Studies of a Tripodal Biomimetic Siderophore Analog: An Efficient Encapsulation for Fe(III) Ion

A new tris-(2-aminoethyl)amine (TREN) capped tripodal Schiff base ligand has been developed by mimicking structural features of a natural siderophore, Bacillibactin, by substituting the catechol units with salicylaldehyde and employing amino acid as spacer. Synthesis of the ligand N-[2-[bis[2-[[2-[(2-hydroxyphenyl)methylamino]acetyl] amino]ethyl]amino]ethyl]-2-[(2-hydroxyphenyl)methylamino]acetamide(TRENglySAL) includes condensation of salicylaldehyde  and amino acid (glycine) followed by an in-situ reduction of the produced Schiff base, followed by further condensation with TREN. The complexation behavior of the ligand with Fe(III) has been investigated by potentiometric and UV-Vis spectrophotometric method at temperature 25±1oC and 0.1 M ionic strength. Seven protonation constants were obtained of the ligand: three each for secondary amine and phenolic protons and one for tertiary nitrogen of capping TREN moiety. The formation constants (log β11n) of different complex species MLH3, MLH2, MLH, ML and MLH-1 are found to be 35.15, 32.09, 27.91, 25.32 and 17.32 respectively. These results indicate that the ligand is an excellent iron binding chelator. Significant electronic spectral variations during complex formation at higher pH also provide a scope for the ligand to act as an optical pH sensor towards Fe (III) metal ion in biological systems. Structures of the ligand and metal complexes are proposed through experimental findings along with the theoretical semi-empirical PM3 calculation. The theoretical spectroscopic results are found to be comparable with the experimental finding.