Influence of H+, OH- and salts on hydrophobic self-assembly

In spite of the ubiquity of acid/base ions and salts in biological systems, their influence on hydrophobic self-assembly remains an open question. Here we use a combined experimental and theoretical strategy to quantify the influence of H+ and OH-, as well as salts containing Li+, Na+, Cl- and Br-, on the hydrophobic self-assembly of micelles composed of neutral oily 1,2-hexanediol surfactants. The distributions of aggregate sizes, both below and above the critical micelle concentration (CMC), are determined using Raman multivariate curve resolution (Raman-MCR) spectroscopy to quantify the multi-aggregation chemical potential surface (MCPS) that drives self-assembly. The results reveal that ions have little influence on the formation of hydrophobic contact dimers but can significantly drive high-order self assembly. Moreover, the hydration-shells of oily solutes are found to expel the above salt ions and OH-, but to attract H+, with wide-ranging implications.

Preparation of Bioderived and Biodegradable Surfactants Based on an Intrinsically Disordered Protein Sequence

Surfactants, block copolymers, and other types of micellar systems are used in a wide variety of biomedical and industrial processes. However, most commonly used surfactants are synthetically derived and pose environmental and toxicological concerns throughout their product life cycle. Because of this, bioderived and biodegradable surfactants are promising alternatives. For biosurfactants to be implemented industrially, they need to be produced on a large scale and also have tailorable properties that match those afforded by the polymerization of synthetic surfactants. In this paper, a scalable and versatile production method for biosurfactants based on a hydrophilic intrinsically disordered protein (IDP) sequence with a genetically engineered hydrophobic domain is used to study variables that impact their physicochemical and self-assembling properties. These amphiphilic sequences were found to self-assemble into micelles over a broad range of temperatures, pH values, and ionic strengths. To investigate the role of the IDP hydrophilic domain on self-assembly, variants with increased overall charges and systematically decreased IDP domain lengths were produced and examined for their sizes, morphologies, and critical micelle concentrations (CMCs). The results of these studies indicate that decreasing the length of the IDP domain and consequently the molecular weight and hydrophilic fraction leads to smaller micelles. In addition, significantly increasing the amount of charged residues in the hydrophilic IDP domain results in micelles of similar sizes but with higher CMC values. This represents an initial step in developing a quantitative model for the future engineering of biosurfactants based on this IDP sequence.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO₄) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K⁺ to the fatty sulfate interface showed 10 times greater affinity than for Li⁺ and approximately 3 times greater than for Na⁺. All three cations formed solvent shared ion pairs when the mole fraction of ESO₄ was 0.33 or lower. Above this threshold charge density, Li⁺ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K⁺ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na⁺ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Structural analysis of a calix[4]arene-based Platonic Micelle

We have recently introduced the concept of “Platonic micelles”, the preference of spherical micelles to specific aggregation numbers mostly coinciding with the number of faces of platonic solids. This effect was observed on bulky, mostly calix[4]arene-based surfactant systems with small aggregation numbers. The preferred aggregation numbers result in better sphere coverage, highliting the packing and the “protection” of hydrophobic cores from the aqueous solvent as the most important factor for this preference. In the present study we further explore the interactions that drive the packing of the highly charged PACaL3 surfactant into highly symmetrical hexameric micelles. We performed a series of molecular dynamics simulations that yielded a large set of structures and an ensemble in good agreement with the experimental Small Angle X-ray Scattering data was selected. The geometry and the rigidity of the calix[4]arene group with proper tail length and headgroup volume are the driving forces for the high symmetry and monodispersity of the micelle. The charge of the headgroups is mainly responsible for inhibiting the formation of higher order structures. Sodium, shown to be important for the stability of the micelle, is not directly interacting with the micelle implying that the calix[4]arene ring is a C2ν symmetry conformation.

Platonic Micelles: Monodisperse Micelles with Discrete Aggregation Numbers Corresponding to Regular Polyhedra

The concept of micelles was first proposed in 1913 by McBain and has rationalized numerous experimental results of the self-aggregation of surfactants. It is generally agreed that the aggregation number (Nagg) for spherical micelles has no exact value and a certain distribution. However, our studies of calix[4]arene surfactants showed that they were monodisperse with a defined Nagg whose values are chosen from 6, 8, 12, 20, and 32. Interestingly, some of these numbers coincide with the face numbers of Platonic solids, thus we named them "Platonic micelles". The preferred Nagg values were explained in relation to the mathematical Tammes problem: how to obtain the best coverage of a sphere surface with multiple identical circles. The coverage ratio D(N) can be calculated and produces maxima at N = 6, 12, 20, and 32, coinciding with the observed Nagg values. We presume that this "Platonic nature" may hold for any spherical micelles when Nagg is sufficiently small.

Supramolecular Architectures of Dendritic Amphiphiles in Water

Dendritic molecules are an exciting research topic because of their highly branched architecture, multiple functional groups on the periphery, and very pertinent features for various applications. Self-assembling dendritic amphiphiles have produced different nanostructures with unique morphologies and properties. Since their self-assembly in water is greatly relevant for biomedical applications, researchers have been looking for a way to rationally design dendritic amphiphiles for the last few decades. We review here some recent developments from investigations on the self-assembly of dendritic amphiphiles into various nanostructures in water on the molecular level. The main content of the review is divided into sections according to the different nanostructure morphologies resulting from the dendritic amphiphiles' self-assembly. Finally, we conclude with some remarks that highlight the self-assembling features of these dendritic amphiphiles.

Carboxylate Ion Pairing with Alkali-Metal Ions for β-Lactoglobulin and Its Role on Aggregation and Interfacial Adsorption

We report a combined experimental and computational study of the whey protein β-lactoglobulin (BLG) in different electrolyte solutions. Vibrational sum-frequency generation (SFG) and ellipsometry were used to investigate the molecular structure of BLG modified air-water interfaces as a function of LiCl, NaCl, and KCl concentrations. Molecular dynamics (MD) simulations and thermodynamic integration provided details of the ion pairing of protein surface residues with alkali-metal cations. Our results at pH 6.2 indicate that BLG at the air-water interface forms mono- and bilayers preferably at low and high ionic strength, respectively. Results from SFG spectroscopy and ellipsometry are consistent with intimate ion pairing of alkali-metal cations with aspartate and glutamate carboxylates, which is shown to be more effective for smaller cations (Li(+) and Na(+)). MD simulations show not only carboxylate-alkali-metal ion pairs but also ion multiplets with the alkali-metal ion in a bridging position between two or more carboxylates. Consequently, alkali-metal cations can bridge carboxylates not only within a monomer but also between monomers, thus providing an important dimerization mechanism between hydrophilic surface patches.

Synthesis of fluorinated oxadiazoles with gelation and oxygen storage ability

A new family of fluorinated low molecular weight (LMW) gelators has been synthesized through SNAr substitution of 5-polyfluoroaryl-3-perfluoroheptyl-1,2,4-oxadiazoles with glycine ester. The obtained compounds give thermal and pH-sensitive hydrogels or thermo-reversible organogels in DMSO. Oxygen solubility studies showed the ability to maintain high oxygen levels in solution and in gel blend with plate counter agar (PCA).

Hydrophobically modified dendrons: developing structure-activity relationships for DNA binding and gene transfection

This paper develops a structure-activity relationship understanding of the way in which surfactant-like dendrons with hydrophilic spermine surface groups and a variety of lipophilic units at their focal points can self-assemble and subsequently bind to DNA with high affinity. The choice of functional group at the focal point of the dendron and the high tunability of the molecular structure have a very significant impact on DNA binding. Mesoscale modeling of the mode of dendron self-assembly provides a direct insight into how the mode of self-assembly exerts its effect on the DNA binding process. In particular, the hydrophobic unit controls the number of dendrons in the self-assembled micellar structures, and hence their diameters and surface charge density. The DNA binding affinity correlates with the surface charge density of the dendron aggregates. Furthermore, these structure-activity effects can also be extended to cellular gene delivery, as surface charge density plays a role in controlling the extent of endosomal escape. It is reported that higher generation dendrons, although binding DNA less strongly than the self-assembling lower generation dendrons, are more effective for transfection. The impact of the lipophilic group at the focal point is less significant for the DNA binding ability of these larger dendrons, which is predominantly controlled by the spermine surface groups, but it does modify the levels of gene transfection. Significant synergistic effects on gene delivery were observed when employing combinations of the dendrons and polyethyleneimine (PEI, 25 kDa), with transfection becoming possible at low loading levels where the two components would not transfect individually, giving practically useful levels of gene delivery.

Self-assembly of structurally persistent micelles is controlled by specific-ion effects and hydrophobic guests

A combined study using cryo-TEM experiments and molecular dynamics simulations reveals remarkable details of the factors that affect the self-organization of specifically designed T-shaped amphiphilic dendrimers upon treatment of an aqueous solution with ultrasound under a layer of hexane. This treatment leads to dodecameric, structured micelles rather than the heptameric ones observed without hexane. Three-dimensional reconstruction of the cryo-TEM images provides very detailed structures of the micelles, and molecular dynamics simulations suggest that approximately 36 hexane molecules are needed to stabilize the dodecameric micelles. Sodium counterions are found to exert a significant stabilizing effect that results in an apparent attraction between the highly negatively charged polycarboxylate headgroups. DFT calculations support the observation that the formation of ion multiplets is especially crucial for this stabilizing counterion effect, which reduces headgroup repulsion. This and the increased hydrophobic stabilization that results from the hexane-enlarged core of the micelle lead to stable dodecameric micelles. The specific effects found for sodium counterions are largely absent for potassium.