Betanin (betanidin 5-O-β-D-glucoside) is a water-soluble plant pigment used as a color additive in food, drugs and cosmetic products. Despite its sensitivity to light and heat, betanin maintains appreciable tinctorial strength in low acidic and neutral conditions, where the color of other plant pigments, such as anthocyanins, quickly fades. However, betanin is an iminium natural product that experiences acid- and base-catalyzed hydrolysis to form the fairly stable betalamic acid and cyclo-DOPA-5-O-β-D-glucoside. Here, we show that the decomposition of betanin in aqueous phosphate solution pH 2-11 is subject to general base catalysis by hydrogen phosphate ion and intramolecular general acid and base catalysis, providing new insights on the mechanism of betanin hydrolysis. UV/Vis absorption spectrophotometry, 1 H NMR spectroscopy and mass spectrometry were used to investigate product formation. Furthermore, theoretical calculations support the hypothesis that the nitrogen atom of the tetrahydropyridine ring of betanin is doubly protonated, as observed for structurally simpler amino dicarboxylic acids. Our results contribute to the study of betanin and other pigments belonging to the class of betalains and to deepen the knowledge on the chemical properties of imino acids as well as on iminium-catalyzed modifications of carbonyl compounds in water.

Revisiting the Mechanism of Hydrolysis of Betanin.

Stability constants of the ternary nickel(II) complexes with salicylic acid and selected amino acids

In the present work the chemical speciation of ternary complexes systems formed by VO2+ ion with picolinic acid and the amino acids = α-alanine (HαAla), glycine (HGly), serine (HSer), threonine (HT...

Speciation studies of binary and ternary complexes formed with oxidovanadium(IV) ion picolinic acid and some amino acids

In this work, the ligand 2-methylquinoline-8-carboxylic acid (HL) was prepared using a modification of a previous protocol, and its protonation constants were studied. The stability constants of th...

Binary and ternary nickel (II) complexes with 2-methylquinoline-8-carboxylic acid and some amino acids

In this work we report the results of a speciation study on the ternary complexes present in the ternary Ni(II)- 1,10 phenanthroline (Phen)-amino acids and Ni(II)- 2,2-bipyridine (Bipy)-amino acids...

Ternary complex formation of the nickel(II), 2,2’-bipyridine, 1,10’-Phenanthroline and some aminoacids

Solution equilibria in the binary systems of Ni(II)–amino acid {aspartic acid (H2asp), glutamic acid (H2glu), cysteine (H2cys) and histidine (Hhis)} and in the ternary systems of Ni(II)–dipicolinic acid (H2dipic)–amino acid {aspartic acid (H2asp), glutamic acid (H2glu), cysteine (H2cys) and histidine (Hhis)} have been studied pH-potentiometrically. The formation constants of the resulting mixed ligand complexes have been determined at 25 °C and ionic strength 1.0 mol·dm−3 NaCl. In the Ni(II)–H2asp system the data indicated the formation of the [NiHL]+, NiL and [NiL2]2− complexes. For the Ni(II)–H2glu system, the formation of the species NiL and [NiL2]2− are indicated. In the Ni(II)–H2cys system the complexes [NiHL]+, NiL and [NiHL2]− were detected, while in the Ni(II)–Hhis system the species [NiH2L]3+ [NiHL]2+, NiL+, [Ni(HL)2]2+, [NiHL2]+ and NiL2 were detected. In the ternary system Ni(II)–H2dipic–H2asp, the species Ni(dipic)(H2L), [Ni(dipic)(HL)]−, [Ni(dipic)(L)]2− and [Ni(dipic)(L)(OH)]3− were detected. In the Ni(II)–H2dipic–H2glu system, the species Ni(dipic)(H2L), [Ni(dipic)(HL)]−, [Ni(dipic)(L)]2− and [Ni(dipic)(L)(OH)]3− were detected. In the Ni(II)–H2dipic–H2cys system, the species Ni(dipic)(H2L), [Ni(dipic)(HL)]− and [Ni(dipic)(L)]2− were detected. Finally, in the Ni(II)–H2dipic–Hhis system, the species [Ni(dipic)(L)]− and [Ni(dipic)(L)(OH)]2− were detected. Species distribution diagrams as a function of pH are briefly discussed.

Solution Equilibria and Stabilities of Binary and Ternary Systems of Nickel(II) Complexes with Dipicolinic Acid and the Amino Acids (Histidine, Cysteine, Aspartic and Glutamic Acids)

José Lema

Papers

Solution Equilibria and Stabilities of Binary and Ternary Systems of Nickel(II) Complexes with Dipicolinic Acid and the Amino Acids (Histidine, Cysteine, Aspartic and Glutamic Acids)