Rodlike Complexes of a Polyelectrolyte (Hyaluronan) and a Protein (Lysozyme) observed by SANS

We study by Small Angle Neutron Scattering (SANS) the structure of Hyaluronan -Lysozyme complexes. Hyaluronan (HA) is a polysaccharide of 9 nm intrinsic persistence length that bears one negative charge per disaccharide monomer (Mmol = 401.3 g/mol); two molecular weights, Mw = 6000 and 500 000 Da were used. The pH was adjusted at 4.7 and 7.4 so that lysozyme has a global charge of +10 and + 8 respectively. The lysozyme concentration was varied from 3 to 40 g/L, at constant HA concentration (10 g/L). At low protein concentration, samples are monophasic and SANS experiments reveal only fluctuations of concentration although, at high protein concentration, clusters are observed by SANS in the dense phase of the diphasic samples. In between, close to the onset of the phase separation, a distinct original scattering is observed. It is characteristic of a rod-like shape, which could characterize "single" complexes involving one or a few polymer chains. For the large molecular weight (500 000) the rodlike rigid domains extend to much larger length scale than the persistence length of the HA chain alone in solution and the range of the SANS investigation. They can be described as a necklace of proteins attached along a backbone of diameter one or a few HA chains. For the short chains (Mw ~ 6000), the rod length of the complexes is close to the chain contour length (~ 15 nm)

Structure Transition in PSS/Lysozyme Complexes: A Chain-Conformation-Driven Process, as Directly Seen by Small Angle Neutron Scattering

Measurements of chain conformation in proteins/polyelectrolytes complexes (lysozyme and PSSNa) show that the crossover observed between an open structure -a chain network crosslinked by the proteins, and a globular one - dense globules of ~ 10 nm aggregated in a fractal way, results from a conformation modification prior to the transition. Before showing this, we have widened the parameters range for the observation of the transition. We had shown before that the two structures can be formed depending on chain length (for a given [PSS]/[lysozyme] ratio): gel for large chains, globules for short chains. We show here that the crossover between these two regimes can also be reached as a function of chains concentration or salinity of the buffer. Since all these crossover parameters act on chains overlapping concentration c*, we reinforce the idea of a transition from the dilute to the semi-dilute regime, but c* is shifted compared to pure PSS solutions. In order to understand this, we have measured by SANS the conformation of a single chain of PSS in presence of proteins within the complexes. This is achieved by a specific labeling trick where we take advantage of the fact that lysozyme and hydrogenated PSS chains have the same neutron scattering length density. In the gel structure, the PSS chains keep a wormlike structure as in pure solutions, but their persistence length is strongly reduced, from 50 {\AA} without proteins to 20 {\AA} in average with lysozyme. With this value of 20 {\AA}, we calculate new overlapping thresholds (concentration, mass, ionic strength) in agreement with observed ones. In a second stage, after the globular structure is formed, the PSS chains get a third conformation, no longer wormlike, but more collapsed, within the globules

Spatial structure and composition of polysaccharide-protein complexes from Small Angle Neutron Scattering

We use Small Angle Neutron Scattering (SANS), with an original analysis method, to obtain both the characteristic sizes and the inner composition of lysozyme-pectin complexes depending on the charge density. Lysozyme is a globular protein and pectin a natural anionic semiflexible polysaccharide with a degree of methylation (DM) 0, 43 and 74. For our experimental conditions (buffer ionic strength I = 2.5 10-2 mol/L and pH between 3 and 7), the electrostatic charge of lysozyme is always positive (from 8 to 17 depending on pH). The pectin charge per elementary chain segment is negative and can be varied from almost zero to one through the change of DM and pH. The weight molar ratio of lysozyme on pectin monomers is kept constant. The ratio of negative charge content per volume to positive charge content per volume, -/+, is varied between 10 and 0.007. On a local scale, for all charged pectins, a correlation peak appears at 0.2 {\AA}-1 due to proteins clustering inside the complexes. On a large scale, the complexes appear as formed of spherical globules with a well defined radius of 10 to 50 nm, containing a few thousands proteins. The volume fraction Phi of organic matter within the globules derived from SANS absolute cross-sections is around 0.1. The protein stacking, which occurs inside the globules, is enhanced when pectin is more charged, due to pH or DM. The linear charge density of the pectin determines the size of the globules for pectin chains of comparable molecular weights whether it is controlled by the pH or the DM. The radius of the globules varies between 10 nm and 50 nm. In conclusion the structure is driven by electrostatic interactions and not by hydrophobic interactions. The molecular weight also has a large influence on the structure of the complexes since long chains tend to form larger globules. This maybe one reason why DM and pH are not completely equivalent in our system since DM 0 has a short mass, but this may not be the only one. For very low pectin charge (-/+ = 0.07), globules do not appear and the scattering signals a gel-like structure. We did not observe any beads-on-a-string structure

Adsorption of MultiLamellar tubes with a temperature tunable diameter at the air-water interface: a process driven by the bulk properties

The behavior at the air/water interface of multilamellar tubes made of the ethanolamine salt of the 12-hydroxy stearic acid as a function of the temperature has been investigated using Neutron Reflectivity. Those tubes are known to exhibit a temperature tunable diameter in the bulk. We have observed multilamellar tubes adsorbed at the air/water interface by specular neutron reflectivity. Interestingly, at the interface, the adsorbed tubes exhibit the same behavior than in the bulk upon heating. There is however a peculiar behavior at around 50\degree for which the increase of the diameter of the tubes at the interface yields an unfolding of those tubes into a multilamellar layer. Upon further heating, the tubes re-fold and their diameter re-decrease after what they melt as observed in the bulk. All structural transitions at the interface are nevertheless shown to be quasi-completely reversible. This provides to the system a high interest for its interfacial properties because the structure at the air/water interface can be tuned easily by the temperature

Une brève introduction à la matière molle

A short introduction to soft condensed matter (polymers, colloids, surfactants) is presented, with particular emphasis to recent progres and applications of small angle scattering. The text is in French

Surface oxide on thin films of yttrium hydride studied by neutron reflectometry

The applicability of standard methods for compositional analysis is limited for H-containing films. Neutron reflectometry is a powerful, non-destructive method that is especially suitable for these systems due to the large negative scattering length of H. In this work we demonstrate how neutron reflectometry can be used to investigate thin films of yttrium hydride. Neutron reflectometry gives a strong contrast between the film and the surface oxide layer, enabling us to estimate the oxide thickness and oxygen penetration depths. A surface oxide layer of 5-10 nm thickness was found for unprotected yttrium hydride films

A Novel Method for Studying the Dynamics of Confined Polymers in Nanoparticles in Nanoblends

The advances in new technologies have prompted the need for functional systems smaller than the gyration radius of polymer chains. Thus, understanding how nanoconfinement affects polymer properties has been the focus of a lot of research for over a decade. Polystyrene in particular has been reported to be strongly affected when nanoconfined as a thin film and specifically its glass transition temperature (Tg) is reported to decrease with decreasing film thickness. Tremendous effort has been dedicated to developing methods for quantifying the large-scale dynamic of nanoconfined polymers: film dewetting, film contraction, nanobubble inflation, nanoparticle imbedding and healing of deformed surfaces etc. In this work we describe a novel method to study the large scale dynamic and nanomechanical properties of nanoconfined polymers in nanoparticles in nanoblends. Nanoblends of dPS/PBMA were prepared from a mixture of colloidal suspensions of cross-linked PBMA and traces of dPS nanoparticles via water evaporation. The polymer blends were prepared at temperatures well below the glass transition of PS (TgPS) and above the Tg of cross-linked PBMA particles (TgPBMA). In these conditions we expect the PBMA particles to deform under capillary pressure to fill the interstices between them and the glassy PS nanoparticles to remain spherical. During the preparation of the nanoblends the elastic energy is stored within the deformed cross-linked PBMA nanoparticles. Upon annealing the films above TgPS, the PBMA nanoparticles regain their spherical shape and release the stored elastic energy, which induces the deformation of the PS nanoparticles. Small angle neutron scattering is then used to monitor the shape evolution of the PS nanoparticles and to quantify the relaxation dynamics of the polystyrene nanoparticles

Structure investigation of nanohybrid PDMA/silica hydrogels at rest and under uniaxial deformation

International audienceNano-hybrid hydrogels were prepared by cross-linking polymerization of N,N-dimethylacrylamide (DMA) within a dispersion of silica nano-particles. Working at constant polymer/water ratio, the mechanical properties of hydrogels can be finely tuned by changing either the level of covalent cross-linker and/or the amount of particles that act as physical cross-linkers through specific adsorption of PDMA chains. Whatever is the cross-linking ratio (from 0 to 1 mol%), the introduction of silica nano-particles dramatically improves the mechanical behavior of hydrogels with a concomitant increase of stiffness and nominal strain at failure. The physical interactions being reversible in nature, the dynamics of the adsorption/desorption process of PDMA chains directly controls the time-dependence of the mechanical properties. Small angle neutron scattering experiments, performed in contrast matching conditions, show that silica particles, which repel themselves at short range, remain randomly dispersed during the formation of the PDMA network. Although PDMA chains readily interact with silica particles, no significant variation of the polymer concentration was observed in the vicinity of silica surfaces. Together with the time dependence of physical interactions pointed out by mechanical analyses, this result is attributed to the moderate adsorption energy of PDMA chains with silica surfaces at pH 9. From 2D SANS experiments, it was shown that strain rapidly gives rise to a non affine deformation of the hybrid network with shearing due to the transverse compression of the particles. After loading at intermediate deformation, the particles recover their initial distribution due to the covalent network that is not damaged in these conditions. That is no longer true at high deformation where residual anisotropy is observed

Small-Angle Neutron Scattering Reveals the Structural Details of Thermosensitive Polymer-Grafted Cellulose Nanocrystal Suspensions.

Thanks to the use of small-angle neutron scattering (SANS), a detailed structural description of thermosensitive polymer-grafted cellulose nanocrystals (CNCs) was obtained and the behavior of aqueous suspensions of these derivatized biosourced particles upon temperature increase was revealed. Although literature data show that the surface grafting of thermosensitive polymers drastically enhances the colloidal properties of CNCs, direct space microscopic investigation techniques fail in providing sufficient structural information on these objects. In the case of CNCs decorated with temperature-sensitive polyetheramines following a peptide coupling reaction, a qualitative and quantitative analysis of SANS spectra shows that CNCs are homogeneously covered by a shell comprising polymer chains in a Gaussian conformation with a thickness equal to their radius of gyration in solution, thus revealing a mushroom regime. An increase of the temperature above the lower critical solution temperature (LCST) of the polyetheramine results in the formation of finite size bundles whose aggregation number depends on the particle concentration and suspension temperature deviation from the LCST. SANS analysis further reveals local changes at the CNC surface corresponding to a release of water molecules and a related denser polymer shell conformation. Noticeably, data show a full reversibility at all length scales when a sample was cooled down to below the LCST after being heated above it. Overall, the results obtained by SANS allow an in-depth structural investigation of derivatized CNCs, which is of high interest for the design of functional materials comprising these biosourced colloids.Institut Carnot PolyNat (ANR N° 16-CARN-025-01), France