Streptavidin tetramerization and 2D crystallization: A mean-field approach

A mean-field theoretical approach is applied to streptavidin tetramerizatio n and two-dimensional (2D) crystallization. This theory includes, in partic ular, solvent-residue interactions following the inhomogeneous Flory-Huggin s model for polymers. It also takes into account residue-residue interactio ns by using tabulated pair interaction parameters. This theory allows one t o explicitly calculate the entropy of the inhomogeneous system. We show tha t hydrophobic interactions are responsible for the stability of tetrameriza tion. Within the present theory, the equilibrium distance between the two d imers is the same as that determined experimentally. The free energy of tet ramerization (i.e,, dissociation of the two dimers) is 50 k(B)T. Unlike tet ramerization, hydrophobic interactions alone are not sufficient to stabiliz e the 2D crystal C-222, but solvent-mediated residue-residue interactions g ive the most important contribution.