Apparent Specific Volume Measurements of Poly(ethylene oxide), Poly(butylene oxide), Poly(propylene oxide), and Octadecyl Chains in the Micellar State as a Function of Temperature

Interfacial Behavior of Biodegradable Poly(lactic-co-glycolic acid)-Pluronic F127 Nanoparticles and Its Impact on Pickering Emulsion Stability

Biodegradable nanoparticle-based emulsions exhibit immense potential in various applications, particularly in the pharmaceutical, cosmetic, and food industries. This study delves into the intricate interfacial behavior of Pluronic F127 modified poly(lactic-co-glycolic acid) (PLGA-F127) nanoparticles, a crucial determinant of their ability to stabilize Pickering emulsions. Employing a combination of Langmuir balance, surface tension, and diffusion coefficient measurements, we investigate the interfacial dynamics of PLGA-F127 nanoparticles under varying temperature and ionic strength conditions. Theoretical calculations are employed to elucidate the underlying mechanisms governing these phenomena. Our findings reveal a profound influence of temperature-dependent Pluronic layer behavior and electrostatic and steric interactions on the interfacial dynamics. Nonlinear changes in surface tension are observed, reflecting the interplay of these factors. Particle aggregation is found to be prevalent at elevated temperatures and ionic strengths, compromising the stability and emulsification efficiency of the formed emulsions. This work provides insights into the rational design of stable and efficient biodegradable nanoparticle-based Pickering emulsions, broadening their potential applications in various fields.

Small Angle X-Ray Scattering Data Analysis and Theoretical Modelling for the Size and Shape Characterization of Drug Delivery Systems Based on Vitamin E TPGS Micelles

We developed a simple two-dimensional/two-components theoretical model that describes the structure and functionality of a VitE-TPGS system of micelles assuming a hydrophobic inner core and an outer hydrated hydrophilic shell. We then conceptually applied the developed methodology to a simple system of VitE-TPGS micelles unloaded and loaded with an active pharmaceutical ingredient, eltrombopag, to verify if the model could reliably monitor the size change of the micelle upon loading. The fit of laboratory Small Angle X-Ray Scattering data against such model allows us to extract absolute values of the micelles size under a spherical shape hypothesis as well as the distribution within the system between components and level of hydration. The intensity scale of the SAXS experimental data needs to be normalized to a reference standard (pure water) to get absolute scattered intensities. The mathematical model which has been developed under a general hypothesis of ellipsoidal micelles, is applied to our experimental data under the simplified spherical assumption, which suitably fits our experimental data.

Investigations to Explore Molecular Interactions and Sweetness Response of Polyhydroxy Compounds with Amino Acids in Aqueous Systems

In conjunction with the development of people's living standards, the modern world demands good-quality food such as sweets, candies, chocolates, diet drinks, beverages, and so on, but because of obesity and other health issues people concentrate more on sugar-free or low-calorie products. Polyols are such a kind of food with desirable qualities, and they play a role in controlling the blood glucose level in diabetic patients. The density (ρ) and sound speed (u) of sugar alcohol in water and in (0.02, 0.04, and 0.06) mol kg^-1l-arginine solutions at different temperatures (293.15-318.15 K) and atmospheric pressure were measured by using Anton Paar DSA5000M. Experimental density and sound velocity data were further used to compute volumetric and acoustic parameters such as apparent molar volume (Ø_V), partial molar volume (Ø _V ⁰), compressibility (Ø _k ⁰), expansibility (Ø _E ⁰), and so on. The positive trends of apparent molar volume (Ø _V ), and partial molar volume Ø _V ⁰), values indicate strong hydrophilic interactions in ternary solutions. These interactions give a complete picture about solvation behavior, the effect of temperature, and hydrogen bonding present among (galactitol + l-arginine) mixtures. The apparent specific volume values were calculated, and it was found that these values of the investigated mixtures lie on the borderline with the reported values of sweeteners. This study may offer a new vision in elucidation of mechanistic modifications between sugar alcohol, amino acid, and their mode of interactions.

Asymmetric Vesicles Self-Assembled by Amphiphilic Sequence-Controlled Polymers

The asymmetric distribution of lipids on the inner and outer membranes of a cell plays a pivotal role in the physiological and immunological activities of life. It has inspired the elaboration of synthetic asymmetric vesicles for the discovery of advanced materials and functions. The asymmetric vesicles were generally prepared by amphiphilic block copolymers. We herein report on the formation of asymmetric vesicles self-assembled by amphiphilic sequence-controlled polymers with two hydrophilic segments SU and TEO. We also developed an efficient fluorescence titration method with europium(III) ions (Eu³⁺) to determine the uneven distribution of SU and TEO. SU units are preferentially located on the outer membrane and TEO on the inner membrane of the resulting vesicles, which is facilitated by the electrostatic repulsion of SU and the U-shaped folding of the hydrophobic backbone of the resulting polymers. This work shows that sequence-controlled polymers with alternating monomer sequence provide a powerful toolbox for the elaboration of important yet challenging self-assembled structures for emerging functions and properties.

A high-flux automated laboratory small-angle X-ray scattering instrument optimized for solution scattering

A commercially available small-angle X-ray scattering (SAXS) NanoSTAR instrument (Bruker AXS) with a liquid-metal-jet source (Excillum) has been optimized for solution scattering and installed at iNANO at Aarhus University. The instrument (named HyperSAXS) employs long high-quality parabolic Montel multilayer optics (Incoatec) and a novel compact scatterless pinhole slit with Ge edges, which was designed and built at Aarhus University. The combination of the powerful source and optimized geometry gives an integrated X-ray intensity close to 109 photons s−1 for a standard range of scattering vector moduli q = 0.0098–0.425 Å−1, where q = (4πsinθ)/λ and λ is the Ga Kα wavelength of 1.34 Å. The high intensity of the instrument makes it possible to measure dilute samples of, for example, protein or surfactant with concentrations of 1 mg ml−1 in a few minutes. A flow-through cell, built at Aarhus University, in combination with an automated sample handler has been installed on the instrument. The sample handler is based on the commercial Gilson GX-271 injection system (Biolab), which also allows samples to be stored under thermostatted conditions. The sample handler inserts and removes samples, and also cleans and dries the sample cell between measurements. The minimum volume of the flow-through capillary is about 20 µl. The high intensity additionally allows time-resolved measurements to be performed with a temporal resolution of seconds. For this purpose a stopped-flow apparatus, (SFM-3000, Bio-Logic) was connected to the flow-through cell by high-performance liquid chromatography tubing. This configuration was chosen as it allows vacuum around the sample cell and thus maintains a low background. The instrument can readily be converted into a low-q setup with a q range of 0.0049–0.34 Å−1 and an X-ray intensity of about 5 × 107 photons s−1.

A complete picture of protein unfolding and refolding in surfactants

Interactions between proteins and surfactants are of relevance in many applications including food, washing powder formulations, and drug formulation. The anionic surfactant sodium dodecyl sulfate (SDS) is known to unfold globular proteins, while the non-ionic surfactant octaethyleneglycol monododecyl ether (C₁₂E₈) can be used to refold proteins from their SDS-denatured state. While unfolding have been studied in detail at the protein level, a complete picture of the interplay between protein and surfactant in these processes is lacking. This gap in our knowledge is addressed in the current work, using the β-sheet-rich globular protein β-lactoglobulin (bLG). We combined stopped-flow time-resolved SAXS, fluorescence, and circular dichroism, respectively, to provide an unprecedented in-depth picture of the different steps involved in both protein unfolding and refolding in the presence of SDS and C₁₂E₈. During unfolding, core-shell bLG-SDS complexes were formed within ∼10 ms. This involved an initial rapid process where protein and SDS formed aggregates, followed by two slower processes, where the complexes first disaggregated into single protein structures situated asymmetrically on the SDS micelles, followed by isotropic redistribution of the protein. Refolding kinetics (>100 s) were slower than unfolding (<30 s), and involved rearrangements within the mixing deadtime (∼5 ms) and transient accumulation of unfolded monomeric protein, differing in structure from the original bLG-SDS structure. Refolding of bLG involved two steps: extraction of most of the SDS from the complexes followed by protein refolding. These results reveal that surfactant-mediated unfolding and refolding of proteins are complex processes with rearrangements occurring on time scales from sub-milliseconds to minutes.

Structural Characterization of Pluronic Micelles Swollen with Perfume Molecules

The insertion in nonionic polymer micelles (Pluronics F127) of seven essential oils and some of the pure compounds that compose them was investigated by complementary differential scanning calorimetry, small-angle X-ray, and neutron scattering (SAXS and SANS). The study revealed various insertion and swelling behaviors for the different oil molecules, an evidence of different interaction mechanisms involved between oils and Pluronic monomers. Thermodynamically, the addition of oil increased the micellization enthalpy due to an enhanced release of water molecules, leading subsequently to a decrease of the critical micellar temperature (CMT). Structurally, with oil, SANS revealed the presence of large aggregates at lower temperature than the CMT for which their size is maximal. Above the CMT, the size decreased and the equilibrium was reached a few degrees after the temperature corresponding to the maximum of the endothermic peak. At 37 °C, the detailed combined SANS and SAXS analysis demonstrated a partial phase separation between the oil and the poly(propylene oxide) core. The hydrophilic stabilizing poly(ethylene oxide) shell remains unchanged.

Insight into the molecular mechanism behind PEG-mediated stabilization of biofluid lipases

Bioconjugates established between anionic polyethylene glycol (PEG) based polymers and cationic proteins have proven to be a promising strategy to engineer thermostable biocatalysts. However, the enzyme activity of these bioconjugates is very low and the mechanism of non-covalent PEG-stabilization is yet to be understood. This work presents experimental and molecular dynamics simulation studies, using lipase-polymer surfactant nanoconjugates from mesophile Rhizomucor miehei (RML), performed to evaluate the effect of PEG on enzyme stability and activity. Results demonstrated that the number of hydrogen bonds between the cationized RML and PEG chain correlates with enzyme thermostability. In addition, an increase of both the number of PEG-polymers units and cationization degree of the enzyme leads to a decrease of enzyme activity. Modelling with SAXS data of aqueous solutions of the biofluid lipases agrees with previous hypothesis that these enzymes contain a core constituted of folded protein confined by a shell of surfactants. Together results provide valuable insight into the mechanism of non-covalent PEG mediated protein stabilization relevant for engineering active and thermostable biofluids. Furthermore, the first biofluids RML with activity comparable to their cationized counterpart are presented.

Injectable synthetic hydrogel for bone regeneration: Physicochemical characterisation of a high and a low pH gelling system

Hybrid poly(ethylene glycol)-co-peptide hydrogels are a versatile platform for bone regeneration. For the use as injectable scaffolds, a good understanding of reaction kinetics and physical properties is vital. However, these factors have not yet been comprehensively illuminated. We show that gelation time can be effectively controlled by pH without affecting the elasticity of the formed hydrogels. Maleimide functionalised PEG gels at lower pH and produces more densely cross-linked networks than vinylsulfone functionalised PEG. Both form non-ideal networks. The elastic moduli on the order of a few kPa are in good agreement with the structural characterisation. Primary human osteoblasts cultured in proximity to bulk gels were not adversely affected in vitro. The results demonstrate that hybrid PEG-peptide hydrogels can be tailored to the requirements of in situ gelation. Attributed to their increased structural properties and a higher tolerance towards low pH, maleimide functionalised hydrogels might provide a better alternative for injectable applications.

Grafting Commercial Surfactants (Brij, CiEj) and PEG to Electrodes via Aryldiazonium Salts

Grafting commercial surfactants appears to be a simple way to modify electrodes and conducting interfaces, avoiding the synthesis of complex organic molecules. A new surface functionalization route is presented to build surfactant coatings with monolayer thickness grafting molecules considered as nonreactive. A monolayer of -SO₂Cl functions (from a p-benzenesulfonyl chloride) was first electrografted. It showed a high reactivity toward weak nucleophiles commonly found on surfactant end-moieties such as hydroxyl groups (-OH), and it was used to covalently graft the following: (1) nonionic diblock oligomers (Brij or CiEj, C_xH_2x + (OCH₂CH₂)_nOH with x = 16 and n = 23 for Brij58, x = 16 and n = 10 for Brij C10, and x = 16 and n = 2 for Brij52); (2) poly(ethylene glycol) (PEG) short chains (PEO₉ for (OCH₂CH₂)_nOH with n = 9) and mixed formula. The surface modification due to these molecular coatings was investigated in terms of wetting properties and interfacial electrochemistry characteristics (charge transfer resistivity, capacity, and ions dynamics). Built on flat and transparent thin chromium films, Brij and PEO mixed coatings have been proven to be promising coatings for electrochemical biosensor application such as for stabilizing a partially tethered supported biomimetic membrane.

Tilia tomentosa pectins exhibit dual mode of action on phagocytes as β-glucuronic acid monomers are abundant in their rhamnogalacturonans I

Silver linden flowers contain different pectins (PSI-PSIII) with immunomodulating properties. PSI is a low-esterified pectic polysaccharide with predominant homogalacturonan region, followed by rhamnogalacturonan I (RGI) with arabinogalactan II and RGII (traces) domains. PSII and PSIII are unusual glucuronidated RGI polymers. PSIII is a unique high molecular weight RGI, having almost completely O-3 glucuronidated GalA units with >30% O-3 acetylation at the Rha units. Linden pectins induced reactive oxygen species (ROS) and NO generation from non-stimulated whole blood phagocytes and macrophages, resp., but suppressed OZP-(opsonized zymosan particles)-activated ROS generation, LPS-induced iNOS expression and NO production. This dual mode of action suggests their anti-inflammatory activity, which is known for silver linden extracts. PSI expressed the highest complement fixation and macrophage-stimulating activities and was active on intestinal Peyer's patch cells. PSIII was active on non-stimulated neutrophils, as it induced ß₂-integrin expression, revealing that acetylated and highly glucuronidated RGI exhibits immunomodulating properties via phagocytes.

Interaction of poly(lactic-co-glycolic acid) nanoparticles at fluid interfaces

HYPOTHESIS

Adsorption and localization of nanoparticles at fluid interfaces are key factors in processes like transport through membranes or emulsion stabilization. Adsorption of poly(lactic-co-glycolic acid) (PLGA) and Pluronic coated PLGA nanoparticles (NPs) were studied at three different fluid interfaces. The effect of particle surface modification and type of interface was investigated with the aim of fine tuning interfacial interaction of the nanoparticles.

EXPERIMENTS

Surface tension measurements were carried out to determine the surface activity and adsorption kinetics of the particles. Particles layers at the air/water interface were further studied using the Langmuir balance technique by recording the surface pressure-area isotherms. Interfacial rheological measurements were performed to characterize the structural properties of the nanoparticle interfacial films.

FINDINGS

Interfacial adsorption and its kinetics were explained by the diffusion controlled adsorption theory and considering the energy barrier of particle transport to the interface. Surface modification by Pluronic increased the interfacial activity of nanoparticles at all interfaces. Surface activity of PLGA-Pluronic particles could be described by the contributions of both the PLGA NPs and the effective portion of their Pluronic shell. Both particle films present mainly elastic dilatational properties suggesting that particles are in kinetically separated state.

Phase State and Saturation Vapor Pressure of Submicron Particles of meso-Erythritol at Ambient Conditions

meso-Erythritol is a sugar alcohol identified in atmospheric aerosol particles. In this work, evaporation of submicron-sized particles of meso-erythritol was studied in a TDMA system including a laminar flow tube under dry conditions at five temperatures (278-308 K) and ambient pressure. A complex behavior was observed and attributed to the formation of particles of three different phase states: (1) crystalline, (2) subcooled liquid or amorphous, and (3) mixed. With respect to saturation vapor pressure, the subcooled liquid and amorphous states are treated to be the same. The particle phase state was linked to initial particle size and flow tube temperature. Saturation vapor pressures of two phase states attributed to the crystalline and subcooled liquid state respectively are reported. Our results suggest a mass accommodation coefficient close to one for both states.

Small-angle X-ray scattering as a useful supplementary technique to determine molecular masses of polyelectrolytes in solution

Determination of molecular masses of charged polymers is often nontrivial and most methods have their drawbacks. For polyelectrolytes, a new possibility for the determination of number-average molecular masses is represented by small-angle X-ray scattering (SAXS) which allows fast determinations with a 10% accuracy. This is done by relating the mass to the position of a characteristic peak feature which arises in SAXS due to the local ordering caused by charge-repulsions between polyelectrolytes. Advantages of the technique are the simplicity of data analysis, the independency from polymer architecture, and the low sample and time consumption. The method was tested on polyelectrolytes of various structures and chemical compositions, and the results were compared with those obtained from more conventional techniques, such as asymmetric flow field-flow fractionation, gel permeation chromatography, and classical SAXS data analysis, showing that the accuracy of the suggested method is similar to that of the other techniques. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1913-1917.

Core Freezing and Size Segregation in Surfactant Core-Shell Micelles

Nonionic surfactants containing poly(ethylene oxide) are chemically simple and biocompatible and form core-shell micelles at a wide range of conditions. For those reasons, they and their aggregates have been widely investigated. Recently, irregularities that were observed in the low-temperature behavior of surfactants of the kind [CH3(CH2)(n)O(CH2CH2O)(m)H], (abbreviated CnEm) were assigned to a freezing-melting phase transition in the micellar core. In this work we expand the focus from the case of single component systems to binary surfactant systems at temperatures between 1 and 15 °C. By applying small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), and density measurements in pure C18E20 and C18E100 solutions and their mixtures, we show that core freezing/melting is also present in mixtures. Additionally, comparing SAXS data obtained from the mixture with those from the single components, it was possible to demonstrate that the phase transition leads to a reversible segregation of the surfactants from mixed micelles to distinct kinds of micelles of the two components.

Studying the concentration dependence of the aggregation number of a micellar model system by SANS

We present a small-angle neutron scattering (SANS) structural characterization of n-alkyl-PEO polymer micelles in aqueous solution with special focus on the dependence of the micellar aggregation number on increasing concentration. The single micellar properties in the dilute region up to the overlap concentration ϕ* are determined by exploiting the well characterized unimer exchange kinetics of the model system in a freezing and diluting experiment. The micellar solutions are brought to thermodynamic equilibrium at high temperatures, where unimer exchange is fast, and are then cooled to low temperatures and diluted to concentrations in the limit of infinite dilution. At low temperatures the kinetics, and therefore the key mechanism for micellar rearrangement, is frozen on the experimental time scale, thus preserving the micellar structure in the dilution process. Information about the single micellar structure in the semidilute and concentrated region are extracted from structure factor analysis at high concentrations where the micelles order into fcc and bcc close packed lattices and the aggregation number can be calculated by geometrical arguments. This approach enables us to investigate the aggregation behavior in a wide concentration regime from dilute to 6·ϕ*, showing a constant aggregation number with concentration over a large concentration regime up to a critical concentration about three times ϕ*. When exceeding this critical concentration, the aggregation number was found to increase with increasing concentration. This behavior is compared to scaling theories for star-like polymer micelles.

Self-assembly of a hydrophobically end-capped charged amphiphilic triblock copolymer: effects of temperature and salinity

This study elucidates the intricate interplay between hydrophobic and electrostatic interactions in aqueous solutions of a responsive charged triblock copolymer.

Kinks in experimental diffusion profiles of a dissolving semi-crystalline polymer explained by a concentration-dependent diffusion coefficient

Although we observe sharp diffusion fronts, our experimental neutron radiography data can be explained using Fick's laws without resorting to non-Fickian – such as Case II – arguments.

Nanoscopic confinement through self-assembly: crystallization within micellar cores exhibits simple Gibbs-Thomson behavior

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Less is known about liquids spontaneously confined through self-assembly into micellar structures. Here we demonstrate, using a very well-defined n-alkyl-poly(ethylene oxide) polymer system with a tunable structure, that n-alkane(s) forming 2-3 nm small micellar cores are affected considerably by confinement in the form of melting point depressions. Moreover, comparing the reduction in melting points, ΔT_{m}, determined through volumetric and calorimetric methods with the micellar core radius, R_{c}, obtained from small-angle x-ray scattering, we find excellent agreement with the well-known Gibbs-Thomson equation, ΔT_{m}∼R_{c}^{-1}. This demonstrates that the reduced size, i.e., the Laplace pressure, is the dominant parameter governing the melting point depression in micellar systems.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical star-like micelles i.e. a constant core density profile, ∼r(0), and a diffuse corona density profile, ∼r(-4/3). Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, Nagg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, Nagg ∼ n(2) similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the Cn-PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

Temperature-induced attractive interactions of PEO-containing block copolymer micelles

Interactions in a temperature sensitive-colloidal model system are investigated over a wide range of temperatures and concentrations to characterize the interparticle interactions within the system. This model system is composed of poly(ethylene oxide) end-capped with an octadecyl chain (C18E100), which by small-angle X-ray scattering (SAXS) have been shown to form spherical micelles in an aqueous salt solution. In the present study a 0.9 M NaF solution is used to shift the cloud point into the experimentally convenient temperature range. Densitometry and SAXS have shown no indication of specific interactions between the salt ions and the micelles. The spherical micelles are found to persist at elevated temperatures and a change in interparticle interaction is observed by viscometry and SAXS. The results are all consistent with the decreased solvent quality of water toward poly(ethylene oxide) with increasing temperature and it is seen that attractive interparticle interactions emerge in the vicinity of the cloud point.

Tailoring the amphiphilicity and self-assembly of thermosensitive polymers: end-capped PEG-PNIPAAM block copolymers

In this work we report on the synthesis and self-assembly of a thermo-sensitive block copolymer system of n-octadecyl-poly(ethylene glycol)-block-poly(N-isopropylacrylamide), abbreviated as C18-PEGn-b-PNIPAAMm. We present a facile synthetic strategy for obtaining highly tunable thermo-responsive block copolymers starting from commercial PEG-based surfactants (Brij®) or a C18 precursor and conjugating with PNIPAAM via an Atom Transfer Radical Polymerization (ATRP) protocol. The self-assembly and detailed nanostructure were thoroughly investigated in aqueous solutions using both small-angle X-ray and neutron scattering (SAXS/SANS) combined with turbidity measurements. The results show that the system forms rather well defined classical micellar structures at room temperature that first undergo a collapse, followed by inter-micellar aggregation upon increasing the temperature. For the pure C18-PNIPAAM system, however, rather ill-defined micelles were formed, demonstrating the important role of PEG in regulating the nanostructure and the stability. It is found that the PEG content can be used as a convenient parameter to regulate the thermoresponse, i.e., the onset of collapse and aggregation. A detailed theoretical modeling analysis of the SAXS/SANS data shows that the system forms typical core-shell micellar structures. Interestingly, no evidence of back folding, where PEG allows PNIPAAM to form part of the C18 core, can be found upon crossing the lower critical solution temperature (LCST). This might be attributed to the entropic penalty of folding a polymer chain and/or enthalpic incompatibility between the blocks. The results show that by appropriately varying the balance between the hydrophobic and hydrophilic content, i.e. the amphiphilicity, tunable thermoresponsive micellar structures can be effectively designed. By means of SAXS/SANS we are able to follow the response on the nanoscale. These results thus give considerable insight into thermo-responsive micellar systems and provide guidelines as to how these systems can be tailor-made and designed. This is expected to be of considerable interest for potential applications such as in nanomedicine where an accurate and tunable thermoresponse is required.

Controlling assembly of helical polypeptides via PEGylation strategies

Recent studies in our laboratories have demonstrated that a helical polypeptide (17H6), equipped with a histidine tag and a helical alanine-rich, glutamic-acid-containing domain, exhibits pH-responsive assembly behavior useful in the production of polymorphological nanostructures. In this study, the histidine tag in these polypeptides was replaced by polyethylene glycol (PEG) with different molecular masses (5 kDa, or 10 kDa), and the self-association behavior of 17H6 and the PEGylated conjugates was characterized via dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM). DLS experiments illustrated that the polypeptide and its PEG-conjugates undergo reversible assembly under acidic conditions, suggesting that the aggregation state of the polypeptide and the conjugates is controlled by the charged state of the glutamic acid residues. Nanoscale aggregates were detected at polypeptide/conjugate concentrations as low as 20 μM (∼0.3-0.5 mg ml-1) at physiological and ambient temperatures. Scattering and microscopy results showed that the size, the aggregation number, and the morphology of the aggregates can be tuned by the size and the nature of the hydrophilic tag. This tunable nature of the morphology of the aggregates, along with their low critical aggregation concentration, suggests that PEG-alanine-rich polypeptide conjugates may be useful as drug delivery vehicles in which the alanine-rich block serves as a drug attachment domain.

Structure of PEP–PEO block copolymer micelles: exploiting the complementarity of small-angle X-ray scattering and static light scattering

The structure of large block copolymer micelles is traditionally determined by small-angle neutron scattering (SANS), covering a large range of scattering vectors and employing contrast variation to determine the overall micelle morphology as well as the internal structure on shorter length scales. The present work shows that the same information can be obtained by combining static light scattering (SLS) and small-angle X-ray scattering (SAXS), which provide information on, respectively, large and short length scales. Micelles of a series of block copolymers of poly(ethylene propylene)-b-poly(ethylene oxide) (PEP–PEO) in a 70% ethanol solution are investigated. The polymers have identical PEP blocks of 5.0 kDa and varying PEO blocks of 2.8–49 kDa. The SLS contrasts of PEP and PEO are similar, providing a homogeneous contrast, making SLS ideal for determining the overall micelle morphology. The SAXS contrasts of the two components are very different, allowing for resolution of the internal micelle structure. A core–shell model with a PEP core and PEO corona is fitted simultaneously to the SAXS and SLS data using the different contrasts of the two blocks for each technique. With increasing PEO molecular weight, a transition from cylindrical to spherical micelles is observed. This transition cannot be identified from the SAXS data alone, but only from the SLS data.

Effects of temperature and salt concentration on the structural and dynamical features in aqueous solutions of charged triblock copolymers

Effects of temperature and salt addition on the association behavior in aqueous solutions of a series of charged thermosensitive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide)-block-poly(4-styrenesulfonic acid sodium) triblock copolymers (MPEG(45)-b-P(NIPAAM)(n)-b-P(SSS)(22)) with different lengths of the PNIPAAM block (n=17, 48, and 66) have been studied with the aid of turbidity, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). Increasing temperature and salinity as well as longer PNIPAAM blocks are all factors that promote the formation of association structures. The SAXS data show that, for the copolymers with n=48 and n=66, increasing temperature and salt concentration induce interchain associations and higher values of the aggregation number, whereas no aggregation was observed for the copolymer with the shortest PNIPAAM chain. However, DLS measurements reveal the presence of larger association clusters. The cloud point is found to decrease with raising salinity and longer PNIPAAM block. The general picture that emerges is the delicate interplay between repulsive electrostatic forces and hydrophobic interactions and that this balance can be tuned by changing the temperature, salinity, and the length of the PNIPAAM block.

An analytical model for the small-angle scattering of polyethylene glycol-modified liposomes

Polyethylene glycol (PEG) modification of liposomes is one of the most commonly applied ways of increasing bothin vitroandin vivostability of liposomes. The formed liposomes are commonly referred to as stealth liposomes because the PEG corona renders the liposomes invisible to the macrophages in the bloodstream. The first detailed small-angle neutron scattering analysis of PEG-modified liposomes is presented here. An analytical model for the PEG-modified liposomes is derived, where the liposomes are described as a water core surrounded by a bilayer lipid film with grafted polymer chains in a Gaussian random coil conformation attached to the inside and the outside lipid leaflets. There is an excellent agreement between the obtained experimental data and the proposed structural model of the liposomes. These results are the most direct proof of the structure of the PEG-modified liposomes presented so far, and the described formalism may easily be generalized to more complex liposome structures such as synaptic vesicles.

Streptavidin-biotin binding in the presence of a polymer spacer. A theoretical description

The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tethered to a planar surface is studied using molecular theory. The theoretical model is applied to mimic experiments (Langmuir 2008, 24, 2472) performed using drop-shape analysis to study receptor-ligand binding at the oil/water interface. Our theoretical predictions show very good agreements with the experimental results. Furthermore, the theory enables us to study the thermodynamic and structural behavior of the PEO-biotin + streptavidin layer. The interfacial structure, shown by the volume fraction profiles of bound proteins and polymers, indicates that the proteins form a thick layer supported by stretched polymers, where the thickness of the layer is greater than the height of the protein. When the polymer spacer is composed of PEO (3000), a thick layer with multilayers of proteins is formed, supported by the stretched polymer chains. It was found that thick multilayers of proteins are formed when long spacers are present or at very high protein surface coverages on short spacers. This shows that the flexibility of the polymer spacer plays an important role in determining the structure of the bound proteins due to their ability to accommodate highly distorted conformations to optimize binding and protein interactions. Protein domains are predicted when the amount of bound proteins is small due to the existence of streptavidin-streptavidin attractive interactions. As the number of proteins is increased, the competition between attractive interactions and steric repulsions determines the stability and structure of the bound layer. The theory predicts that the competition between these two forces leads to a phase separation at higher protein concentrations. The point where this transition happens depends on both spacer length and protein surface coverage and is an important consideration for practical applications of these and other similar systems. If the goal is to maximize protein binding, it is favorable to be above the layer transition, as multiple layers can accommodate greater bound protein densities. On the other hand, if the goal is to use these bound proteins as a linker group to build more complex structures, such as when avidin or streptavidin serves as a linker between two biotinylated polymers or proteins, the optimum is to be below the layer transition such that all bound linker proteins are available for further binding.

Hierarchical micellar structures from amphiphilic invertible polyesters: 1H NMR spectroscopic study

The environment-dependent behavior of invertible polyesters has been studied by 1H NMR spectroscopy. In dilute toluene solutions, the micelle exterior is made up of the lipophilic fragments, and the interior consists of the hydrophilic constituents. The polyester inverts the structure in an aqueous medium to form micelles with a hydrophobic inner part and a hydrophilic outer part. Increasing polyester concentration leads to the formation of hierarchical structures both in toluene and in an aqueous medium as a result of the aggregation of unimolecular micelles and the formation of hydrophilic and lipophilic domains. On the contrary, no unimolecular micelles or micellar aggregation has been observed in acetone or chloroform.

Structure and viscoelasticity of mixed micelles formed by poly(ethylene oxide) end capped with alkyl groups of different length

Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (Tc) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.

The role of hydrogen bonding in tethered polymer layers

A molecular theory to study the properties of end-tethered polymer layers, in which the polymers have the ability to form hydrogen bonds with water, is presented. The approach combines the ideas of the single-chain mean-field theory to treat tethered layers with the approach of Dormidontova (Macromolecules, 2002, 35, 987.) to include hydrogen bonds. The generalization includes the consideration of position-dependent polymer-water and water-water hydrogen bonds. The theory is applied to model poly(ethylene oxide) (PEO), and the predictions are compared with equivalent polymer layers that do not form hydrogen bonds. It is found that increasing the temperature lowers the solubility of the PEO and results in a collapse of the layer at high enough temperatures. The properties of the layer and their temperature dependence are shown to be the result of the coupling between the conformational entropy of the chains, the ability of the polymer to form hydrogen bonds, and the intermolecular interactions. The structural and thermodynamic properties of the PEO layers, such as the lateral pressure-area isotherms and polymer chemical potentials, are studied as a function of temperature and type of tethering surface. The possibility of phase separation of the PEO layer at high enough temperature is predicted due to the reduced solubility induced by breaking of polymer-water hydrogen bonds. A discussion of the advantages and limitations of the theory, together with how to apply the approach to different hydrogen-bonding polymers, is presented.