INTERACTION OF THE TETRAMETHYLAMMONIUM ION WITH THE LYSOZYME MOLECULE, STUDIED USING NEUTRON-DIFFRACTION

Using single-crystal neutron diffraction, the interaction between the deuteriated tetramethylammonium (TMA) ion and the surface of lysozyme has been studied, in triclinic crystals of hen egg-white lysozyme soaked in a 30% volume concentration of TMACI. Four partially occupied TMA sites were located, and refined to a final crystallographic R factor of 0.135. Although the average B factor of the TMA-soaked lysozyme was higher than in the unsoaked crystals, no correlation between enhanced (or reduced) B factors and the TMA positions was found. There were also no significant differences induced in the protein conformation. The close environment of the TMA positions was found to be predominantly polar, in contrast to earlier related studies of the interactions of lysozyme with ethanol and DMSO, where the environment was found to be predominantly non-polar. That the last two molecules act as protein denaturants, while TMA has protectant properties, raises interesting issues covering the mechanisms of the stabilisation and destabilisation of protein structures that warrant further study.