Can electrospray ionization help the characterization of iron gall inks? Investigation on the interactions of gallic acid with iron ions in aqueous solutions

Chelation of iron(II) ions by ellagitannins-Effects of hexahydroxydiphenoyl and nonahydroxytriphenoyl groups

Tannins represent secondary plant metabolites that are used to control bacterial populations by chelation of essential metal ions. Their presence in food also affects the bioavailability of iron. This study investigates the influence of ellagitannins (vescalin, castalin, vescalagin, castalagin) structure and pH on the stoichiometry and formation constants of ellagitannin-Fe(II) coordination compounds. We demonstrated that ellagitannins are stable for at least one hour at pH values lower than 7.25. The spectra of neutral compounds were measured and explained with the help of TDDFT calculations. Furthermore, the pH-dependence of the ellagitannins UV-Vis spectra was examined to obtain insight into their protolytic equilibrium. Using Job's method in the pH range 3.50-5.51, the stoichiometry of the formed ellagitannin-Fe(II) ions complexes was determined. A model explaining interactions between ellagitannins and Fe(II) ions, that took into account the protolytic equilibrium of ellagitannins, was fitted globally to all four Job plots, whereby the corresponding formation constants were obtained.

Dependence of the Fe(II)-Gallic Acid Coordination Compound Formation Constant on the pH

One important property of tannins involves their ability to form coordination compounds with metal ions, which is vital for the bioavailability of these ions, as well as for the antibacterial and antioxidative activities of tannins. In this study, the pH dependence of interactions between gallic acid, one of the basic building blocks of tannins, and Fe(II) ions, was investigated using UV/Vis spectroscopy, in conjunction with density functional theory (DFT) calculations. Moreover, two models were developed to explain the processes taking place in the solution. The first model treated the reaction as a simple bimolecular process while the second also considered the protolytic equilibrium, which was proven very successful in discerning the pH dependence of formation constants, and whose assumptions were well supported by DFT calculations. We showed that the two-time deprotonated gallic acid species forms the coordination compound with Fe(II) ions in a 1:1 molar ratio. To gain better insight into the process, the coordination compound formation was also studied using various DFT functionals, which further supported the model results. Furthermore, due to the relatively low sample amounts needed, the methodology developed here will be useful to study compounds that are more difficult to isolate.

Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications

Oxidative stress, a result of an overproduction and accumulation of free radicals, is the leading cause of several degenerative diseases such as cancer, atherosclerosis, cardiovascular diseases, ageing and inflammatory diseases.

Review of the bulk and surface chemistry of iron in atmospherically relevant systems containing humic-like substances

The current state of knowledge and future research directions of the bulk and surface chemistry of iron relevant to atmospheric surfaces are reviewed.

Kinetic study of the complexation of gallic acid with Fe(II)

Kinetic study on the complexation of gallic acid with ferrous sulfate was performed using UV-Vis absorption spectroscopy. Under the experimental conditions, the stoichiometric composition of the formed complex is 1:1. The complexation reaction was found to be a second-order one. The influences of temperature, ionic strength and solvents on the complexation reaction were investigated. According to the Arrhenius equation, the apparent activation energy of the complexation reaction was evaluated to be 71.64 kJ x mol(-1). A three-step reaction mechanism was proposed, which can well explain the kinetic results obtained.