Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session K14: Bio-inspired Phase Separation II |
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Sponsoring Units: DSOFT DPOLY Chair: Sam Wilken, University of California, Santa Barbara; Omar Saleh, University of California, Santa Barbara Room: Room 206 |
Tuesday, March 7, 2023 3:00PM - 3:12PM |
K14.00001: Solvent reorganization leads to enthalpy–entropy compensation in biomolecular recognition Shensheng Chen, Zhen-Gang Wang In aqueous solutions at room temperature, the association between two macromolecules (such as protein–protein and protein–ligand binding) exhibits enthalpy–entropy compensation (EEC) behavior: While the overall binding Gibbs free energy remains nearly constant, there is great variation in the entropic and enthalpic contributions, depending on the species of the paired polymers. The molecular origin of the EEC remains controversial. Here, using coarse-grained simulations and thermodynamics analysis, we extract the entropic and enthalpic components in the thermodynamic driving forces for the association between two biomolecules. We find that the solvent reorganization is the major source of the EEC during the binding process, arising from the temperature-dependent nonelectrostatic and electrostatic interactions. For systems with a lower critical solution temperature (LCST) below the room temperature, the solvent reorganization entropically dominates the favorable free energy change in the nonelectrostatic interaction, at the expense of energy. For systems with an upper critical solution temperature (UCST) above the room temperature, the nonelectrostatic part of the free energy change is dominated by the energy due to solvent reorganization, at the expense of entropy. The solvent reorganization due to the electrostatic attraction also has EEC behavior: The entropic and enthalpic contributions to the free energy change are of opposite signs. |
Tuesday, March 7, 2023 3:12PM - 3:24PM |
K14.00002: Complexation of oppositely-charged polyelectrolytes in the presence of divalent ions Jacob D Horne, Jian Qin, Yan Xia, Kayla P Barker Polyelectrolyte solutions exhibit a demixing transition known as polyelectrolyte complexation (PEC). Recent interest in PEC has been spurred by its relevance in organizing biomolecules in intracellular environments, and by the advancement of theoretical treatments. A range of molecular parameters have been shown to influence the demixing transition, including charge patterning, polarity, stoichiometry, etc. We present a study on the effects of ion valency, which remains largely unexplored. We leverage our previous expertise in synthesizing homologous polyelectrolytes to investigate how the addition of divalent ions (Ca2+, Mg2+) affects their complexation behavior. Dye-labeling and ICP-MS measurements enable the construction of accurate complexation phase diagrams as well as quantification of ion partitioning. The two-phase window is found to be substantially narrower than the case of monovalent ions, while the ion and polymer partitioning remained similar. The results are rationalized by analyzing how ion valency reduces the ionic correlations, which enables the collapse of phase diagrams for both monovalent and divalent ions. |
Tuesday, March 7, 2023 3:24PM - 3:36PM |
K14.00003: Factors Governing Polyelectrolyte Complexation in Non-Ideal Environments Divya Iyer, Holly Senebandith, Samanvaya Srivastava, Lucas Willey, Peter Goh, Vanessa Huaco The phase behavior and viscoelastic properties of polyelectrolyte complexes (PECs) comprising oppositely charged polyelectrolytes (PEs) are influenced by solution conditions (ionic strength, pH) and properties of the charged PEs (concentration, molecular weight, chemistry). These studies are, however, performed in model systems, where properties of the solution and polymers are well-controlled. In real systems where PEs are employed, they are often exposed to non-ideal conditions, and little to no control can be exercised over the solution conditions and PE properties. The factors influencing PE complexation under these conditions remain to be probed. Previously, we showed that introduction of divalent ions (often encountered in biological and wastewater systems) in charge-matched PECs reverses the phase composition and induces hindered chain relaxation. In this talk, non-idealities in PE composition and architecture will be addressed to mimic and represent realistic conditions under which PE complexation occurs. These non-idealities will be shown to influence the extent of complexation, salt resistance, phase composition, and viscoelastic behavior in PECs. Further, a design guide for inducing complexation and altering the processability of these PE complexes will be discussed. |
Tuesday, March 7, 2023 3:36PM - 3:48PM |
K14.00004: Internal structure in systems of self-coacervating polymers Zuzanna Jedlinska, Robert A Riggleman Liquid-liquid phase separation (LLPS) has been shown to play a central role in many |
Tuesday, March 7, 2023 3:48PM - 4:00PM |
K14.00005: Transfer Matrix Theory of Complex Coacervation for Patterned Polyelectrolytes Ashley R Knoerdel, Charles E Sing Polymeric complex coacervation is an associative phase separation process of oppositely charged polyelectrolytes. This is driven by electrostatic attraction between the polycation and polyanion species leading to polymer dense and polymer dilute phases. By understanding how charged monomer sequence plays a role in complex coacervation, we aim to gain a greater understanding of the physics involved with biological condensates. These intracellular compartments are formed from intrinsically disordered proteins and are sensitive to charged amino acid sequence. We have developed a theoretical approach to model polyelectrolytes that have different charged monomer sequences, as analogues to these biological macromolecules. We can predict how changes in the electrostatics of arbitrarily patterned polyelectrolytes affects the phase boundary and electrostatics of the system. Using a transfer matrix approach, we construct a statistical mechanical model that incorporates local correlations associated with nearest-neighbor monomer interactions. We show that predictions using this theoretical model are consistent with previous simulation-informed theory, both in terms of the phase behavior of sequence-defined polyelectrolytes as well as their sequence-dependent effective interaction energy. |
Tuesday, March 7, 2023 4:00PM - 4:12PM |
K14.00006: pH Effects on Conformation and Ionization of Charge Sequence Varied Polyampholytes Winnie H Shi Polyampholytes (PAs) are polymers containing both positively and negatively charged groups along their backbone. Previous studies have found pH and salt affect phase behavior and viscoelastic properties of PA solutions through electrostatic interactions. From weak polyelectrolyte (PE) pH studies, the increase in charge fraction has been shown to decrease ionization of charged groups due to electrostatic repulsion of neighboring like charges. Conformation has also been shown to affect ionization of charged groups in PEs. Electrostatic attraction between oppositely charged groups in weak PAs, in contrast to PEs, create a favorable ionization environment that counter electrostatic repulsion between like charges. Hence, using solid phase peptide synthesis, we synthesized a set of poly-L-(lysine, glutamic acid) peptides with neutral net charge consisting of 32 residues arranged in increasing block sizes to evaluate the effect of increasing neighboring electrostatic repulsion with increasing block size with oppositely charged groups. From pH titrations and small angle X-ray (SAXS) experiments, we find increasing buffering behavior with growing block length as well as extension of the molecule, approaching PE behavior at extreme pHs. The PAs do not demonstrate clear pKas, unlike expectations from the titration of individual monomers. We further discuss effects of pH on ionization states and hydrogen bonding of charge sequence varied PA systems and its implications in conformation and solution phase behavior using NMR and SAXS. |
Tuesday, March 7, 2023 4:12PM - 4:24PM |
K14.00007: Structure and Viscoelasticity of Hybrid Colloid-Polyelectrolyte Coacervates Artem M Rumyantsev, Oleg Borisov, Juan J De Pablo We theoretically consider complex coacervation between polyelectrolytes (PEs) and oppositely charged colloids, such as globular proteins, solid nanoparticles, or spherical micelles of ionic surfactants. A scaling theory of stoichiometric colloid-PE (“hybrid”) coacervates is developed to predict their structure, viscoelasticity, and critical concentration of coacervation. At low concentrations, PEs adsorb at the colloids to form electrically neutral finite-size complexes. Attractions of these clusters arise due to bridging between the adsorbed PE layers and, above the threshold concentration, macroscopic phase separation sets in. The internal structure of hybrid coacervate is controlled by (i) the strength of the PE adsorption and (ii) the ratio of the PE shell thickness to the colloid radius. Different scaling regimes of hybrid coacervates are distinguished, and the respective scaling diagram is constructed in the coordinates of the colloid charge Q and radius R. At the high charge of colloids, the shell is thick and the most volume of the hybrid coacervate is occupied by PEs, which define its osmotic and rheological properties. In contrast to usual PE-PE coacervates, hybrid coacervates have an inhomogeneous structure: The local polymer density decreases with the distance from the colloid surface. Hybrid coacervates have higher average density but simultaneously lower surface tension as compared to usual PE-PE counterparts. Rouse and reputation models are applied to describe PE dynamics and diffusion of colloids in the condensed phase. In Θ solvent, the coacervate viscosity increases with the colloid charge as ηRouse ∼Q4/5 when PEs are unentangled and as ηrep ∼Q28/15 for entangled systems. The diffusion coefficients of colloids are strongly decreasing functions of Q and R. Obtained predictions are compared to the experimental results on hybrid coacervates formed from globular supercationic green fluorescent proteins (GFPs) and RNA polyanions. |
Tuesday, March 7, 2023 4:24PM - 4:36PM |
K14.00008: Phase Separation with Elasticity: Interplay of Droplet Morphology and Thermodynamic Stability Shichen Wang, Peter D Olmsted Phase separation in an elastic environment such as a polymer network is not very well understood due to the long range and non-additive nature of elasticity, as might occur during condensate formation in the gel-like regions of the nucleus or the cytosol. In this study, I propose a model that comprises elastic and mixing energies to describe the liquid-network system. The presence of elasticity results in a dependency of phase behavior, or inclusion morphology, on the dimensionality of deformation as well as on the shape of the material: a lower deformational dimension (e.g. swelling a thin slab) engenders higher free energy density (e.g. compared to cylindrical swelling), yet is easier to undergo macroscopic phase separation. If the network is completely excluded from one phase, phase separation can be modeled as a Gent type cavity growth of incompressible one component material, but with growth driven by the de-mixing free energy. In a finite system, the size of the cavities in equilibrium increases with the stiffness of the network and the size of the system. A larger cavity in a stiffer network yields higher free energy, suggesting that stiffer networks will break up into smaller sub-systems with multiple cavities. We also study ternary systems with un-crosslinked polymer in addition to the network and solvent. The addition of a second liquid component with the same chemical properties as the network can reduce the elastic energy incurred upon forming the deformation of the network, and thus influence the phase separation. |
Tuesday, March 7, 2023 4:36PM - 4:48PM |
K14.00009: Effect of Polymer Gel Elasticity on Coacervate Phase Behavior Kathryn G Wilcox, Brittany K Roopnarine, Kai Yamagami, Adam Linscott, Svetlana Morozova Polymer gels make up many biological systems and tissues such as the extracellular matrix in cartilage, skin, and vitreous in the eye. Their network structure and internal pressure have the capacity to influence structure formation, tissue biomechanics, and influence liquid phase separation. In order to investigate the effect of the elastic environment on biomacromolecular assembly, we have studied how varying gel modulus changes the phase behavior and radius of coacervate droplets. Poly-L-lysine (PLL) and hyaluronic acid (HA) coacervate phases were prepared in polyacrylamide gels of moduli varying from ~0.035 - 6.2 kPa. The size of the coacervate droplets is reported from Bright Field Microscopy and Confocal Microscopy. Fluorescence Microscopy is used to determine the concentration of fluorescently tagged HA in the coacervate and supernatant phases as a function of pH (2 – 10) and ionic strength (25 - 800 mM). Overall, with increasing shear modulus of the polyacrylamide gel, the coacervate droplet size decreases. By understanding how an elastic environment influences simple electrostatic assembly, we can further understand more complicated biomacromolecular assemblies. |
Tuesday, March 7, 2023 4:48PM - 5:00PM |
K14.00010: Mixing and solvation transitions in aqueous mixtures of free polymers and microgels Senthilkumar Duraivel, Sofia Goodrich, Brent Sumerlin, Thomas E Angelini Interest in liquid-liquid phase separation (LLPS) within complex materials has grown recently, driven by the discovery of LLPS in living cells where it has been found that phase separated liquid condensates of biomolecules play a vital role in numerous intracellular processes. Inspired by these discoveries in living cells, investigations have expanded beyond basic aqueous two-phase systems to studies of LLPS in more complex systems like crosslinked networks of solvated polymers. Inspired by these studies, we hypothesized that aqueous solutions of free polymers and microparticles made from crosslinked solvated polymers (microgels), should exhibit transitions in the intermixing of the two polymer species across the microgel surfaces. In this presentation, we will describe how solvated poly(N-isopropylacrylamide), or PNIPAM, does not penetrate into polyethylene glycol (PEG) microgels dispersed into an aqueous PNIPAM solution at low temperatures. At slightly elevated temperatures, a clear mixing transition occurs where PNIPAM enters the PEG microgels, depending on the PNIPAM molecular weight and the relative polymer concentrations. In addition to this mixing transition, we observe aggregation and precipitation of PNIPAM at even higher temperatures, corresponding to the upper-critical solvation temperature of PNIPAM in water. We demonstrate that this phase behavior can be leveraged in applications such as synthesizing PEG microgels with a well-defined PNIPAM coating. We will describe the rheological properties of these coated microgels over a range of temperatures |
Tuesday, March 7, 2023 5:00PM - 5:12PM |
K14.00011: Charge-Pattern Dependent Sequestration of Globular Proteins in Membraneless Organelles Heyi Liang, Artem Rumyantsev, Juan J De Pablo Condensation of polyampholytes into coacervates is driven by sequence-dependent charge correlations. It is proposed to be one of the mechanisms underlying the formation of membraneless organelles (MLOs) through liquid-liquid phase separation of intrinsically disordered proteins (IDPs), which are rich in charged residues and lack ordered structures. Unlike IDPs, globular proteins fold into complicated 3d structures and expose their charged residues to the surface to form specific surface charge patterns. We use coarse-grained molecular dynamics simulations to study the role of surface charge patterns in the sequestration of globular proteins in MLOs. In our coarse-grained model, globular proteins are modeled by spherical nanoparticles with patterned surface charges, while MLOs are represented by sequence-controlled polyampholyte coacervates. The free energy landscape of a globular protein partitioning between two MLOs is calculated by umbrella sampling. We have shown that the MLO formed by blocky polyampholytes prefers to uptake globular proteins with patchy surface charge, while the MLO formed by random polyampholytes shows a stronger affinity to globular proteins with random surface charge. Such "like dissolves like" behavior is consistent with the theoretical picture of the sequence/pattern-dependent electrostatic correlation. |
Tuesday, March 7, 2023 5:12PM - 5:24PM |
K14.00012: Liquid-liquid phase separation within fibrillar networks Jason X Liu, Mikko Haataja, Andrej Kosmrlj, Sujit S Datta, Craig Arnold, Rodney Priestley Liquid-liquid phase separation of protein condensates within the cell has recently been observed in a multitude of biological mechanisms. Ranging from dynamic biomolecular condensates such as stress granules and P-bodies to fibril-forming liquid protein aggregates, these liquid phase droplets are relevant to a variety of cellular mechanisms relating to health as well as to pathology. Within the cell, phase separation occurs within a complex viscoelastic medium which mediates the growth of such droplets. However, the physical principles underlying the droplets' formation, dynamics, and interaction with the viscoelastic medium are only now being developed. Here, we introduce a synthetic analog to biomolecular condensates within the cell. In this system, we demonstrate the formation of mesh-scale condensates constrained by a fibrillar network. We show that the competition between condensate capillarity and network elasticity dictates the relevant droplet mechanics. In particular, we demonstrate that droplet deformation due to confinement by the network may be a universal phenomenon in the precipitation of condensed liquid phases within fibrillar networks. Such results are of relevance to the formation of biomolecular condensates within the complex cellular interior. |
Tuesday, March 7, 2023 5:24PM - 5:36PM |
K14.00013: Liquid state theory of microstructure and local clustering of biomolecular condensates and its consequences on slow dynamics Guang Shi, Ken S Schweizer Biomolecular condensates formed through phase separation of proteins and nucleic acids are ubiquitous, which provide a fundamental way to organize the intracellular material in a membrane-less manner. It is believed that these condensates are typically in a homogeneous isotropic liquid state. However, their internal microstructures are incompletely understood. Here, we use the Polymer Reference Interaction Site Model liquid state integral equation theory that is highly developed for synthetic homopolymer and copolymer melts and solutions to study biomolecular condensates of periodic and aperiodic sequences in polymeric microemulsion-like states of organization. An associating polymer/sticker-spacer minimal model is employed to establish the effect of polymer packing fraction, sequence, and the strength and range of intermolecular attractions on the internal organization of condensates from the local to the microdomain/macromolecular length scale in real and Fourier space. The structural results are used as input in microscopic dynamical approaches to address the slowing down of diffusion due to clustering and physical bond formation including possible kinetic arrest into structured physical gels. |
Tuesday, March 7, 2023 5:36PM - 5:48PM |
K14.00014: Upper Critical Solution Temperature Phase Behavior of Polyguanidinium Simple Coacervation Seunghwan Oh, SooHyung Choi Coacervation is liquid-liquid phase separation (LLPS) of polyelectrolytes in aqueous media, which is usually induced by electrostatic, cation-π and π-π interactions. The coacervation process is actively observed in bio-inspired phase-separating systems including cellular compartmentation of membraneless organelles. Recently, an arginine residue, containing a π-conjugated and positively charged guanidinium group, has drawn great attention to provide non-aromatic π-π stacking of like-charged guanidinium pairs in aqueous media. In this study, we demonstrate that the polyguadinium in aqueous media undergoes LLPS when the electrostatic repulsion is sufficiently reduced by monovalent or multivalent salts, and the simple coacervation shows the upper critical solution temperature (UCST) phase behavior. Remarkably, salt concentration plays a crucial role to regulate the phase behavior of polyguanidinium aqueous solutions. Using neutron and light scattering measurements, the thermodynamic properties including critical and theta temperature were extracted, resulting in understanding phase behavior of π-π interaction in aqueous media as a function of salt concentration, temperature, and polymer molecular weight. Our findings shed new light on the π-π association-induced phase transition to characterize thermodynamics of bio-inspired polymer solutions. |
Tuesday, March 7, 2023 5:48PM - 6:00PM |
K14.00015: Molecular mechanism of selective partitioning of medicinal lipids into lipid droplets Chang Yun Son In guiding lipid droplets (LDs) to serve as storage vessels that insulate high-value lipophilic compounds in cells, we demonstrate that chain flexibility of lipids determines their selective migration in intracellular LDs. Focusing on commercially important medicinal lipids with biogenetic similarity but structural dissimilarity, we have recently validated computationally and experimentally that LD remodeling should be differentiated between overproduction of structurally flexible squalene and that of rigid zeaxanthin and β-carotene. In this talk, we present a molecular dynamics simulation study revealing that worm-like flexible squalene is readily deformed to move through intertwined chains of triacylglycerols in the LD core, whereas rod-like rigid zeaxanthin is trapped on the LD surface due to a high free energy barrier in diffusion. The simulation was consistent with experimental observation that intracellular storage of squalene significantly increases with LD volume expansion, but that of zeaxanthin and β-carotene is enhanced through LD surface broadening; as visually evidenced, the outcomes represent internal penetration of squalene and surface localization of zeaxanthin and β-carotene. Our study shows the computational and experimental validation of selective lipid migration into a phase-separated organelle and reveals LD dynamics and functionalization. |
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