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

Time-resolved small-angle neutron scattering for characterization of molecular exchange in lipid nanoparticle therapeutics

HYPOTHESIS

Nano-scale dynamics of self-assembled therapeutics play a large role in their biological function. However, assessment of such dynamics remains absent from conventional pharmaceutical characterization. We hypothesize that time-resolved small-angle neutron scattering (TR-SANS) can reveal their kinetic properties. For lipid nanoparticles (LNP), limited molecular motion is important for avoiding degradation prior to entering cells while, intracellularly, enhanced molecular motion is then vital for effective endosomal escape. We propose TR-SANS for quantifying molecular exchange in LNPs and, therefore, enabling optimization of opposing molecular behaviors of a pharmaceutical in two distinct environments.

EXPERIMENTS

We use TR-SANS in combination with traditional SANS and small-angle x-ray scattering (SAXS) to experimentally quantify nano-scale dynamics and provided unprecedented insight to molecular behavior of LNPs.

FINDINGS

LNPs have molecular exchange dynamics relevant to storage and delivery which can be captured using TR-SANS. Cholesterol exchanges on the time-scale of hours even at neutral pH. As pH drops below the effective pKa of the ionizable lipid, molecular exchange occurs faster. The results give insight into behavior enabling delivery and provide a quantifiable metric by which to compare formulations. Successful analysis of this multi-component system also expands the opportunities for using TR-SANS to characterize complex therapeutics.

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.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

The change in the transport properties (i.e., water diffusivity, shear viscosity, etc.) when adding salts to water has been used to classify ions as either being chaotropic or kosmotropic, a terminology based on the presumption that this phenomenon arises from respective breakdown or enhancement of the hydrogen-bonding network structure. Recent quasi-elastic neutron scattering measurements of the collective structural relaxation time, τC, in aqueous salt solutions were interpreted as confirming this proposed origin of ion effects on the dynamics of water. However, we find similar changes in τC in the same salt solutions based on molecular dynamics (MD) simulations using a coarse-grained water model in which no hydrogen bonding exists, challenging this conventional interpretation of mobility change resulting from the addition of salts to water. A thorough understanding of specific ion effects should be useful in diverse material manufacturing and biomedical applications, where these effects are prevalent, but poorly understood.

Ionic Strength-Dependent Structure of Complex Coacervate Core Micelles

Salt concentration-dependent structure of complex coacervate core micelles (C3Ms), formed by polyether-based block copolyelectrolytes containing cationic ammonium (A) or anionic sulfonate (S) groups in aqueous media, is investigated by light scattering and small-angle X-ray/neutron scattering (SAX/NS). As the salt concentration increases, both a core radius (Rcore) and an aggregation number (Nagg) significantly decrease, but a corona thickness (Lcorona) is nearly unchanged. Larger salt concentrations can lower the interfacial tension between the coacervate cores and aqueous media, resulting in an increased interfacial area per chain and a more relaxed conformation of the core blocks. Based on the structure characterization, the scaling relationship between structure parameters (i.e., Rcore, Nagg, and Lcorona) and salt concentration is obtained and compared to the theoretical description estimated by the free energy balance between the entropic penalty of core stretching and the interfacial energy. We propose that the free energy contribution of the core block stretching is not negligible in C3Ms because of the highly swollen cores caused by water.

Scaling Relationship of Complex Coacervate Core Micelles: Role of Core Block Stretching

The scaling relationship of complex coacervate core micelles (C3Ms) has been investigated experimentally and theoretically. The C3Ms are formed by mixing two oppositely charged block copolyelectrolyte solutions (i.e., AB + AC system) and are characterized by small-angle neutron (SANS) and X-ray scattering (SAXS). Scaling relationships for micellar structure parameters, including core radius, total radius, corona thickness, and aggregation number, all with respect to the core block length, are determined. A scaling theory is also proposed by minimizing the free energy per chain, leading to four regimes depending on the core and corona chain conformations. Although the corona block is significantly longer than the core block, the structure of our C3Ms is consistent with that of the crew-cut I regime. A highly swollen core by water enables the core blocks to be stretched significantly and corona chains to be minimally overlapped.

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.

Controlling Trapping, Release, and Exchange Dynamics of Micellar Core Components

Whereas the formation and overall stability of hierarchically organized self-assembled supramolecular structures have been extensively investigated, the mechanistic aspects of subcomponent dynamics are often poorly understood or controlled. Here we show that the dynamics of polyamidoamine (PAMAM) dendrimer based micelles can be manipulated by changes in dendrimer generation, pH, and stoichiometry, as proven by NMR and FRET. For this, dendrimers were functionalized with either fluorescein (donor) or rhodamine (acceptor) and encapsulated into separate micelles. Upon mixing, exchange of dendrimers is revealed by an increase in FRET. While dendrimicelles based on dendrimer generations 4 and 5 show a clear increase in FRET in time, revealing the dynamic exchange of dendrimers between micellar cores, generation 6 based micelles appear to be kinetically trapped systems. Interestingly, generation 6 based dendrimicelles prepared at a pH of 7.8 rather than 7.0 do show exchange dynamics, which can be attributed to about 25% less charge of the dendrimer, corresponding to the charge of a virtual generation 5.5 dendrimer at neutral pH. Changing the pH of dendrimicelle solutions prepared at a pH of 7.8 to 7.0 shows the activated release of dendrimers. High-resolution NMR spectra of the micellar core are obtained from a 1.2 GHz spectrometer with sub-micromolar sensitivity, with DOSY discriminating released dendrimers from dendrimers still present in the micellar core. This study shows that dendrimer generation, charge density, and stoichiometry are important mechanistic factors for controlling the dynamics of complex coacervate core micelles. This knowledge can be used to tune micelles between kinetically trapped and dynamic systems, with tuning of exchange and/or release speeds, to be tailored for applications in, e.g., material science, sensors, or drug delivery.

An Overview of Coacervates: The Special Disperse State of Amphiphilic and Polymeric Materials in Solution

Individual amphiphiles, polymers, and colloidal dispersions influenced by temperature, pH, and environmental conditions or interactions between their oppositely charged pairs in solvent medium often produce solvent-rich and solvent-poor phases in the system. The solvent-poor denser phase found either on the top or the bottom of the system is called coacervate. Coacervates have immense applications in various technological fields. This review comprises a concise introduction, focusing on the types of coacervates, and the influence of different factors in their formation, structures, and stability. In addition, their physicochemical properties, thermodynamics of formation, and uses and multifarious applications are also concisely presented and discussed.

Ionic group-dependent structure of complex coacervate hydrogels formed by ABA triblock copolymers

This study investigates the nanostructure of complex coacervate core hydrogels (C3Gs) with varying compositions of cationic charged groups (i.e., ammonium and guanidinium) using small-angle X-ray/neutron scattering (SAX/NS). C3Gs were prepared by stoichiometric mixing of two oppositely charged ABA triblock copolymers in aqueous solvents, in which A end-blocks were functionalized with either sulfonate groups or a mixture of ammonium and guanidinium groups. Comprehensive small-angle X-ray/neutron scattering (SAX/NS) analysis elucidated the dependence of C3Gs structures on the fraction of guanidinium groups in the cationic end-block (x) and salt concentration (c_s). As x increases, the polymer volume fraction in the cores, and interfacial tension (γ_core) and salt resistance (c*) of the coacervate cores increase, which is attributed to the greater hydrophobicity and non-electrostatic association. Furthermore, we observed that the salt dependence of the interfacial tension follows γ_core ∼ (1 - c_s/c*)^3/2 in all series of x. The results show that the variation of the ionic group provides a powerful method to control the salt-responsiveness of C3Gs as stimuli-responsive materials.

DNA dynamics in complex coacervate droplets and micelles

Single stranded DNA (ssDNA), or another polyanion, can be mixed with polycations to form liquid-like complex coacervates. When the polycations are replaced by cationic-neutral diblock copolymers, complex coacervate core micelles (C3Ms) can be formed instead. In both complex coacervates and C3Ms, dynamics plays an important role. Yet, to date, the effect of chain length on the dynamics effect is still not fully understood. The DNA complexes provide a versatile platform to further elucidate these chain length effects because the DNA is monodisperse and its length can be easily adapted. Therefore, we study in this paper the dynamics of fluorescently labelled ssDNA in both complex coacervate droplets and micelles. The DNA dynamics in the complex coacervate droplets is probed by fluorescence recovery after photobleaching (FRAP). We observe that the DNA diffusion coefficient depends more strongly on the DNA length than predicted by the sticky Rouse model and we show that this can be partly explained by changes in complex coacervate density, but that also other factors might play a role. We measure the molecular exchange of C3Ms by making use of Förster resonance energy transfer (FRET) and complement these measurements with Langevin dynamics simulations. We conclude that chain length polydispersity is the main cause of a broad distribution of exchange rates. We hypothesise that the different exchange rates that we observe for the monodisperse DNA are mainly caused by differences in dye interactions and show that the dye can indeed have a large effect on the C3M exchange. In addition, we show that a new description of the C3M molecular exchange is required that accounts among others for the effect of the length of the oppositely charged core species. Together our findings can help to better understand the dynamics in both specific DNA systems and in complex coacervate droplets and micelles in general.