Molecular dynamics simulation of the effect of ligand homogeneity on protein behavior in hydrophobic charge induction chromatography.

Hydrophobic charge induction chromatography (HCIC) is an adsorption chromatography combining hydrophobic interaction in adsorption with electrostatic repulsion in elution. Ligand density has significant effects on protein adsorption behavior, but little is understood about the effect of ligand homogeneity on surface morphology of ligands, protein conformational transition and dynamics within adsorbent pore due to the lack of microscopic experimental techniques. In the present study, a coarse-grained adsorbent pore model established in an earlier work is used to represent the actual porous adsorbent composed of matrix and immobilized HCIC ligands. Two adsorbent pores with different ligand distributions are constructed by adjusting the coupling sites, denoted as L1 and L2. In L1 the ligands are bonded uniformly while in L2 the ligands are arranged in lines in the axial direction and thus exhibit a heterogeneous distribution. Protein adsorption, desorption, and conformational transition in both L1 and L2 are shown by molecular dynamics simulations of a 46-bead beta-barrel coarse-grained model protein within the adsorbent pore models. The simulations indicate that ligand homogeneity has significant effect on both the irreversibility and the dynamics of adsorption while no obvious effect on protein conformation distribution. In comparison with L1, L2 leads to irreversible and slow adsorption, indicating the strict requirement of a suitable protein orientation to reach stable adsorption. The simulations have provided new insight into the microscopic behavior of HCIC, which would be beneficial to the rational design of adsorbents and parameter optimization for high-performance HCIC.