The University of Massachusetts Amherst
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PhD Defense, Li-Wei Chang, “Sequence Control of Complex Coacervates”


Thursday, February 13, 2020 - 1:00pm


N410 LSL


Committee Chair:  Sarah Perry

Complex coacervation is a liquid-liquid phase separation phenomenon driven by the electrostatic and entropically-driven complexation of oppositely charged polyelectrolytes. The resulting coacervate phase retains significant amounts of water and ions and displays an extremely low surface tension that has enabled the use of these materials for many applications, such as underwater adhesives, drug delivery, food and personal care products. There also has been increasing interest in coacervate-like droplets occurring in biological systems. The majority of these so-called membraneless organelles involve a combination of intrinsically-disordered proteins and RNA, and phase separate due to a combination of long-range charge effects and short-range hydrophobic effects. While evolution has optimized the self-assembly of these types of biological polymers over millions of years, our ability to design such materials remains limited, in part because the relevant interactions that occur over a wide range of different length scales. 
The goal of this research is to establish molecular-level design rules as to how chemical sequence can modulate the formation and properties of complex coacervates. While studies to date have focused on the effect of parameters such as the charge stoichiometry, temperature, pH, salt concentration, stereochemistry, polymer architecture, and the density of charges present, the ability to pattern the sequence of charges and other chemistries has been rarely studied. The scarcity of such studies is due largely to the difficulty of synthesizing polyelectrolytes with equal chain length and charge density, but different distributions of charge or other functionalities. However, polypeptides represent a model platform for the synthesis and study of polyelectrolytes with precisely controlled polymer architecture and sequence patterning at the molecular level, while retaining relevance to a variety of biological, medical, and industrial applications. Experimental measures such as turbidimetry and optical microscopy, coupled with isothermal titration calorimetry were coupled with theoretical and computational approaches to study how variations in the patterning and overall fraction of charged groups along the polymer affect the resulting coacervate phase behavior. Increasing the number of charged residues increased the salt resistance and the size of the two-phase region. More interestingly, a comparison between polypeptides with the same overall charge fraction, but different periodic repeating patterns of charged monomers (e.g., alternating, every two residues, every four, etc.) showed an increase in coacervate stability with increasing charge block size. Thermodynamic data, coupled with insights from simulation showed that the increase in stability was entropic in nature, resulting from differences in the one-dimensional confinement of counterions along the patterned polymer. Expanding these efforts to consider the effect of arbitrary sequence, incorporating hydrophobic residues, and varying the identify of salt, we are looking to understand the physics driving sequence-specific complex coacervation as a composition-independent strategy for modulating the phase behavior and physical properties of designer materials for a range of applications. Furthermore, we have extended this work to characterize the effect of charge patterning on self-coacervating polyampholytes in anticipation of deepening insight into phase separation in biological systems


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