Semi-Synthetic Proteins as Metal Ion Capture Agents: Catch and Release of Ni(II) and Cu(II) with Myoglobin Bioconjugates

Wereport the synthesis, characterization, and metal ioncapturing ability of a novel semi-synthetic protein, Mb-SG-20. Mb-SG-20 shows strong affinities for Ni(II) and Cu(II)and was able to capture super-stoichiometric amounts of Ni(II) andCu(II) from aqueous solutions in a recyclable manner. Furthermore, Mb-SG-20 captures Ni(II) and Cu(II) selectively from a complexmixture of metal ions in a CFA leachate solution. Metals play a vital role in manyindustries, but their extractionthrough traditional mining methods poses significant environmentalchallenges. To address these issues, alternative sources such as coalfly ash (CFA) need to be explored for the recovery of transition metals.In this study, we describe a novel green strategy for the selectiveseparation and recovery of valuable metal ions from complex solutionsusing semi-synthetic proteins. Myoglobin (Mb), a small protein availablefrom renewable sources, was conjugated to the high-affinity metalchelator SG-20 to create the semi-synthetic protein Mb-SG-20. Mb-SG-20wascharacterized by gel electrophoresis, mass spectrometry, and proteomicanalysis and found to contain a distribution of 4-7 equiv ofSG-20 conjugated to Mb surface lysine residues. Binding and recoveryproperties of Mb-SG-20 for nickel (Ni) and copper (Cu) ions were characterized.The results showed that Mb-SG-20 captures super-stoichiometric amountsof Ni or Cu while maintaining a strong affinity for Ni or Cu ions.Furthermore, we demonstrate the recyclability of Mb-SG-20 as a Niand Cu ion capture agent and provide proof-of-concept experimentsusing CFA leachate solution. This study presents a promising approachfor the sustainable recovery of valuable metals from non-traditionalsources, such as CFA.