Wolfgang Eimer

Influence of charge density on the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels

Abstract The volume phase transition of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels depends in a complex way on the effective charge density within the polymer network. A series of monodisperse PNIPAM/AAc microgels with different content of acrylic acid were synthesized by surfactant-free emulsion polymerization employing sonication instead of a conventional stirring technique. Subsequently, the colloids were characterized by dynamic light scattering and electron microscopy. Potentiometric titrations provided the amount of carboxyl groups incorporated into the copolymer. The effective charge density was systematically controlled by the content of acrylic acid monomers, the pH value of the suspension, and the salt concentration. The hydrodynamic dimensions of the microgels have been measured by dynamic light scattering. The swelling/deswelling behavior is determined by the delicate balance between hydrophobic attraction of NIPAM and the repulsive electrostatic interactions of the carboxylate group of the acrylic acid moieties. Compared to their macroscopic counterparts the charged microgel particles show a significantly different swelling/deswelling behavior. This manifests in the occurrence of a two-step volume phase-transition process with increasing acrylic acid content. Hydrogen bonding has to be considered to understand this two step volume phase transition uniquely observed for colloidal microgels. Another interesting phenomenon presented here is the reversible formation of well-defined aggregates at low pH and under high salt conditions.

Structural changes in PNIPAM microgel particles as seen by SANS, DLS, and EM techniques

Abstract Thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels with different cross-linker density have been investigated by complementary scattering methods and high-resolution imaging techniques. The swelling behavior of the colloidal particles as probed by dynamic light scattering (DLS), is discussed together with information on local structure and dynamics of the particles as obtained from small angle neutron scattering (SANS). Below the volume phase transition the static structure factor reveals liquid-like contributions. This is similar to macroscopic gels, but for low q the scattering behavior is different because of the influence of the particle form factor P(q). Beyond the phase transition temperature, Tc, the system becomes completely phase separated and interfacial scattering dominates the measured SANS profiles (I( q )∝ q −4 ). Slight deviations from the ideal Porod behavior can be interpreted in terms of surface roughness of the collapsed particles. This corresponds to structures visible on AFM micrographs.

The Ca(2+)-induced conformational change of gelsolin is located in the carboxyl-terminal half of the molecule.

We have purified the two functionally distinct domains of gelsolin, a Ca(2+)-dependent actin binding protein, by proteolytic cleavage and characterized their size and shape in solution by dynamic light scattering. In the absence of calcium we obtained the same translational diffusion coefficient for both fragments which are of approximately equal molecular mass. The frictional ratio fo/fexp (1.33-1.39) is similar to the value as obtained for intact gelsolin (1.37) in aqueous solution (Patkowski, A., J. Seils, H. Hinssen, and T. Dorfmüller. 1990. Biopolymers. 30:427-435), indicating a similar molecular shape for the native protein as well as for the two subdomains. Upon addition of Ca2+ the translational diffusion coefficient of the carboxyl-terminal half decreased by almost 10%, while there was no change observed for the amino terminus. This result indicates that the ligand-induced conformational change as seen for intact gelsolin is probably located on the carboxyl-terminal domain of the protein. Since gelsolin has binding sites in both domains, and the isolated amino terminus binds and severs actin in a calcium-independent manner, our results suggests that the structural transition in the carboxyl-terminal part of intact gelsolin also affects the actin binding properties of the amino-terminal half.

Internal dynamics in colloidal PNIPAM microgel particles immobilised in mesoscopic crystals

Abstract Internal dynamics of colloidal microgels based on N -isopropyl-acrylamide is investigated by means of neutron spin-echo spectroscopy. To avoid interference of the relaxation of interest with the centre of mass diffusion, the microgels were immobilised in a colloidal crystal. Due to the q -range used in these quasi-elastic neutron scattering experiments, it is possible to obtain nearly completely decaying ensemble averaged intermediate scattering functions 〈ISF〉 E . These can be fitted by single exponential forms, simplifying largely the data analysis compared with photon correlation spectroscopy. This is due to the fact that for light q 2 〈 ξ 2 〉→∞ can not be reached, whereas for neutrons this condition is approximately fulfilled. The obtained network diffusion coefficients decrease linearly with increasing cross-linker concentration. This is in contrast to previous results from photon correlation spectroscopy obtained for macroscopic poly( N -isopropyl-acrylamide) gels [Macromolecules, 29 (1996) 8746].

The micro-structures formed by Ni2+-AOT/cyclohexane/water microemulsions: a light scattering study

Abstract In this study we present dynamic light scattering (DLS) measurements on the system cyclohexane/Ni(AOT)2 (AOT=bis-2-ethylhexylsulfosuccinate)/water. It is shown that the reverse micelles formed in the L2-phase exhibit the scattering behavior predicted for stiff elongated scatterers. The rotational and the translational diffusion coefficient can be obtained at the same time. The transport properties are used to derive a structural model for the reverse micelles formed in the system. The determination of the radius of gyration by dissymmetry measurements of the static scattering intensity is also presented.

Hydrodynamic properties of nitrogenase - The MoFe protein from Azotobacter vinelandii studied by dynamic light scattering and hydrodynamic modelling

Experimental results for the nitrogenase MoFe protein from Azotobacter vinelandii obtained by dynamic light scattering (DLS) are presented. The translational diffusion coefficient was determined to D = (4.0 +/- 0.2) x 10(-7) cm2/s. Complementary, we have performed hydrodynamic model calculations based on the X-ray crystallographic data of the MoFe protein. The calculated transport coefficient suggests that the size and shape of the protein in solution is consistent with that in the crystal structure.

Dynamic light scattering in sooting premixed atmospheric-pressure methane-, propane-, ethene-, and propene-oxygen flames

In a systematic investigation under well-defined flame conditions, dynamic light scattering (DLS) was applied to the determination of soot particle radii with the aim of examining the suitability of this technique for accurate soot particle sizing. In particular, flat premixed methane-, propane-, ethene-, and propene-oxygen flames at atmospheric pressure were investigated, and particle sizes were obtained as a function of stoichiometry and height above the burner surface. In combination with absorption measurements, soot volume fraction and particle number density were determined; also, the temperature was measured at each flame condition. In comparison to absorption techniques, attractive features of DLS are its independence of the particle refractive index and its insensitivity to fluorescence interference; also, it offers spatial resolution. In principle, additional information on the particle size distribution as well as on the global shape of the particles may be obtained from DLS experiments. This study is therefore an evaluation of the potential of DLS as a complement to other soot diagnostic techniques.

Interaction of a G-DNA quadruplex with mono- and divalent cations. A force field calculation.

The formation and stability of four-stranded DNA in solution is specifically dependent on the type of cations present. The interaction potential of a model quadruplex structure with different mono- and divalent ions was determined by force field calculations. Though the electrostatic contribution to the total energy is mainly responsible for the stabilisation of the cations within the quadruplex channel, it is the van der Waals interaction at short distances that determines the specific characteristics of the different cations. An explicit consideration of the solvent indicates that the position of water molecules in close proximity to the DNA channel have a strong influence on the form of the potential, and hence on the capability of the cations for leaving and re-entering the cavity. The effect of cation size, as expressed through their Lennard-Jones parameters, is discussed.

The Size and Shape of Caldesmon and Its Fragments in Solution Studied by Dynamic Light Scattering and Hydrodynamic Model Calculations

The size and the shape of caldesmon as well as its 50-kDa central and 19-kDa C-terminal fragments were investigated by photon correlation spectroscopy. The hydrodynamic radii, which have been calculated from the experimentally obtained translational diffusion coefficients, are 9.8 nm, 6.0 nm, and 2.9 nm, respectively. Moreover, the experimental values for the translational diffusion coefficients are compared with results obtained from hydrodynamic model calculations. Detailed models for the structure of caldesmon in solution are derived. The contour length is about 64 nm for all of the models used for caldesmon.

Neutron Spin Echo Study of the Dynamics in BIS Cross-Linked Poly(N-Isopropyl Acrylamide) Microgels: Dependence on the Cross-Linker Concentration

Neutron spin echo spectroscopy (NSE) is an excellent tool to access the internal dynamics in gels and microgels. NSE directly provides the ensemble averaged intermediate scattering function 〈ISF〉E and due to the Q-range quasi-ergodic scattering behaviour is recovered. In the present study the cross-linker dependence of the internal dynamics of the gel network inside poly(N-isopropyl acrylamide) microgel particles was investigated by means of neutron spin-echo spectroscopy. The measured intermediate scattering functions are well described by single exponential decays, which is in agreement with the model derived by Tanaka et al. [1]. The computed collective diffusion coefficients of the gel network decrease linearly with the number of cross-links introduced in the network for low cross-linker concentrations. This is in contrast with dynamic light scattering (DLS) results obtained for macroscopic gels based on the same polymer by Shibayama et al.[2].