Synthesis, crystal structures and protease activity of amino acid Schiff base iron(III) complexes

Abstract

Iron(III) complexes, (NHEt³)[Fe(III)(sal-met)₂] and (NHEt₃)[Fe(III)(sal-phe)₂], of amino acid Schiff base ligands, viz., N-salicylidene-L-methionine and N-salicylidene-L-phenylalanine, have been prepared and their binding to bovine serum albumin (BSA) and photo-induced BSA cleavage activity have been investigated. The complexes are structurally characterized by single crystal X-ray crystallography. The crystal structures of the discrete mononuclear monoanionic complexes show FeN₂O₄ octahedral coordination geometry in which the tridentate dianionic amino acid Schiff base ligand binds through phenolate and carboxylate oxygen and imine nitrogen atoms. The imine nitrogen atoms are trans to each other. The Fe-O and Fe-N bond distances range between 1.9 and 2.1 Å. The sal-met complex has two pendant thiomethyl groups. The high-spin iron(III) complexes (μ_eff~5.9μ_B) exhibit quasi-reversible Fe(III)/Fe(II) redox process near -0.6 V vs. SCE in water. These complexes display a visible electronic band near 480 nm in tris-HCl buffer assignable to the phenolate-to-iron(III) charge transfer transition. The water soluble complexes bind to BSA giving binding constant values of ~10⁵ M^-1. The complexes show non-specific oxidative cleavage of BSA protein on photo-irradiation with UV-A light of 365 nm.