Molecular Modelling Analysis of the Interaction Between Iron(III) and L-Cysteine in Solution in Water at Low pH

Abstract

The interaction between iron(III) and L-cysteine, an amino acid, in solution in water at low pH has been investigated by UV-Visible spectrophotometry using the technique of continuous variation and molecular modelling. The absorbance value at λ_max was plotted against concentrations of reactants. The maximum in the absorbance vs. concentration graph gave the ratio at which Fe³⁺ and L-cysteine combined. At low pH, iron(III) is found to form predominantly 1 : 1 complex with L-cysteine in which iron has a distorted tetrahedral coordination geometry. In the complex, cysteine acts as a tridentate ligand being bonded to Fe(III) through amino nitrogen, carboxyl oxygen and deprotonated sulfhydryl group. The fourth position in the coordination geometry is occupied by a water molecule. The proposed structure of the complex has been optimized and its electronic spectrum generated based on molecular mechanics and semi-empirical calculations. Comparison of the observed and predicted electronic spectra of the complex indicates that iron(III) in the complex is in the high spin state with a spin multiplicity of 6.