Selective extraction of histidine derivatives by metal affinity with a copper(II)–chelating ligand complex in an aqueous two-phase system

• Metal affinity extraction of histidine derivatives in an ATPS was studied.
• A complex between Cu and a commercial chelating ligand was used as the extractant.
• His was extracted with the Cu(II) complex under neutral or weakly basic conditions.
• Dominant parameters for extracts on the metal affinity extraction were clarified.
• His and His derivatives were selectively extracted from other amino acids.

Affinity extraction based on the interaction between a target molecule and a specific affinity ligand offers a novel separation system for biomolecules in an aqueous two-phase system, however, most of affinity ligands are expensive. In the present study, metal affinity extraction of histidine (His) derivatives using a complex between Cu(II) and a commercially available chelating ligand was studied in a poly(ethylene glycol) (PEG)/Li2SO4 ATPS. Alizarin complexone (3-[N,N-bis(carboxymethyl)amino methyl]-1,2-dihydroxy anthraquinone, AC) was selected as the chelating ligand because of the good extractability of Cu(II) into the upper PEG-rich phase. On the basis of coordinate bonding with Cu(II), the extraction of His in the presence of the Cu(II)–AC complex under neutral condition was 73%, which was much higher than that under Cu(II) free condition (13%). Among a series of divalent transition metal ions (Cu(II), Ni(II), Co(II), and Zn(II)), Cu(II) was the most effective for the extraction of His. Derivatives of His were selectively extracted in the presence of many other amino acids because of the specificity of the interaction between Cu(II) and imidazole group of His. Extracted His was quantitatively stripped from the Cu(II)–AC complex using competitive complexation with agents such as iminodiacetic acid and imidazole.