Structure and Dynamics of Associative Water-Soluble Polymer Aggregates As Seen by 19F NMR Spectroscopy

Decoupling between Translational Diffusion and Viscoelasticity in Transient Networks with Controlled Network Connectivity

The mobility of sustained molecules is influenced by viscoelasticity, which is strongly correlated with the diffusional property in polymeric liquid. However, the study of transient networks formed by a reversible crosslink, which is the viscoelastic liquid, was insufficient due to the absence of a model system. We compare the viscoelastic and diffusional properties of the transient networks, using the model system with controlled network connectivity (Tetra-PEG slime). According to independent measurements of viscoelasticity and diffusion, the root-mean-square distance the polymer diffuses during the viscoelastic relaxation time shows a large deviation from the self-size of the polymer, which is contrary to the conventional understanding. This decoupling between viscoelasticity and diffusion is unique for transient networks, suggesting that the viscoelastic relaxation is not induced by the diffusion of one prepolymer, particularly in the network with low connectivity. These findings will provide a definite basis for discussion to understand the viscoelasticity in transient networks.

Understanding the self-assembly of the polymeric drug solubilizer Soluplus®

HYPOTHESIS

Soluplus® is one of the most widely used amphiphilic copolymers in drug delivery and has been reported to strongly enhance the adsorption of model drugs. However, there is still a limited understanding of its micellar behavior as it responds to the different routes of administration, which involve important changes in concentration.

EXPERIMENTS

The microstructure of Soluplus aqueous solutions has been investigated at a wide range of polymer concentrations (2 × 10^-6 - 0.2 g/mL) by a combination of diffusion NMR (dNMR), small angle X-ray scattering (SAXS), static (SLS) dynamic (DLS) light scattering and viscosity measurements. These techniques have been coupled with surface tension measurements to frame the polymer's critical micellar concentration (cmc).

FINDINGS

We demonstrate the presence at all tested concentrations of two forms of Soluplus, with hydrodynamic radii of 3 and 26 nm, where the fraction of smaller objects accounts for as much as 60-70%. dNMR, SAXS, DLS and SLS indicate that Soluplus spontaneously self-assembles into large spherical particles with a core-shell structure. However, self-assembly takes place three orders of magnitude above the cmc evaluated via surface tension measurements. Instead of the traditional cooperative micellization process, we propose a thermal-activated isodesmic self-assembly of the small aggregates into core-shell micelles.

Single Molecule Nanopore Spectrometry for Peptide Detection

Sensing and characterization of water-soluble peptides is of critical importance in a wide variety of bioapplications. Single molecule nanopore spectrometry (SMNS) is based on the idea that one can use biological protein nanopores to resolve different sized molecules down to limits set by the blockade duration and noise. Previous work has shown that this enables discrimination between polyethylene glycol (PEG) molecules that differ by a single monomer unit. This paper describes efforts to extend SMNS to a variety of biologically relevant, water-soluble peptides. We describe the use of Au25(SG)18 clusters, previously shown to improve PEG detection, to increase the on- and off-rate of peptides to the pore. In addition, we study the role that fluctuations play in the single molecule nanopore spectrometry (SMNS) methodology and show that modifying solution conditions to increase peptide flexibility (via pH or chaotropic salt) leads to a nearly 2-fold reduction in the current blockade fluctuations and a corresponding narrowing of the peaks in the blockade distributions. Finally, a model is presented that connects the current blockade depths to the mass of the peptides, which shows that our enhanced SMNS detection improves the mass resolution of the nanopore sensor more than 2-fold for the largest cationic peptides studied.

Self-aggregation of gel forming PEG-PLA star block copolymers in water

The aggregation behavior and dynamics of poly(ethylene glycol) (PEG) and poly(lactide) (PLA) chains in a homologous series of eight-armed PEG-PLA star block copolymers ((PEG(65)-NHCO-PLA(n))(8) with n = 11, 13, and 15) in water at different concentrations and temperatures were studied by means of (1)H and (13)C NMR spectroscopy and (1)H longitudinal relaxation time analysis. The state of water in these systems was also investigated through the combined use of (1)H and (2)H longitudinal relaxation time measurement. On the basis of the NMR experimental findings and of dynamic light scattering measurements, (PEG(65)-NHCO-PLA(n))(8) in water can be described as self-aggregated systems with quite rigid hydrophobic domains made of PLA chains and aqueous domains where both PEG chains and water molecules undergo fast dynamics. A smaller number of rigid domains was found for (PEG(65)-NHCO-PLA(11))(8) with respect to the homologous copolymers with longer PLA chains. At low concentrations, the PLA domains are mainly formed by chains belonging to the same molecule, thus giving rise to unimolecular micelles. At intermediate concentrations, that is, above the critical association concentration (CAC) but below the critical gel concentration (CGC), nanogels are formed by interconnection of several PLA domains through shared unimers. Above the CGC, the network is extended to the entire system, giving rise to macroscopic gels. In all cases, a fraction of PLA chains remains quite mobile and exposed to water due to topological constraints of the star architecture.

Mixed micelles of fluorinated and hydrogenated surfactants

The model mixed surfactant system of sodium perfluorooctanoate and sodium decyl sulfate was carefully reexamined by a combination of nuclear magnetic resonance methods. Over a wide range of sample compositions, detailed (19)F and (1)H chemical shift data in combination with self-diffusion coefficients for the perfluorooctanoate and decyl sulfate ions are collected. All data are analyzed together in a framework that uses a minimal number of initial assumptions to extract the monomer concentrations of both surfactants and the micellar chemical shifts of (19)F and (1)H as a function of relative concentration. The main conclusion drawn from this analysis is that there exists neither complete demixing nor complete mixing on molecular or micellar levels. Instead, the experimental data favor a single type of micelles within which fluorinated surfactants are preferentially coordinated by fluorinated ones and hydrogenated surfactants by hydrogenated ones. The data are quantitatively interpreted in the framework of the first approximation of the regular solution theory (also called the quasi-chemical treatment) leading to an energy of mixing of omega = W/kT = 0.98 between the constituting surfactant types. These findings may help to resolve a long controversy about micellar mixing-demixing in this particular mixture and in its relatives.

Mixed adsorption of fluorinated and hydrogenated surfactants

The adsorption isotherms of sodium perfluorooctanoate and sodium decyl sulfate and their 1:1 mixture on gamma-alumina are recorded by depletion-type experiments with (1)H and (19)F NMR spectroscopy as the detection tool. The isotherms of the different surfactant species, obtained with and without added salt, closely resemble each other. Salt addition changes the isotherms from stepwise to the familiar S-shaped. After having reached saturation, a further increase of surfactant concentration in the mixed system leads to decyl sulfate desorption and increased perfluorooctanoate adsorption. The (19)F chemical shift of adsorbed perfluorooctanoate suggests that, for saturated surfaces, the two sorts of adsorbed surfactants form molecularly mixed surface aggregates.

Micellar kinetics of a fluorosurfactant through stopped-flow NMR

19F NMR chemical shifts and transverse relaxation times T2 were measured as a function of time after quick stopped-flow dilution of aqueous solutions of sodium perfluorooctanoate (NaPFO) with water. Different initial concentrations of micellar solution and different proportions of mixing were tested. Previous stopped-flow studies by time-resolved small-angle X-ray scattering (TR-SAXS) detection indicated a slow (approximately 10 s) micellar relaxation kinetics in NaPFO solutions. In contrast, no evidence of any comparable slow (>100 ms) relaxation process was found in our NMR studies. Possible artifacts of stopped-flow experiments are discussed as well as differences between NMR and SAXS detection methods. Upper bounds on the relative weight of a slow relaxation process are given within existing kinetic theories of micellar dissolution.

19F fast magic-angle spinning NMR studies of perfluoroalkanoic acid self-assembled monolayers

The bonding and dynamic properties of perfluoroalkanoic acid self-assembled monolayers (SAMs) on zirconia and titania powders were characterized by Fourier transform infrared and solid-state 19F magic-angle spinning NMR spectroscopy. The perfluoro fatty acids investigated included C(n)F(2n+1)CO2H, n = 7, 13, 15 and 17. The acids bind to both metal oxides via ionic carboxylate bonds, but complete monolayers are only formed on the zirconia. The shift of the CF3 group from -83 ppm in the bulk state to -85 ppm for the adsorbed monolayers is assigned to CF3 groups located at the air/monolayer interface. With the exception of the perfluorooctanoic acid, 19F spin lattice relaxation measurements indicate that the fluorocarbon chains of the adsorbed acids, even in the case of densely packed monolayers, are significantly more mobile than those in the bulk state. The motions associated with the enhanced mobility of the adsorbed acids are proposed to involve reorientations along the long chain axes. No evidence for chain melting in the fluorocarbon SAMs is found for temperatures well above the melting temperature of the bulk acids.

Well-defined nanoparticles formed by hydrophobic assembly of a short and polydisperse random terpolymer, amphipol A8-35

Amphipols are short amphilic polymers designed for applications in membrane biochemistry and biophysics and used, in particular, to stabilize membrane proteins in aqueous solutions. Amphipol A8-35 was obtained by modification of a short-chain parent polymer (poly(acrylic acid); PAA) with octyl- and isopropylamine, to yield an amphiphilic product with an average molar mass of 9-10 kg x mol(-1) (sodium salt form) and a polydispersity index of 2.0 to 3.1, depending on the source of PAA. The behavior of A8-35 in aqueous buffers was studied by size exclusion chromatography, static and dynamic light scattering, equilibrium and sedimentation velocity analytical ultracentrifugation, and small angle neutron scattering. Despite the variable length of the chains and the random distribution of hydrophobic groups along them, A8-35 self-organizes into well-defined assemblies. The data are best compatible with most of the polymer forming compact assemblies (particles) with a molar mass of approximately 40 kg x mol(-1), a radius of gyration of approximately 2.4 nm, and a Stokes radius of approximately 3.15 nm. Each particle contains, on average, four A8-35 macromolecules and 75-80 octyl chains. Neutron scattering reveals a sharp interface between the particles and water. A minor (approximately 0.1%) mass fraction of the material forms much larger aggregates, whose proportion may increase under certain conditions of preparation or handling, such as low pH. They can be removed by gel filtration.