These observations can be rationalized by the strong negative cooperativity for the Tris-HCl buffer (the metal-templated dimer formation would be disfavoured, i.e., the initial metal-binding event outcompetes the formation of templated dimer) by considering Tris-HCl interacts strongly with Cu II , retaining it in solution. This maximises the effective Cu II concentration able to bind dHis motifs, and since for templated dimer formation, one monomer must have an unoccupied dHis motif, Tris-HCl buffer disfavours this. On the contrary, precipitation of Cu II as Cu3(PO4)2 reduces the effective Cu concentration in solution. Lower Cu II availability reduces the apparent affinity of dHis motifs, fewer dHis motifs are occupied, and metal-templated dimer formation is favoured.