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Orientation of a Y-shaped biomolecule adsorbed on a charged surface.

The adsorption and orientation properties of a Y-shaped biomolecule, which models an immunoglobulin (Ig), on a charged surface are analyzed mesoscopically by Monte Carlo simulations. The orientation is a consequence of the interplay between van der Waals interactions and electrostatic interactions. For adsorption dominated by van der Waals attraction, the molecule prefers lying flat on the surface. For weak attraction, we observe a depletion zone in the concentration profile, which can result in a negative surface excess. A secondary peak is found for strong adsorption. For electrostatically dominated adsorption, the orientation is mainly determined by electric dipole and a vertically adsorbed molecule can be attained as it possesses strong electric dipole. Our study provides an explanation for experimental observations of preferential orientation.