Abstract
Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (I d mechanism) of water molecule from the Cr(H2O)5OH2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are ΔH 2(obs) ≠ = 28.6 ± 2.9 kJ mol−1, ΔS 2(obs) ≠ = −220 ± 10 J K−1mol−1, ΔH 3(obs) ≠ = 62.9 ± 6.7 kJ mol−1 and ΔS 3(obs) ≠ = −121 ± 22 J K−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.
Highlights
Polyphenols are widely found in plants and are present in all plant-derived systems [1,2,3]
Hydroxy-cinnamates, especially caffeic acid and its derivatives, are widely distributed and their presence in fruit juices is due to their easy extractability
Our goal was to study the kinetics and mechanism of the complexation of caffeic acid with chromium(III) under weak acidic aqueous conditions that exclude the autoxidation of the ligand
Summary
Polyphenols are widely found in plants and are present in all plant-derived systems [1,2,3]. Hydroxy-cinnamates, especially caffeic acid (see Figure 1) and its derivatives, are widely distributed and their presence in fruit juices is due to their easy extractability. Foods containing polyphenols undergo enzymatic and nonenzymatic browning due to autoxidation reactions [4, 5]. Initial oxidation of compounds like caffeic acid results in the formation of their quinone form [6,7,8], which, being strongly electrophilic, undergoes nucleophilic attack [9,10,11]. The above reaction takes place under acidic conditions and is catalysed in the presence of metals [12,13,14]. Food processing that includes treatment in alkaline conditions results in decreased nutritional value due to phenol oxidation reactions with amino acids and proteins through their nitrogen [15]
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