Abstract
The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of [Cr(H2O)5(OH)]2+ with the ligand in weak acidic aqueous solutions that follows an I d mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are ΔH ≠ 2(obs) = 61.2 ± 3.1 kJmol−1, ΔS ≠ 2(obs) = −91.1 ± 11.0 JK−1mol−1, ΔH ≠ 3(obs) = 124.5 ± 8.7 kJmol−1, and ΔS ≠ 3(obs) = 95.1 ± 29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value of ΔS ≠ 3(obs) can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease.
Highlights
Pathogens have developed many strategies to cope with iron limitation caused by plants and mammals in order to restrict their unwanted growth in them
One of the best-known methods is through the synthesis of small molecules that can act as iron chelators known as siderophores [1]. 2,3-Dihydroxybenzoic acid (2,3-DHBA) is a monocatechol siderophore
The form of the metal ion that reacts with the ligand 2,3DHBA is [Cr(H2O)5OH]2+ since it is well known that the hydroxy complex [Cr(H2O)5(OH)]2+ is highly more reactive than [Cr(H2O)6]3+
Summary
Pathogens have developed many strategies to cope with iron limitation caused by plants and mammals in order to restrict their unwanted growth in them. 2,3-Dihydroxybenzoic acid (2,3-DHBA) is a monocatechol siderophore. One of the best-known methods (strategies) is through the synthesis of small molecules that can act as iron chelators known as siderophores [1]. This ligand is a triprotic acid H3L, and the values of K1 and K2 refer to the protonation constants of the two hydroxylate groups and K3 refers to that of the carboxyl group. At 25◦C and Ionic Strength 0.2 M the average values of the accepted constants for 2,3-DHPA are log(K2/M−1) = 9.86 ± 0.04 and log(K3/M−1) = 3.00 ± 0.27. Catecholic type of coordination was suggested for the Fe(III)-2,3-DHBA complex [4, 5]
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