Kinetics and thermodynamics of complex formation with iron of a new series of dicatecholspermidine siderophore-like ligands

This article deals with the kinetics and thermodynamics of complex formation between Fe(3+) and a series of four synthetic chelators of the 1,2-dicatecholspermidine family (LA5, LA3, LE5 and LE3). LA5 and LA3 bear a carboxylic moiety linked to the central nitrogen by either a C(5) or a C(3) chain, whereas LE5 and LE3 bear an ethyl ester moiety. The following data concern LE5, LE3, LA5 and LA3, respectively. Each species undergoes four acid-base dissociations of the hydroxyls of the catechols with, for the two hydroxyls in position 1; average pK(2a)=7.30, 7.25, 7.45, 7.34 and, for the two hydroxyl in position 2; average pK(3a)=12.35, 12.65, 12.10, 12.60. The LA5 and LA3 species also undergo proton-dissociations of their carboxylic moieties; pK(1a)=5.20 and 5.10. The four species form one-to-one iron complexes with, for the 1-hydroxyl; an average pK(22a)=2.65, 2.25, 2.95, 2.80, for the 2-hydroxyl; pK(33a)=5.20, 5.40, 6.10, 5.40 and, for the carboxylic moieties; pK(11a)=3.90 and 4.45. In the vicinity of pH 5, Fe(3+) is rapidly exchanged between FeNta and the four ligands. This occurs with direct rate constants: k(1)=(1.3+/-0.1)x10(4), (1.4+/-0.2)x10(4), (3.3+/-0.2)x10(4), (1.4+/-0.1)x10(4)M(-1)s(-1), and reverse rate constants: k(-1)=(7+/-0.5)x10(4), (9+/-1)x10(4), (1.15+/-0.15)x10(5), (7+/-0.5)x10(4)M(-1)s(-1). The kinetic data, the pK(a) values of the free ligands, those of the iron complexes and the beta value of FeNta allow us to determine the affinity constants of the four ligands for iron: logbeta(1)=33, 34, 33, 34, and pFe=23.3, 24.6, 22.2, 24.3. This implies that these ligands of the dicatecholspermidine family may act as siderophores. They may also be used as drug carriers which can utilize the bacterial iron-acquisition paths.