Acylated Anthocyanins from Red Cabbage and Purple Sweet Potato Can Bind Metal Ions and Produce Stable Blue Colors

Abstract

Red cabbage (RC) and purple sweet potato (PSP) are naturally rich in acylated cyanidin glycosides that can bind metal ions and develop intramolecular π-stacking interactions between the cyanidin chromophore and the phenolic acyl residues. In this work, a large set of RC and PSP anthocyanins was investigated for its coloring properties in the presence of iron and aluminum ions. Although relatively modest, the structural differences between RC and PSP anthocyanins, i.e., the acylation site at the external glucose of the sophorosyl moiety (C2-OH for RC vs. C6-OH for PSP) and the presence of coordinating acyl groups (caffeoyl) in PSP anthocyanins only, made a large difference in the color expressed by their metal complexes. For instance, the Al3+-induced bathochromic shifts for RC anthocyanins reached ca. 50 nm at pH 6 and pH 7, vs. at best ca. 20 nm for PSP anthocyanins. With Fe2+ (quickly oxidized to Fe3+ in the complexes), the bathochromic shifts for RC anthocyanins were higher, i.e., up to ca. 90 nm at pH 7 and 110 nm at pH 5.7. A kinetic analysis at different metal/ligand molar ratios combined with an investigation by high-resolution mass spectrometry suggested the formation of metal–anthocyanin complexes of 1:1, 1:2, and 1:3 stoichiometries. Contrary to predictions based on steric hindrance, acylation by noncoordinating acyl residues favored metal binding and resulted in complexes having much higher molar absorption coefficients. Moreover, the competition between metal binding and water addition to the free ligands (leading to colorless forms) was less severe, although very dependent on the acylation site(s). Overall, anthocyanins from purple sweet potato, and even more from red cabbage, have a strong potential for development as food colorants expressing red to blue hues depending on pH and metal ion.

Identification of Anthocyanin Compounds in Butterfly Pea Flowers (Clitoria ternatea L.) by Ultra Performance Liquid Chromatography/Ultraviolet Coupled to Mass Spectrometry

“…The chemical structures and molecular weights of six common anthocyanidins and the most common sugars and acylation groups are shown on the basis of preferences. The study of anthocyanin composition in butterfly pea flowers was started by using the basic anthocyanin structure as a reference, taking into account previous literature data on anthocyanin structure [29][30][31][32][33]. More recently, faster methods have been proposed for the quantification of polyphenols using more powerful chromatographic systems such as UHPLC in combination with a UV detector [25,26] or mass analyzer [27].…”

Addition of Chelators Increased the Stability of Black Rice Anthocyanins against the Metallic Ions in Tap Water and Improved the Coloration of Steamed Cold Noodles

“…Anthocyanins could bind metallic ions via the catechol B-ring in neutral or mildly acidic solutions [ 26 ]. As shown in Figure 1 a, the BRA solution exhibited an attractive red color in deionized water; however, the color turned black in tap water or deionized water containing the nine metals allowed by GB 5749-2006 [ 9 ], indicating that the presence of metallic ions could cause color decay of the BRA solution, which could limit the application of BRA as a food colorant.…”

Anthocyanins as Natural Food Colorings: The Chemistry Behind and Challenges Still Ahead

“…1 min after mixing (green) and at equilibrium (blue, 15−20 min after mixing). 21 In the competition between water addition (color loss) and metal binding, the former prevails with P1, while the latter prevails with P2, resulting in a huge bathochromic shift of the visible band (>100 nm). Upon binding, the iron center undergoes autoxidation to Fe 3+ .…”

“…This monoacylated ACN not only is exceptionally resistant to hydration but also forms highly stable blue complexes with Al 3+ and Fe 3+ . 18,21 The complexes, which consist in 3 cyanidin ligands in octahedral coordination with the metal center, are efficiently stabilized by interligand cyanidin− sinapoyl π-stacking interactions. Unfortunately, this remarkable ACN is only a minor component of the pool of red cabbage pigments, and its extraction/purification does not meet the economic criteria of industrial development.…”