  We have theoretically studied the uptake of a non-uniformly charged
biomolecule, suitable to represent a globular protein or a drug, by a charged
hydrogel carrier in the presence of a 1:1 electrolyte. Based on the analysis of
a physical interaction Hamiltonian including monopolar, dipolar and Born
(self-energy) contributions derived from linear electrostatic theory of the
unperturbed homogeneous hydrogel, we have identified five different sorption
states of the system, from complete repulsion of the molecule to its full
sorption deep inside the hydrogel, passing through meta- and stable surface
adsorption states. The results are summarized in state diagrams that also
explore the effects of varying the electrolyte concentration, the sign of the
net electric charge of the biomolecule, and the role of including
excluded-volume (steric) or hydrophobic biomolecule-hydrogel interactions. We
show that the dipole moment of the biomolecule is a key parameter controlling
the spatial distribution of the globules. In particular, biomolecules with a
large dipole moment tend to be adsorbed at the external surface of the
hydrogel, even if like-charged, whereas uniformly charged biomolecules tend to
partition towards the internal core of an oppositely-charged hydrogel.
Hydrophobic attraction shifts the states towards internal sorption of the
biomolecule, whereas steric repulsion promotes surface adsorption for
oppositely-charged biomolecules, or the total exclusion for likely-charged
ones. Our results establish a guidance for the spatial partitioning of proteins
and drugs in hydrogel carriers, tuneable by hydrogel charge, pH and salt
concentration.
