Thermodynamic and IR Spectral Study of Metal Cations–Anthocyanin Chelation: Mechanism of Formation of Pigments

Abstract

Metal ions chelation of some anthocyanins [ACN-Al3+/Ga3+/Cr3+/Fe3+/Mg2+] chelation in aqueous medium were investigated to discover the color fluctuation of different derivatives of these compounds at low pH. Chelation of Mn+ by ACN also improved the color expression range of ACN in more different pH environments. In this verdict, it has been studied the metal ions diffusing of deprotonating for the ACN B-ring of cyanidin (Cy), delphinidin (Dp), and petunidin (Pt) in vacuum and water media, transforming flavylium cations to the blue quinonoidal bases at lower pH using the infrared method though pursuing Beer–Lambert law for finding the physical and chemical properties of frequency, intensity, and absorbance of the compounds, respectively. In this wok, it has been illustrated that the important factor for increasing the absorbance in a positive non-linear fashion due to deviating from the Beer–Lambert law is the self-association of anthocyanins of Cy, Dp, and Pt anthocyanins. The difference of ∆HR among [ACN-Al3+/Ga3+/Cr3+/Fe3+ /Mg2+] chelation complexes has been discussed the double bonds and carbonyl groups through the chelation of B ring for Cy, Dp, and Pt anthocyanins in vacuum and water media that explains the stability and color of [ACN-Al3+/Ga3+/Cr3+/Fe3+ /Mg2+] chelation of Cy, Dp, and Pt pigments in a weak acidic condition. The ACNs of Cy, Dp, and Pt with highest chelate in the active part of these structures by metal ions of Al3+/Ga3+/Cr3+/Fe3+/Mg2+ produce a variety range of colors under acidic pH; as divalent Mg2+ has shown various physicochemical properties considering its optimized geometry optimization as the bond length of O $$\underline{\cdots \cdots}\,$$ Mg2+ ≈1.9 A and the bond angle of O $$\underline{\cdots \cdots}\,$$ Mg2+ $$\underline{\cdots \cdots}\,$$ O ≈112° while it has been indicated the bond length of O $$\underline{\cdots \cdots}\,$$ M3+ ≈1.8 A and bond angle of O $$\underline{\cdots \cdots}\,$$ M3+ $$\underline{\cdots \cdots}\,$$ O ≈120° for trivalent metal cations of Al3+/Ga3+/Cr3+/Fe3+ with more electron rich metal ions caused shifts and hue changes in the stability of color and structure. Moreover, we have found that [ACN-Al3+/Ga3+/Cr3+/Fe3+/Mg2+] chelation is dependent on the position of active cites of labeled oxygen atoms and cations in these structures which transfer the charge of electrons in aromatic cyclic chains because of water polar medium in contrast to vacuum. The spin density and partial charges have been obtained by fitting the electrostatic potential to fixed charge of $${\text{O}}_{{17}}^{ + }$$ , $${\text{O}}_{{16}}^{ + }$$ , and $${\text{O}}_{7}^{ + }$$ cations for Cy-Mn+ (31), Dp-Mn+ (32), and Pt-Mn+ (35), respectively based on the electrophilic groups of Cy, Dp, and Pt anthocyanin pigments which unravel the activity and the stability of these compounds in the natural products.