Biophys J, April 2001, p. 2004-2010, Vol. 80, No. 4

Streptavidin Tetramerization and 2D Crystallization: A Mean-Field Approach

T. Coussaert,^* A. R. Völkel, J. Noolandi, and A. P. Gast^*

 ^*Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 USA, and  Xerox Research Center of Canada, Mississauga, Ontario L5K 2L1, Canada

A mean-field theoretical approach is applied to streptavidin tetramerization and two-dimensional (2D) crystallization. This theory includes, in particular, solvent-residue interactions following the inhomogeneous Flory-Huggins model for polymers. It also takes into account residue-residue interactions by using tabulated pair interaction parameters. This theory allows one to explicitly calculate the entropy of the inhomogeneous system. We show that hydrophobic interactions are responsible for the stability of tetramerization. Within the present theory, the equilibrium distance between the two dimers is the same as that determined experimentally. The free energy of tetramerization (i.e., dissociation of the two dimers) is 50 k_BT. Unlike tetramerization, hydrophobic interactions alone are not sufficient to stabilize the 2D crystal C₂₂₂, but solvent-mediated residue-residue interactions give the most important contribution.

Biophys J, April 2001, p. 2004-2010, Vol. 80, No. 4
© 2001 by the Biophysical Society   0006-3495/01/04/2004/07  $2.00

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