Engineered Synthetic Polymer Nanoparticles as IgG Affinity Ligands

A process for the preparation of an abiotic protein affinity
ligand
is described. The affinity ligand, a synthetic polymer hydrogel nanoparticle
(NP), is formulated with functional groups complementary to the surface
presentation of the target protein. An iterative process is used to
improve affinity by optimizing the composition and proportion of functional
monomers. Since the polymer NPs are formed by a kinetically driven
process, the sequence of functional monomers in the polymer chain
is not controlled; only the average composition can be adjusted by
the stoichiometry of the monomers in the feed. To compensate for this
the hydrogel NP is lightly cross-linked resulting in chain flexibility
that takes place on a submillisecond time scale allowing the polymer
to “map” onto a protein surface with complementary functionality.
In this study, we report a lightly cross-linked (2%) N-isopropyl acrylamide (NIPAm) synthetic polymer NP (50–65
nm) incorporating hydrophobic and carboxylate groups that binds with
high affinity to the Fc fragment of IgG. The affinity and amount of
NP bound to IgG is pH dependent. The hydrogel NP inhibits protein
A binding to the Fc domain at pH 5.5, but not at pH 7.3. A computational
analysis was used to identify potential NP–protein interaction
sites. Candidates include a NP binding domain that overlaps with the
protein A–Fc binding domain at pH 5.5. The computational analysis
supports the inhibition experimental results and is attributed to
the difference in the charged state of histidine residues. Affinity
of the NP (3.5–8.5 nM) to the Fc domain at pH 5.5 is comparable
to protein A at pH 7. These results establish that engineered synthetic
polymer NPs can be formulated with an intrinsic affinity to a specific
domain of a large biomacromolecule.