Synthesis and properties of polypeptide-based coacervates
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Synthesis and properties of polypeptide-based coacervates


Coacervation is a process involving the associative liquid-liquid phase separation (LLPS) of macromolecules in solution to form separate macromolecular-enriched and dilute liquid phases. The phenomenon of coacervation was first observed empirically as flocculation that occurs upon mixing aqueous solutions of oppositely charged macromolecules. Subsequently, synthetic, charged polyelectrolytes (PE) were used to fundamentally understand how changes to the solution environment and PE molecular features could influence phase separation. Interest in coacervation has increased dramatically in recent years since the discovery that proteins lacking defined structures (i.e., intrinsically disordered proteins (IDPs)) facilitate many complex biological processes via coacervation as membraneless organelles (MLOs) within cells. In recent years, the study of polypeptide and protein-based coacervates has been pursued as model systems to learn more about these biological processes. Currently, these MLO model systems utilize charged, synthetic homopolypeptides, short peptide motifs, and recombinantly expressed proteins. These approaches each have their own advantages and disadvantages, and there remains a need for MLO models that bridge the gap between all of them. This dissertation will focus on new class of synthetic poly(S-alkyl-L-homocysteine)s that demonstrate many of the features prevalent in all three types of MLO models. A family of new amino acid functionalized cationic poly(S-alkyl-L-homocysteine)s were synthesized and found to undergo stimuli responsive reversible coacervation in aqueous media. These polypeptides, prepared by derivatization of a common poly(L-methionine) precursor by reaction with different amino acid-functionalized alkylating agents, exhibited phase behavior that was dependent on the identity of amino acids at their side-chain termini. These side-chain functional groups allowed for the multi-stimuli responsive control of coacervation via changes to counterions, pH, temperature as well via oxidation of side-chain thioether groups. Despite retaining stable α-helical secondary structures over a range of pH in aqueous solution, molecular dynamics simulations of these polypeptides revealed a high degree of side-chain conformational disorder and hydration around the ordered backbones, which may explain the ability of these polypeptides to form coacervates instead of precipitates upon phase separation. The modular design, uniform nature, and ordered chain conformations of these polypeptides show their potential as MLO models since they allow deconvolution of molecular elements that influence biopolymer coacervation. The modular nature of these poly(S-alkyl-L-homocysteine)s was then employed to study how alteration of various molecular features of polypeptide side-chains influences coacervate formation. The initial series of cationic polypeptides was expanded to include both anionic and cationic homopolypeptides with varying side-chain lengths. It was found that the poly(L-homocysteine) backbone was optimal for coacervate formation. Increased spacing of the thioether linkage from the polypeptide backbone resulted in polypeptides that phase separated from water as precipitates. These results helped define the molecular features that need to be conserved for coacervate formation. Pegylated cationic, α-helical poly(S-alkyl-L-homocysteine)s were also prepared for studies on oligonucleotide complexation and release, to determine if the coacervate forming polypeptide segments would provide advantages for oligonucleotide delivery. Both sodium tripolyphosphate (TPP) and a model RNA sequence polyadenylic acid (poly(A)) were used to identify the optimal polypeptide composition and length for the formation of uniform complex coacervate core micelles (C3Ms). These C3Ms were prepared and characterized for RNA encapsulation efficiency and micelle stability as model carriers for gene delivery. This preliminary study demonstrated that the coacervate forming poly(S-alkyl-L-homocysteine)s can be optimized for encapsulation of biological cargos.

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