Salt crystallizing agents studied to date generally induce a protein salting-out phenomenon at high concentrations, whereas the addition of NiCl2 over 0.5 M induces an increase in the solubility of the tetragonal form crystals of hen egg-white lysozyme. To elucidate this particular effect of NiCl2 on the solubility, the structures of the crystals grown at pH 4.7 in the presence of 0.25, 0.31, 0.50, 1.05, and 1.27 M NiCl2 have been determined. The protein molecules in these crystals were shown to have an identical main chain and side chain conformations. For each crystal structure, one Ni2+ binding with the O delta atom of Asp52 in the active site, and one Cl- interacting with the O eta atom of Tyr23 have been identified. The a (or b) of unit cell dimensions, the unit cell volume, and the distance between the Ni2+ and the O delta atom of Asp52 have minimum values at 0.5 M NiCl2, indicating that the crystals are the most compact at the solubility minimum. Also, the B-factors had the least values. Electrostatic and preferential protein-solvent interactions between NiCl2 solution and lysozyme due to the binding of Ni2+ to the active site of lysozyme are suggested to be responsible to the solubility minimum phenomenon. (c) 2004 Elsevier B.V. All rights reserved.