In the present study, molecular simulations were performed to investigate the chelating mechanisms of various metal ions to the His-tag motifs with various His residues. The chelation mostly involved the i and i+2 His residues for Ni²⁺, Zn²⁺, Cu²⁺, and Co²⁺, while the cooperation of 3 His residues was necessary when Fe³⁺ was involved in chelation with His-tags having more than 4 His residues. Metal ion was best fitted into the pocket formed by the imidazole nitrogens while it was about equally located among these nitrogen atoms. His-tag6 was found to have little effect on the structural integrity while the target protein contains more than 68 amino acid residues. Ni²⁺ interacted with the imidazole nitrogen of His3 in the beginning of chelation, and then entered into the pocket formed by His3 and His5 at 4 ns during the 10 ns molecular dynamics simulations. The fast chelating process resulted in successful application of IMAC techniques in efficient protein purification.

Key words: Molecular simulations, Chelating, His-tag, Molecular dynamics, IMAC.