Raman spectroscopy and DFT calculations of As(III) complexation with a cysteine-rich biomaterial

Arsenite adsorption onto a protein-rich biomass and, more specifically, the chemical groups involved in the uptake were investigated using Raman spectroscopy and DFT calculations. The study was based on spectroscopic analyses of raw and arsenic-loaded biomass as well as standard samples of amino acids and arsenic salts. The predominant secondary structure of the protein was identified as the β-sheet type, with some contribution from α-helix structures. The participation of sulphydryl groups from cystine/cysteine molecules during the adsorption of arsenite was demonstrated. Only the gauche–gauche–gauche (g–g–g) conformation type of the disulfide bonds was involved in arsenic complexation. The formation of a pyramidal trigonal As(HCys) 3 complex was modeled according to the density functional theory (DFT). The agreement of the DFT harmonic frequencies with the RAMAN spectra of the As(HCys) 3 complex demonstrated the relevant features of the cysteine-rich biomaterial regarding arsenic uptake as well as of the mechanism involved in the As(III)/biomass interaction at a molecular level. The results also illustrate that Raman spectroscopy can be successfully applied to investigate the mechanism of metal adsorption onto amorphous biomaterials. During arsenite adsorption onto a protein-rich biomass each As(III) atom reacts with three sulfur atoms from cysteine. Three water molecules are released and a pyramidal As–S complex is formed.