Multimodal chromatography
is a powerful tool which uses multiple
modes of interaction, such as charge and hydrophobicity, to purify
protein-based therapeutics. In this work, we performed molecular dynamics
simulations of a series of multimodal cation-exchange ligands immobilized
on a hydrophilic self-assembled monolayer surface at the commercially
relevant surface density (1 ligand/nm2). We found that
ligands that were flexible and terminated in a hydrophobic group had
a propensity to aggregate on the surface, while less flexible ligands
containing a hydrophobic group closer to the surface did not aggregate.
For aggregating ligands, this resulted in the formation of a surface
pattern that contained relatively large patches of hydrophobicity
and charge whose sizes exceeded the length scale of the individual
ligands. On the other hand, lowering the surface density to 1 ligand/3
nm2 reduced or eliminated this aggregation behavior. In
addition, the introduction of a flexible linker (corresponding to
the commercially available ligand) enhanced cluster formation and
allowed aggregation to occur at lower surface densities. Further,
the use of flexible linkers enabled hydrophobic groups to collapse
to the surface, reducing their accessibility. Finally, we developed
an approach for quantifying differences in the observed surface patterns
by calculating distributions of the patch size and patch length. This
clustering phenomenon is likely to play a key role in governing protein–surface
interactions in multimodal chromatography. This new understanding
of multimodal surfaces has important implications for developing improved
predictive models and designing new classes of multimodal separation
materials.