each pH that result in a loss or gain of conjugation. It was noted, however, that the vacuole containing anthocyanins is only mildly acidic. Given the extreme instability of the positively charged oxonium ion, this seemed to make sense. However, if this were the case, the majority of anthocyanins would be expected to be colorless. It was this observation that led researchers to discover copigments – molecules that stabilized the colored form of the anthocyanin. It has been observed that the addition of tannin to malvidin-3-glucoside, for example, alters its color to a bluer red. Additionally, association with metal cations can promote stability and alter the color of anthocyanins. Recently, a paper was published demonstrating the existence of several of these mechanisms in vivo by X-ray crystallography. Here we see examples of both stabilization by copigments and examples of π-π stacking. This entire complex is coordinated by interactions between the metal cations and hydroxyls on the phenyl group.

Synthesis Pathway in Dahlias

  The anthocyanin pathway is currently unknown in dahlias.