Unravelling the Interactions between Surface-Active Ionic Liquids and Triblock Copolymers for the Design of Thermal Responsive Systems

The correlation between the micelle morphology of surface-active ionic liquids with self-assembly and thermodynamic characteristics: coarse-grained MD simulation and experiment

Surface-active ionic liquids (SAILs) show great promise as novel green solvents due to their low vapor pressure, high thermal stability, high electrical conductivity, and bio-friendly nature to replace traditional volatile organic solvents in industrial processes. In the present work, the combination of coarse-grained (CG) molecular dynamics (MD) simulations with conductivity measurements was employed to explain the correlation between the micelle morphology and physicochemical and thermodynamic properties of self-assembly. A homologous series of SAIL molecules, 1-n-alkyl-3-methylimidazolium bromide [Cnmim][Br] (n = 4, 6, 8, 10, and 12), were chosen at various concentrations to shed light on this issue. Simultaneously two factors of concentration and alkyl chain length affected the morphology to control the physical and thermodynamic features. Moreover, the nature of the headgroup for two SAILs with the longest alkyl chain was assessed by shifting from imidazolium into ammonium. First, the critical micelle concentration (CMC), the degree of counterion dissociation of micelles, and the standard Gibbs energy of micellization of SAILs were determined using conductivity data. The micelle morphology such as the aggregation number, micelle radius, and moment of inertia was computed before, around, and after the CMC by MD simulation. Simulated results in accordance with the experimental measurements provide a quantitative understanding of the micellar properties. Increasing the alkyl chain length was associated with a non-spherical bigger micelle while the ammonium-based surfactant with a lower repulsion between neighboring monomers in micelles induced bigger and more spherical aggregates. Raising the SAIL concentration did not considerably influence the sphericity of the micelle except for the SAIL with the longest tail. The umbrella sampling method calculated the potential of mean force (PMF) for pulling a monomer of SAIL from a pre-assembled micelle into the solution. The dissociation energy of a SAIL monomer from a micelle increased with the tail length or with shifting into the ammonium head group and was substantially influenced by micelle morphology. Comparison between a sphere micelle with an oval one demonstrated that the dissociation of a SAIL monomer from a non-spherical shape needed a higher amount of energy. An improved understanding of how the shape of the SAIL micelles controls the physicochemical properties and stability helps to extend their application to different chemical processes.

Ionic Liquids Mediated Micellization of Pluronic Copolymers: Aggregation Behavior of Amphiphilic Triblock Copolymers

Understanding the micellization of amphiphilic triblock copolymers, especially Pluronics can play a persuasive role in engineering "smart" formulations for drug delivery applications. Their underlying self-assembly in the presence of designer solvents such as ionic liquids (ILs) provides combinatorial benefits of unique munificent properties of ILs and copolymers. The complex molecular interactions in the Pluronic copolymers/ILs mixed system influence the aggregation mechanism of copolymers depending on various aspects with no standardized factors to govern the structure-property relationship, which led to the practical applications. Here, we summarized recent progress in understanding the micellization process of IL-Pluronic mixed systems. Special emphasis was given to pure Pluronic systems (i.e., PEO-PPO-PEO) without any structural modifications, such as copolymerization with other functional groups, and ILs having cholinium and imidazolium groups. We expect that the correlation between existing/developing experimental and theoretical studies will provide the necessary basis and impetus for successful utilization in drug delivery applications.

Rationalizing the Design of Pluronics-Surfactant Mixed Micelles through Molecular Simulations and Experiments

Aqueous systems comprising polymers and surfactants are technologically important complex fluids with tunable features dependent on the chemical nature of each constituent, overall composition in mixed systems, and solution conditions. The phase behavior and self-assembly of amphiphilic polymers can be changed drastically in the presence of conventional ionic surfactants and need to be clearly understood. Here, the self-aggregation dynamics of a triblock copolymer (Pluronics L81, EO₃PO₄₃EO₃) in the presence of three cationic surfactants (with a 12C long alkyl chain but with different structural features), viz., dodecyltrimethylammonium bromide (DTAB), didodecyldimethylammonium bromide (DDAB), and ethanediyl-1,2-bis(dimethyldodecylammonium bromide) (12-2-12), were investigated in an aqueous solution environment. The nanoscale micellar size expressed as hydrodynamic diameter (D_h) of copolymer-surfactant mixed aggregates was evaluated using dynamic light scattering, while the presence of a varied micellar geometry of L81-cationic surfactant mixed micelles were probed using small-angle neutron scattering. The obtained findings were further validated from molecular dynamics (MD) simulations, employing a simple and transferable coarse-grained molecular model based on the MARTINI force field. L81 remained molecularly dissolved up to ∼20 °C but phase separated, forming turbid/translucent dispersion, close to its cloud point (CP) and existed as unstable vesicles. However, it exhibited interesting solution behavior expressed in terms of the blue point (BP) and the double CP in the presence of different surfactants, leading to mixed micellar systems with a triggered morphology transition from unstable vesicles to polymer-rich micelles and cationic surfactant-rich micelles. Such an amendment in the morphology of copolymer nanoaggregates in the presence of cationic surfactants has been well observed from scattering data. This is further rationalized employing the MD approach, which validated the effective interactions between Pluronics-cationic surfactant mixed micelles. Thus, our experimental results integrated with MD yield a deep insight into the nanoscale interactions controlling the micellar aggregation (Pluronics-rich micelles and surfactant-rich micelles) in the investigated mixed system.

Role of protein-copolymer assembly in controlling micellization process of amphiphilic triblock copolymer

HYPOTHESIS

Triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) forms a well-known micellar assembly at a particular temperature. Apart from regular assembly within the copolymer, it is crucial to explore additional assembly behaviour via simple exposure of proteins which unveils biased interactions with blocks of copolymer. The current work focuses on the examination of Pluronic F108 i.e. PEG-PPG-PEG with two different proteins i.e. α-chymotrypsin (CT) and lysozyme (LSZ), aiming at probing the critical micellization temperature (CMT) and molecular level interactions.

EXPERIMENTS

Potential role of protein-copolymer assembly formation at a particular concentration of protein in modulating CMT was shown by a systematic experimental approach combined with a series of physicochemical methods. The sophisticated multiple techniques include fluorescence spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, dynamic light scattering (DLS), zeta potential measurements, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, molecular docking studies were also employed to correlate theoretical insights with experimental findings.

FINDINGS

CT and LSZ decrease CMT in regular concentration-dependent manner except for particular concentration (1.5 mg/mL) of LSZ which shows anomalous behaviour in steady-state fluorescence spectroscopy, temperature dependent fluorescence spectroscopy, Raman spectroscopy and DLS measurements. SEM and TEM results clearly reveal protein-copolymer assembly formation. The assembled structure has different biophysical properties. Docking studies elucidate several bio macromolecular interactions which can be involved in assembly formation. Based on obtained results from biophysical techniques mechanism of CMT variation was deduced. Obtained results can be useful in biosensors and targeted drug delivery systems.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

The Martini Model in Materials Science

The Martini model, a coarse-grained force field initially developed with biomolecular simulations in mind, has found an increasing number of applications in the field of soft materials science. The model's underlying building block principle does not pose restrictions on its application beyond biomolecular systems. Here, the main applications to date of the Martini model in materials science are highlighted, and a perspective for the future developments in this field is given, particularly in light of recent developments such as the new version of the model, Martini 3.

Using coarse-grained molecular dynamics to understand the effect of ionic liquids on the aggregation of Pluronic copolymer solutions

This study is aimed to enhance the understanding of the interaction between ionic liquids (ILs) and non-ionic Pluronic triblock copolymers in aqueous two-phase micellar systems (ATPMS) used for the selective separation/purification of hydrophobic biomolecules. The ILs allow a precise control of the cloud point phase separation temperature (CPT), particularly important when the stability of the molecule is highly dependent on temperature. The effect of choline-based ILs, with two different counter-anions, chloride and hexanoate, was evaluated using molecular dynamics simulations (MD) for F-68 and L-35 Pluronic aqueous solutions. The simulations revealed the role played by the anions during the Pluronic self-assembly, with choline chloride hindering Pluronic aggregation and the choline hexanoate favouring micelle formation and coalescence, in agreement with the experimental data. A detailed study of the accessible surface area of Pluronic showed a progressive dehydration of the Pluronic hydrophilic micelle corona in choline hexanoate mixtures promoting inter-micelle interactions and, consequently, micelle coalescence. With the addition of choline hexanoate, it was observed that the hydrophilic segments, which form the micelle corona, twisted towards the Pluronic micelle core. The electrostatic interaction is also shown to play a key role in this IL-Pluronic aqueous solution, as the hexanoate anions are accommodated in the Pluronic micelle core, while the choline cations are hosted by the Pluronic micelle corona, with the ions interacting with each other during the self-assembly process. In addition, a comparison study of F-68 and L-35 aqueous solutions shows that the IL impact depends on the length of the Pluronic hydrophilic segment. This work provides a realistic microscopic scenario of the complex interactions between Pluronic copolymers and ILs.

Mechanistic Understanding From Molecular Dynamics Simulation in Pharmaceutical Research 1: Drug Delivery

In this review, we outline the growing role that molecular dynamics simulation is able to play as a design tool in drug delivery. We cover both the pharmaceutical and computational backgrounds, in a pedagogical fashion, as this review is designed to be equally accessible to pharmaceutical researchers interested in what this new computational tool is capable of and experts in molecular modeling who wish to pursue pharmaceutical applications as a context for their research. The field has become too broad for us to concisely describe all work that has been carried out; many comprehensive reviews on subtopics of this area are cited. We discuss the insight molecular dynamics modeling has provided in dissolution and solubility, however, the majority of the discussion is focused on nanomedicine: the development of nanoscale drug delivery vehicles. Here we focus on three areas where molecular dynamics modeling has had a particularly strong impact: (1) behavior in the bloodstream and protective polymer corona, (2) Drug loading and controlled release, and (3) Nanoparticle interaction with both model and biological membranes. We conclude with some thoughts on the role that molecular dynamics simulation can grow to play in the development of new drug delivery systems.