Spatiotemporal Formation and Growth Kinetics of Polyelectrolyte Complex Micelles with Millisecond Resolution

Coacervation in Slow Motion: Kinetics of Complex Micelle Formation Induced by the Hydrolysis of an Antibiotic Prodrug

Colistin methanesulfonate (CMS) is the less-toxic prodrug of highly nephrotoxic colistin. To develop and understand highly necessary new antibiotic formulations, the hydrolysis of CMS to colistin must be better understood. Herein, with the addition of poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) to CMS, we show that we can follow the hydrolysis kinetics, employing small-angle X-ray scattering (SAXS) through complex coacervation. During this hydrolysis, hydroxy methanesulfonate (HMS) groups from CMS are cleaved, while the newly formed cationic amino groups complex with the anionic charge from the PMAA block. As the hydrolysis of HMS groups is slow, we can follow the complex coacervation process by the gradual formation of complex micelles containing activated antibiotics. Combining mass spectrometry (MS) with SAXS, we quantify the hydrolysis as a function of pH. Upon modeling the kinetic pathways, we found that complexation only happens after complete hydrolysis into colistin and that the process is accelerated under acidic conditions. At pH = 5.0, effective charge switching was identified as the slowest step in the CMS conversion, constituting the rate-limiting step in colistin formation.

Kinetically Controlled Polyelectrolyte Complex Assembly of microRNA-Peptide Nanoparticles toward Treating Mesothelioma

Broad size distributions and poor long-term colloidal stability of microRNA-carrying nanoparticles, especially those formed by polyelectrolyte complexation, represent major hurdles in realizing their clinical translation. Herein, peptide design is used alongside optimized flash nanocomplexation (FNC) to produce uniform peptide-based miRNA particles of exceptional stability that display anticancer activity against mesothelioma in vitro and in vivo. Modulating the content and display of lysine-based charge from small intrinsically disordered peptides used to complex miRNA proves essential in achieving stable colloids. FNC facilitates kinetic isolation of the mechanistic steps involved in particle formation to allow the preparation of particles of discrete size in a highly reproducible, scalable, and continuous manner, facilitating pre-clinical studies. To the best of the authors knowledge, this work represents the first example of employing FNC to prepare polyelectrolyte complexes of miRNA and peptide. Encapsulation of these particles into an injectable hydrogel matrix allows for their localized in vivo delivery by syringe. A one-time injection of a gel containing particles composed of miRNA-215-5p and the peptide PKM1 limits tumor progression in a xenograft model of mesothelioma.

Recent advances in synchrotron scattering methods for probing the structure and dynamics of colloids

Recent progress in synchrotron based X-ray scattering methods applied to colloid science is reviewed. An important figure of merit of these techniques is that they enable in situ investigations of colloidal systems under the desired thermophysical and rheological conditions. An ensemble averaged simultaneous structural and dynamical information can be derived albeit in reciprocal space. Significant improvements in X-ray source brilliance and advances in detector technology have overcome some of the limitations in the past. Notably coherent X-ray scattering techniques have become more competitive and they provide complementary information to laboratory based real space methods. For a system with sufficient scattering contrast, size ranges from nm to several μm and time scales down to μs are now amenable to X-ray scattering investigations. A wide variety of sample environments can be combined with scattering experiments further enriching the science that could be pursued by means of advanced X-ray scattering instruments. Some of these recent progresses are illustrated via representative examples. To derive quantitative information from the scattering data, rigorous data analysis or modeling is required. Development of powerful computational tools including the use of artificial intelligence have become the emerging trend.

Effect of Charged Block Length Mismatch on Double Diblock Polyelectrolyte Complex Micelle Cores

Polyelectrolyte complex micelles are hydrophilic nanoparticles that self-assemble in aqueous environments due to associative microphase separation between oppositely charged blocky polyelectrolytes. In this work, we employ a suite of physical characterization tools to examine the effect of charged block length mismatch on the equilibrium structure of double diblock polyelectrolyte complex micelles (D-PCMs) by mixing a diverse library of peptide and synthetic charged-neutral block polyelectrolytes with a wide range of charged block lengths (25-200 units) and chemistries. Early work on D-PCMs suggested that this class of micelles can only be formed from blocky polyelectrolytes with identical charged block lengths, a phenomenon referred to as chain length recognition. Here, we use salt annealing to create PCMs at equilibrium, which shows that chain length recognition, a longstanding hurdle to repeatable self-assembly from mismatched polyelectrolytes, can be overcome. Interestingly, D-PCM structure-property relationships display a range of values that vary systematically with the charged block lengths and chemical identity of constituent polyelectrolyte pairings and cannot be described by generalizable scaling laws. We discuss the interdependent growth behavior of the radius, ionic pair aggregation number, and density in the micelle core for three chemically distinct diblock pairings and suggest a potential physical mechanism that leads to this unique behavior. By comparing the results of these D-PCMs to the scaling laws recently developed for single diblock polyelectrolyte complex micelles (S-PCMs: diblock + homopolymer), we observe that D-PCM design schemes reduce the size and aggregation number and restrict their growth to a function of charged block length relative to S-PCMs. Understanding these favorable attributes enables more predictive use of a wider array of charged molecular building blocks to anticipate and control macroscopic properties of micelles spanning countless storage and delivery applications.

Chemical Feedback in Templated Reaction-Assembly of Polyelectrolyte Complex Micelles: A Molecular Simulation Study of the Kinetics and Clustering

The chemical feedback between building blocks in templated polymerization of diblock copolymers and their consecutive micellization was studied for the first time by means of coarse-grained molecular dynamics simulations. Using a stochastic polymerization model, we were able to reproduce the experimental findings on the effect of chemical feedback on the polymerization rates at low and high solution concentrations. The size and shape of micelles were computed using a newly developed software in Python conjugated with graph theory. In full agreement with the experiments, our simulations revealed that micelles formed by the templated micellization are more spherical and have a lower radius of gyration than those formed by the traditional two-step micellization method. The advantage of molecular simulation over the traditional kinetic models is that with the simulation, one studies in detail the heterogeneous polymerization in the presence of the oppositely charged template while also accounting for the incompatibility between reacted species, which significantly influences the reaction process.

Complexes of Charged-Neutral Block Copolymers and Surfactants: Process-Dependent Features and Long-Term Stability of Their Aqueous Dispersions

Aqueous dispersions of charged-neutral block copolymers (poly(acrylamide)-b-poly(acrylate)) complexed with an oppositely charged surfactant (dodecyltrimethylammonium) have been prepared by different approaches: the simple mixing of two solutions (MS approach) containing the block copolymer and surfactant, with their respective simple counterions, and dispersion of a freeze-dried complex salt prepared in the absence of simple counterions (CS approach). The CS particles were investigated under different conditions: dispersion of a CS in salt-free water and dispersion of a CS in a dilute salt solution, the latter condition yielding dispersions with the same composition as the MS process. Additionally, aged dispersions (up to 6 months) and dispersed complexes of the polyacrylate homopolymer and dodecyltrimethylammonium surfactant were evaluated. By employing different characterization techniques, it was seen that dispersions prepared by the MS approach display nanometric spherical particles with disordered cores, and poor colloidal stability, partially caused by the absence of surface charge (ζ-potential close to zero). Oppositely, anisometric particles were formed in CS dispersions and were large enough to sustain micellar cubic cores. The CS particles presented long-time colloidal stability, partially due to a net negative surface charge, but the stability varied with the length of the neutral block composing the corona. Our results demonstrate that all dispersed particles are metastable structures, with physicochemical properties strongly dependent on the preparation procedure, thus making these particles suitable for fundamental studies and potential applications where accurate control of their properties, including size, shape, internal structure, and stability, is desired.

The critical Transition from Soluble Complexes to Colloidal Aggregates of Polyelectrolyte Complexes at Non-stoichiometric Charge Ratios

The transition from soluble to colloidal polyelectrolyte complex normally occurs at a critical non-stoichiometric charge ratio. Here, we demonstrated that the conventional batch mixing produces heterogeneous binding and complexation, which could easily mask this soluble-colloidal complex transition (sol-col transition) even for weakly binding polyelectrolytes like polyacrylic acid (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC). When mixed efficiently using multi-inlet vortex mixer (MIVM), the sol-col transition occurs beyond a critical charge ratio (n-/n+) and the large colloidal complexes are formed through the aggregation of small primary complexes (as revealed by atomic force microscopy). Moreover, the sol-col transition occurs at a constant charge ratio below the overlapping concentration (c*) of the long host polyelectrolyte, but at lower charge ratios above c* due to chain entanglement. When adding NaCl to the solution, the sol-col transition charge ratio first decreases, then remained stable for a period and finally increased and vanished at high ionic strength. When replacing NaCl with chaotropic salts, the sol-col transition occurs at lower charge ratios, while kosmotropes had little impact. The solvent quality and polymer hydrophobicity effects are also discussed. With the assistance of rapid mixing, this study provides a more reliable way of studying the sol-col transition of polyelectrolyte complexes. This article is protected by copyright. All rights reserved.

Liquid-Phase Condensation via Macromolecular Crowding in Polymerization-Induced Electrostatic Self-Assembly

Macromolecular crowding plays a key role in liquid-phase condensation of proteins and membraneless organelles yet is largely unexplored for artificial liquid materials. Herein, we present a strategy for direct access to multiphase liquid condensates with individual charged/neutral subdomains, by introducing macromolecular crowding to our previous protocol of liquid-liquid phase-separation-driven polymerization-induced electrostatic self-assembly (LLPS-PIESA). We show that reversible addition fragmentation chain transfer (RAFT) aqueous dispersion photo-copolymerization of a charged monomer with a specific neutral monomer, in the presence of a polar macrochain transfer agent (CTA) and an oppositely charged polyion, can induce self-sorting and macromolecular crowding. LLPS-PIESA proceeds via liquid-phase condensation of as-assembled nascent clusters up to biologically important nanostructured multiphase condensates with individual charged/neutral subdomains.

Molecular Exchange Kinetics in Complex Coacervate Core Micelles: Role of Associative Interaction

Molecular exchange dynamics between spherical complex coacervate core micelles (C3Ms) are documented using time-resolved small-angle neutron scattering measurements (TR-SANS), and the effects of salt concentration, type of charges, and core block polydispersity to the chain exchange are quantified. Isotopically labeled block copolyelectrolytes were prepared by postpolymerization modification of two nearly identical poly(ethylene oxide-b-allyl glycidyl ether), one with normal and the other with deuterated PEO blocks (i.e., hPEO-PAGE and dPEO-PAGE). The observed rates at multiple salt concentrations are consolidated using time-salt superposition shift factors representing chain exchange rates and analyzed. Our comprehensive analytical relaxation function based on the sticky-Rouse model and the thermodynamic barrier for core block extraction successfully describes the molecular exchange kinetics between the isotopically labeled C3Ms. We believe this work provides fundamental design criteria for C3Ms with engineered chain exchange dynamics.

Nanostructured Multiphase Condensation of Complex Coacervates in Polymerization-Induced Electrostatic Self-Assembly

We present the nanostructured multiphase condensation of complex coacervates in the dynamic evolving aqueous medium of liquid-liquid phase separation driven polymerization-induced electrostatic self-assembly (LLPS-PIESA). We show that the parent droplets evolve into nanostructured coacervate-in-coacervate multiphase condensates with diverse morphologies, such as dandelions, worms, lamellae, vesicles, and vesicle polymers, upon the kinetically dictated recruitment of anionic free chains, cationic growing chains, and nascent clusters from the dynamic evolving aqueous medium of LLPS-PIESA, under the interplay of electrostatic and arginine-like salt bridge interactions.

Early-Stage Aggregation and Crystalline Interactions of Peptoid Nanomembranes

Fully synthetic peptoid membranes are known to mimic important features of biological membranes, with several advantages over other biomimetic membranes. A fundamental understanding of how the individual peptoid amphiphiles assemble in solution to form the bilayer membrane is key to unlocking their versatility for application in a broad range of processes. In this study, in situ X-ray scattering and molecular dynamics simulations are used to understand the early stages of assembly of three different peptoids that exhibit distinctly different crystallization kinetics. The in situ measurements reveal that the peptoids aggregate first into a nascent phase that is less crystalline than the assembled peptoid membrane. Anisotropic aromatic interactions are determined to be the dominant driving force in the early stages of membrane formation. These results provide key insights into how the peptoid assembly may be manipulated during the early stages of assembly and nucleation and growth.

Advances in the Structural Design of Polyelectrolyte Complex Micelles

Polyelectrolyte complex micelles (PCMs) are a unique class of self-assembled nanoparticles that form with a core of associated polycations and polyanions, microphase-separated from neutral, hydrophilic coronas in aqueous solution. The hydrated nature and structural and chemical versatility make PCMs an attractive system for delivery and for fundamental polymer physics research. By leveraging block copolymer design with controlled self-assembly, fundamental structure-property relationships can be established to tune the size, morphology, and stability of PCMs precisely in pursuit of tailored nanocarriers, ultimately offering storage, protection, transport, and delivery of active ingredients. This perspective highlights recent advances in predictive PCM design, focusing on (i) structure-property relationships to target specific nanoscale dimensions and shapes and (ii) characterization of PCM dynamics primarily using time-resolved scattering techniques. We present several vignettes from these two emerging areas of PCM research and discuss key opportunities for PCM design to advance precision medicine.

100th Anniversary of Macromolecular Science Viewpoint: Attractive Soft Matter: Association Kinetics, Dynamics, and Pathway Complexity in Electrostatically Coassembled Micelles

Electrostatically coassembled micelles constitute a versatile class of functional soft materials with broad application potential as, for example, encapsulation agents for nanomedicine and nanoreactors for gels and inorganic particles. The nanostructures that form upon the mixing of selected oppositely charged (block co)polymers and other ionic species greatly depend on the chemical structure and physicochemical properties of the micellar building blocks, such as charge density, block length (ratio), and hydrophobicity. Nearly three decades of research since the introduction of this new class of polymer micelles shed significant light on the structure and properties of the steady-state association colloids. Dynamics and out-of-equilibrium processes, such as (dis)assembly pathways, exchange kinetics of the micellar constituents, and reaction-assembly networks, have steadily gained more attention. We foresee that the broadened scope will contribute toward the design and preparation of otherwise unattainable structures with emergent functionalities and properties. This Viewpoint focuses on current efforts to study such dynamic and out-of-equilibrium processes with greater spatiotemporal detail. We highlight different approaches and discuss how they reveal and rationalize similarities and differences in the behavior of mixed micelles prepared under various conditions and from different polymeric building blocks.