Computational Design of Oligopeptide Containing Poly(ethylene glycol) Brushes for Stimuli-Responsive Drug Delivery

Stimuli-responsive biomaterials are used to facilitate drug and gene delivery by shielding the drug/gene during circulation times and selectively releasing the cargo at the desired target. Within stimuli-responsive materials, pH-responsive materials are exploited for delivery to specific organs, intracellular compartments, cancer cells, site of inflammation or infection as those sites are characterized by pH that is different from the blood pH. In this paper we use molecular dynamics (MD) simulations to design such pH-responsive biomaterials where the balance between the various intermolecular interactions (e.g., electrostatics, van der Waals) within the biomaterials allow biofunctional molecules to be reversibly shielded and exposed to the environment with change in pH. In our model the shielding aspect is imparted by a polyethylene glycol (PEG) brush and the pH-responsive component is a PEG-tethered oligopeptide that undergoes changes in conformations via protonation of residues upon changes in pH. Starting with a PEG-tethered peptide in a monodisperse short PEG brush, we first vary the composition and sequence of histidine (H), lysine (K), and glutamate (E) along the oligopeptide sequence to find the design parameters that maximize the shielding and exposure of the oligopeptide at pH ∼ 7.0 and pH < 7.0, respectively. Then, we probe the effect of the PEG brush on the conformations of the oligopeptides by simulating PEG-tethered peptide in a bimodal PEG brush containing short PEG and long PEG chains. We characterize the intermolecular interactions involving the PEG, peptide, and solvent that influence the shielded and exposed conformations of the oligopeptides at the two different pHs. In a short monodisperse PEG brush, with a longer PEG-tethered peptide containing large blocks of histidines that undergo change in protonation state as a response to pH change, placed between a protonated lysine and deprotonated glutamate, the PEG brush exhibits maximum shielding and exposure with pH change. This change from shielded to exposed state is driven by electrostatic repulsion upon H protonation. The presence of long PEG chains in a bimodal PEG brush leads to dominating PEG-peptide attractive interactions that reduces the contrast in shielded and exposed conformations of the PEG-tethered peptide upon protonation of histidines.