Metal affinity precipitation of proteins

Proteins containing multiple surface-accessible histidine residues can be precipitated using small quantities of bis-copper chelates. The chelates serve to crosslink the proteins, presumably via the accessible histidines, leading to the formation of large, insoluble complexes. When excess copper chelate is used to carry out the precipitation, the resulting precipitate has a stoichiometry of 1:1 copper:accessible histidine. The precipitation is analogous to antibody-antigen precipitin reactions and can be described qualitatively using simple equilibrium theory developed for those systems. Human hemoglobin contains a large number of surface histidines and is efficiently precipitated by the copper salt CuSO4 as well as by bis-copper chelates. Sperm whale myoglobin contains many fewer surface histidines and is precipitated only by the bis-chelates. The effects of the number of accessible histidines on the protein, the chain length separating the two chelates, and the pH on the precipitation reaction have been investigated.