(11i) Explaining Preferential Binding of Proteins to Mica Surfaces By Analysis of Ion-Water-Surface Interactions

The mica class of inorganic minerals are desirable materials for hierarchical assembly due to the property of nearly atomically flat cleavage, providing a lattice of predictably spaced ions onto which proteins or other macromolecules template with preferential orientations. The number of unique axes along which a given protein liquid crystal can grow differs with both the solution ionic strength and the specific type of mica substrate, a tendency that must be carefully understood to control the emergence of order in these systems. To identify the length scales at which Designed Helical Repeat (DHR) mica-binding proteins recognize mica variants muscovite or phlogopite as being laterally anisotropic, atomistic simulations were conducted with restrained protein orientations and ion on-off rates expedited by Parallel-Bias Partitioned Families Metadynamics. Then, the mean-field charge density of water was taken as input for Local Molecular Field (LMF) theory to obtain the long-range electrostatic potential as the protein’s alignment with the binding axes changes. This approach uses the orientation of water molecules from simulation as reporters for the electric field above anisotropic surfaces with and without another charged body’s interference. Due to the high ion concentrations used in experiment, the orientation of water dipoles show damped oscillation with distance from the surface, waves that constructively interfere when the charged face of the protein is oriented parallel to the surface. The length scales at which oriented attraction is felt between the bodies sheds light on the nature of their binding, suggesting that displacing water and ions from the respective surfaces is not only unnecessary, but that these particles mediate the interaction and confer the anisotropic driving force. These findings offer insight into experimental observations of alignment preferences in related charge-screened systems and suggest rethinking the accessibility of mica surface cations.