Reversible and irreversible states of pressure-dissociated casein micelles were studied by in situ light scattering techniques and ex situ atomic force microscopy. AFM experiments performed at ambient pressure reveal heterogeneities across the micelle, suggesting a sub-structure on a 20 nm scale. At pressures between 50 and 250 MPa, the native micelles disintegrate into small fragments on the scale of the observed sub-structure. At pressures above 300 MPa the micelles fully decompose into their monomeric constituents. After pressure release two discrete populations of casein aggregates are observed, depending on the applied initial pressure: Between 160 and 240 MPa stable micelles with diameters near 100 nm without detectable sub-structures are formed. Casein micelles exposed to pressures above 280 MPa re-associate at ambient pressure yielding mini-micelles with diameters near 25 nm. The implications concerning structural models are discussed.

The structure of thin casein films prepared with spin-coating is investigated as a function of the calcium concentration. Grazing incidence small-angle x-ray scattering and atomic force microscopy are used to probe the micelle structure. For comparison, the corresponding casein solutions are investigated with dynamic light-scattering experiments. In the thin films with added calcium three types of casein structures, aggregates, micelles, and mini-micelles, are observed in coexistence with atomic force microscopy and grazing incidence small-angle x-ray scattering. With increasing calcium concentration, the size of the aggregates strongly increases, while the size of micelles slightly decreases and the size of the mini-micelles increases. This effect is explained in the framework of the particle-stabilizing properties of the hairy layer of kappa-casein surrounding the casein micelles.

Casein films were successfully prepared with the spin-coating technique of aqueous casein solutions on base-treated glass surfaces. The film structure is investigated in real space with optical microscopy and atomic force microscopy and for the first time in reciprocal space with grazing incidence small-angle X-ray scattering (GISAXS). The size of the substructures detected in the film increases with pH from 170 nm (pH 5.1) up to 490 nm (pH 9.4). Dynamic light scattering experiments reveal that the average diameters of casein micelles in solution exhibit the same quantitative increase. This result suggests that the substructures detected in the bulklike films with GISAXS reflect intact casein micelles. However, with thin homogeneous casein films, the micelle size diminishes with decreasing film thickness. This indicates that the moderate pressures introduced by spin-coating force the micelles to rearrange into a more compact structure.

No abstract

SummaryCalcium gradients are prepared by piling a micropipette with casein solutions of varying calcium concentration and spreading them on glass slides. The casein film is formed by solution casting process which results in a macroscopically rough surface. We used microbeam grazing incidence small-angle X-ray scattering to investigate the lateral size distribution of three main components in casein films: casein micelles, casein mini-micelles and micellar calcium phosphate. At length scales within the beam size the film surface is flat and detection of size distribution in a macroscopic casein gradient becomes accessible. The model used to analyze the data is based on a set of three log-normal distributed particle sizes.Increasing calcium concentration causes a decrease in casein micelle diameter while the size of casein mini-micelles increases and micellar calcium phosphate particles remain unchanged.3

The formation of thaumatin crystals by Langmuir-Blodgett (LB) film nanotemplates was studied by the hanging-drop technique in a flow-through cell by synchrotron radiation micrograzing-incidence small-angle x-ray scattering. The kinetics of crystallization was measured directly on the interface of the LB film crystallization nanotemplate. The evolution of the micrograzing-incidence small-angle x-ray scattering patterns suggests that the increase in intensity in the Yoneda region is due to protein incorporation into the LB film. The intensity variation suggests several steps, which were modeled by system dynamics based on first-order differential equations. The kinetic data can be described by two processes that take place on the LB film, a first, fast, process, attributed to the crystal growth and its detachment from the LB film, and a second, slower process, attributed to an unordered association and conversion of protein on the LB film.

Tobacco mosaic virus particles can be rapidly assembled into 3D-domains by capillary flow-driven alignment at the triple contact-line of an evaporating droplet. Virus particles of ∼150 Å diameter can be resolved within individual domains at the outer rim of the "coffee-ring" type residue by atomic force microscopy. The crystalline domains can also be probed by X-ray microdiffraction techniques. Both techniques reveal that the rod-like virus particles are oriented parallel to the rim. We further demonstrate the feasibility of collection of hk0 reflection intensities in GISAXS geometry and show it allows calculating a low-resolution electron density projection along the rod axis.

No abstract