Engineering Peptide-Based Polyelectrolyte Complexes with Increased Hydrophobicity

Abstract

Polyelectrolyte complexation is a versatile platform for the design of self-assembled materials. Here we use rational design to create ionic hydrophobically-patterned peptides that allow us to precisely explore the role of hydrophobicity on electrostatic self-assembly. Polycations and polyanions were designed and synthesized with an alternating sequence of d- and l-chiral patterns of lysine or glutamic acid with either glycine, alanine or leucine due to their increasing hydrophobicity index, respectively. Two motifs were considered for the oppositely charged patterned peptides; one with equal residues of charged and uncharged amino acids and the other with increased charge density. Mass spectroscopy, circular dichroism, H- and F-NMR spectroscopy were used to characterize the polypeptides. Polyelectrolyte complexes (PECs) formed using the sequences were characterized using turbidity measurements, optical microscopy and infrared spectroscopy. Our results show that the critical salt concentration, a key measure of PEC stability, increased with both increasing charge density as well as hydrophobicity. Furthermore, by increasing the hydrophobicity, the amount of PEC formed increased with temperature, contrary to purely ionic PECs. Lastly, we assessed the encapsulation behavior of these materials using a hydrophobic dye. Concluding that encapsulation efficiency increased with hydrophobic content of the complexes providing insight for future work on the application of these materials for drug delivery.

Patterning Polyelectrolyte Complexes with Alternating Monomer Sequence Distributions

“…This suggests that by reducing the effective charge density and increasing polymer hydrophobicity, chains can rearrange on slower timescales into monodisperse size distributions in the final state (Figure 2B). This is consistent with reported observations in the other bulk coacervate systems 14,16.…”

Recent progress in the science of complex coacervation

“…For instance, synthetic polypeptides have been used extensively as a model polymer system where different side chain functionalities can be introduced along the same backbone. 35,[43][44][45]55,75,77,136 Additionally, solid-phase synthesis enables precise control over chemical sequence, 70,[137][138][139][140][141][142][143] and can be combined with methods for controlled polymerization to allow for the preparation of well-controlled comb polymer architectures. 46,144 Controlled polymerization also has allowed for the synthesis of random copolymers to facilitate the introduction of multiple functionalities.…”

“…For all of these questions, the challenge is one of trying to understand changes in the structure of water. To date, the majority of approaches rely on indirect phenomenological measurements such as changes in the phase behavior of coacervates, 40,64,139 differences in the amount of water introduced into a sample by different ions, 53 or measurements of "non-freezing," surface-bound water. 89 Experimentally, new techniques such as terahertz dielectric spectroscopy have the potential to help directly access these structural changes, 149,150 and these questions look to be key to future materials design questionseven beyond coacervation.…”

“…45,140,275 A combination of experimental results using polypeptides of controlled chirality and molecular dynamics simulations demonstrated that a continuous sequence of approximately eight amino acids of the same chirality was needed to drive the formation of a stable β -sheet structure, tipping the stability of the system from liquid complex coacervates to solid precipitates. Systematic variation of hydrophobicity in both polypeptides 139 and synthetic polymers 40,53,64 has mostly demonstrated that hydrophobic polymers form denser coacervate phases, 40,53,139 though in some cases these differences were negligible. 64 In a separate study, Hyman, Alberti, and Pappu used mutagenesis studies of the FUS family of IDPs to demonstrate a hierarchy of interactions driving phase separation.…”

“…Atomistic detail has, except for a few studies, 167,200,203,206,207 been largely neglected; yet, variations in polymer chemistry and salt identity lead to significant differences in coacervate phenomena. 40,45,53,139 It is unclear the extent to which coarse-grained models will be able to -even with careful parameterization -capture the physical aspects of these differences. What is more apparent, however, is that there are at least some situations where an atomistic view is crucial.…”