pH-dependent oligomerization of BPTI in undersaturated and supersaturated solutions studied by dynamic light scattering

Bovine pancreatic trypsin inhibitor (BPTI), a small protein molecule, is reported to form several types of oligomers in solution including a decamer. We investigate in this work the relation between the oligornerization and crystallization. The oligornerization and interaction between BPTI molecules in NaCl and NaH2PO4/K2HPO4 solutions were investigated by dynamic light scattering (DLS). In the NaCl solutions, we found that strong electrostatic interaction among BPTI and chloride ions led to the decamer formation only at an acidic pH, whereas the protein solutions were monomeric at a neutral or basic pH, where the electrostatic interaction seemed to be unimportant. The single crystals with a hexagonal truncated bipyramidal shape grew only in the decameric solutions at an acidic pH in the presence of NaCl. On the other hand, the electrostatic interaction was weak in the presence of NaH2PO4/K2HPO4 at all the pH values investigated, and the solutions were always monomeric. In contrast with the NaCl solutions, single crystals with a rod-like shape appeared at a neutral and basic pH in the NaH2P04/K2HPO4 solutions. These results suggest that the crystallization mechanism in the presence of NaCl, where electrostatic interactions seem to drive crystallization, is totally different from that in the presence of NaH2PO4/K2HPO4 where another interaction such as hydrophobic interaction or hydrogen bonding might cause crystallization. (C) 2002 Elsevier Science B.V. All rights reserved.