Aristeidis Papagiannopoulos

The viscoelasticity of self-assembled proteoglycan combs.

Particle tracking microrheology with a fast digital camera allowed the slow and intermediate time regimes (10(-4)-10(1) s) of the linear viscoelasticity of giant aggrecan proteoglycans to be mapped. Combined with diffusing wave spectroscopy experiments this enabled us to probe the linear viscoelasticity of aggrecan over seven orders of magnitude in time (10(-6)-10(1) s) [Palmer et al., Biophys. J., 1999, 76, 1063; Papagiannopoulos et al., Biomacromolecules, 2007, 7, 2162]. When the comb side-groups self-assemble on the hyaluronic acid backbones they cause a dramatic increase in the relaxation time of the solutions and consequently the viscosity of the sample, but leave the elasticity of the solutions relatively unchanged. The experiments illustrate the modular nature of aggrecans viscosity and clearly demonstrate the role of this molecule in vivo in cartilaginous composites, where it dissipates energy. Both one- and two-particle tracking microrheology were used to investigate the length-scale dependent viscoelasticity of the comb superstructures [Lui et al., Phys. Rev. Lett., 2006, 96, 118104] and the errors inherent in the two techniques were quantified [Savin and Doyle, Biophys. J., 2005, 88, 623; Waigh, Rep. Prog. Phys., 2005, 68, 685]. The behaviour of the viscoelasticity is compared with the predictions of dynamic scaling theory, indicating a significant contribution of the side-chain dynamics to the reptative motion of both the aggrecan aggregate and the monomers. The results have important implications for a molecular understanding of tissue function and pathology in osteoarthritis.

Self-Assembled Nanoparticles from a Block Polyelectrolyte in Aqueous Media: Structural Characterization by SANS

We present a small angle neutron scattering (SANS) study of polystyrene-b-sodium (sulfamate/carboxylate) isoprene (PS-PSCI) nanoparticles in aqueous media. The SANS experiments are complemented by static and dynamic light scattering measurements. A detailed analysis of the scattering form factor obtained by SANS for the self-assembled block polyelectrolyte spherical nanoparticles implies a two-region power-law model for the radial volume fraction profiles. The theoretically predicted scaling of the osmotic brush regime phi(r) approximately r(-2) for the inner region and the osmotic annealing brush regime phi(r) approximately r(-8/3) for the outer region are in agreement with our experimental findings. A concentrated shell of PSCI polyelectrolyte chains collapsed on the polystyrene core is needed in the form factor analysis so that the aggregation number of the nanoparticles is self-consistent. The self-assembled nanoparticles are found to be kinetically frozen i.e. their aggregation number is not sensitive to the solution conditions and is defined by the preparation protocol. The size of the spherical nanoparticles tends to decrease upon the addition of salt and the drop of pH.

Optical coherence tomography picorheology of biopolymer solutions

We describe an interferometric optical fiber based dynamic light scattering technique to study the linear viscoelasticity of complex fluids. This digital correlator based method enables viscoelasticity to be probed at high frequencies (10−2–105Hz) over picoliter regions of an aqueous sample. We demonstrate the technique using solutions of the self-assembled biological polymers aggrecan and fibrin. The technique is demonstrated in an optical coherence tomography type mode of action with picorheology measurements at 40μm transverse sections (490pl volumes) over a 1cm sample length. The method allowed the viscoelasticity of aggrecan to be probed and provided a direct measurement of the persistence length of the fibrin fibers (Lp=32±2cm).

Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2

Using a double-disc chopper with a variable slit opening in concert with a velocity selector and the time-of-flight data acquisition mode, controlled variation of the wavelength spread Δλ/λ between 2 and 20% has become routinely possible at the KWS-2 SANS diffractometer of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center.

Complexation of lysozyme with adsorbed PtBS-b-SCPI block polyelectrolyte micelles on silver surface.

We present a study of the interaction of the positively charged model protein lysozyme with the negatively charged amphiphilic diblock polyelectrolyte micelles of poly(tert-butylstyrene-b-sodium (sulfamate/carboxylate)isoprene) (PtBS-b-SCPI) on the silver/water interface. The adsorption kinetics are monitored by surface plasmon resonance, and the surface morphology is probed by atomic force microscopy. The micellar adsorption is described by stretched-exponential kinetics, and the micellar layer morphology shows that the micelles do not lose their integrity upon adsorption. The complexation of lysozyme with the adsorbed micellar layers depends on the micelles arrangement and density in the underlying layer, and lysozyme follows the local morphology of the underlying roughness. When the micellar adsorbed amount is small, the layers show low capacity in protein complexation and low resistance in loading. When the micellar adsorbed amount is high, the situation is reversed. The adsorbed layers both with or without added protein are found to be irreversibly adsorbed on the Ag surface.

Self-assembly of poly(ethylene glycol-b-phenyl oxazoline) diblock copolymers in aqueous media and their interactions with proteins

AbstractAmphiphilic block copolymers composed of poly(ethylene glycol) (PEG) and poly(2-phenyl-2-oxazoline) (PPhOx) with different molecular weights and compositions are synthesized. Their molecular characteristics are determined by size exclusion chromatography (SEC), proton nuclear magnetic resonance (1H NMR), and Fourier transform infrared spectroscopy (FTIR). In aqueous solutions, the size of the formed aggregates is much higher than the one expected from single core-shell micelles and point to larger spherical aggregates as observed by light scattering. The aggregation state is found temperature-sensitive. The internal environment of these aggregates is considerably polar as evidenced by pyrene fluorescence measurements. The interactions of the synthesized diblock copolymers with fetal bovine serum (FBS) proteins are investigated, and further aggregation of the initial structures is observed in the presence of serum proteins. The ability of the PEG-b-PPhOx aggregates to associate with proteins is tested for the case of bovine serum albumin (BSA) where it is found that indeed accumulation of BSA globules occur because of hydrophobic interactions between the protein and the hydrophobic domains of the polymeric aggregates. Graphical abstractEleni Vlassi, Aristeidis Papagiannopoulos, Stergios Pispas.Self-assembly of poly(ethylene glycol-b-phenyl oxazoline) diblock copolymers in aqueous media and their interactions with proteins.The molecular structure of amphiphilic PEG-b-PhOx block copolymers results to formation of aggregates in aqueous solutions as shown by the SEM image, which show interactions with serum proteins.

Scattering investigation of multiscale organization in aqueous solutions of native xanthan

The hierarchical morphology of xanthan solutions is analyzed by light and neutron scattering in a broad range of concentrations in order to connect their morphology to their well-documented dynamic properties. Static light scattering inside the semidilute regime is dominated by the form factor of individual xanthan chains while at higher concentrations chain interconnections appear to modify the low wave vector scattering. Dynamic light scattering reveals the self-similar nature of the solutions caused by interchain associations as intensity autocorrelation functions present power-law behaviour. Small angle neutron scattering is dominated by the fractal scattering from the formed network at intermediate length scales. At small length scales the rigid structure of xanthan is revealed and the molecular weight per unit length is extracted. No detectable morphological alterations for shear rates up to 1000rad/s are observed revealing that the shear thinning behaviour of xanthan is related to the disruption of chain-chain associations.

The high-intensity option of the SANS diffractometer KWS-2 at JCNS – characterization and performance of the new multi-megahertz detection system

A new detection system based on an array of 3He tubes and innovative fast detection electronics has been installed on the small-angle neutron scattering (SANS) diffractometer KWS-2 at the Jülich Centre for Neutron Science (JCNS), Germany. The high counting rates that can be detected and the event-mode capability will enable new scientific opportunities in the field of structural investigation of small soft-matter and biological systems.

Modification of xanthan solution properties by the cationic surfactant DTMAB

The interactions between xanthan gum (XG) and dodecyltrimethylammonium bromide (DTMAB) are studied by static and dynamic light scattering, microrheology and viscometry within the overlapping regime of aqueous xanthan solutions. In the absence of salt mixing with surfactant transforms the complex fluid of interconnected xanthan chains into diffusing aggregates because of the reduction of the polysaccharides effective charge and the introduction of hydrophobic units. In the presence of salt the effect of complexation is weaker. Mixing the two components while XG is in its denatured state results to a network with reduced interchain connections. Linear (microrheological) and nonlinear (steady shear) viscoelastic experiments show that there is a significant compromise of the viscoelastic moduli which is remarkably more intense when mixing DTMAB with XG in its denatured state. These findings prove that xanthans viscoelastic and morphological properties may be tuned by addition of surfactants and that the conformational state of XG during complexation is a defining parameter.

Formation of complexes in aqueous solutions of amphiphilic triblock polyelectrolytes of different topologies and an oppositely charged protein

The complexation of lysozyme with aggregates from two triblock amphiphilic polyelectrolytes of the same blocks but different topologies and block molar masses, namely PS-b-SCPI-b-PEO and SCPI-b-PS-b-PEO, is investigated by scattering and spectroscopy methods. Light scattering reveals that the interaction with lysozyme causes shrinkage of the self-assembled nanoparticles in the case of the hydrophobic-polyelectrolyte-hydrophilic sequence. In the polyelectrolyte-hydrophobic-hydrophilic sequence, the opposite trend is observed. Small angle neutron scattering confirms the existence of micellar and fractal aggregates and the complexation with lysozyme. The pH-dependence of the interactions and the stability of the hybrid protein/polymer nanoparticles upon salt addition are tested. The native conformation of the protein is found to be preserved during complexation. This study reveals that both micellar and fractal aggregates made of amphiphilic triblock polyelectrolytes are capable of loading with oppositely charged proteins in a controllable manner, tuned primarily by the structure of the triblock terpolymer.