Small Angle Scattering techniques are regularly used to probe the structure of proteins and proteins assemblies in a solvent. X-rays enable fast and localized measurements thanks to the high flux and the small beam size, but there is a risk of protein degradation that is prevented with neutron scattering. In order to increase the contrast between proteins and the solvent, deuterated solvents are usually used for SANS measurements. Normally, if the contrast is due to scattering density differences between proteins and the solvent, SAXS and SANS profiles are identical. In this talk, we will present the study of gluten protein gels that display radically different SAXS and SANS profiles when the solvent is deuterated. The shape of the high q signal is identical for both measurements, whereas totally different features are measured at low q (figure). The SAXS profile can be successfully described with a model combining a Lorentzian and a q-2 power law [1]. This model was previously used to describe polymeric gels under good solvent conditions and relates the random coil Gaussian chain conformation of proteins and large-scale concentration heterogeneities. For the range of q where a q-2 power law regime is measured for SAXS, SANS profile displays instead a scattered intensity that scales as q-4 and a shoulder at even smaller q from which a characteristic size can be extracted. Measurements with various levels of solvent deuteration were performed to understand these surprising scattering profiles. Quantitative analysis of the contrast with deuteration indicates a progressive H-D exchange of protein chains. Moreover, an increase of the low q q-4 contribution is observed with solvent deuteration. Results were rationalized considering a heterogeneous H-D exchange of protein protons that would be due to local high concentration of proteins where hydrogen would be involved in hydrogen bonds, preventing thus H-D exchange. In particular, the q-4 behavior would originate from the sharp interfaces between protein rich regions which are more hydrogenated and protein diluted regions which are more deuterated. The cross-over between the q-4 and the quasi-plateau at smaller q correspond to a distance of 60 nm, similar to the size of the protein assemblies measured in the dilute regime, reflecting the characteristic size over which H-D exchange can take place.

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Producing wheat gluten gels with tunable mechanical properties via simple sol-gel routes would facilitate their processing into plant protein-rich food products. However, standard gluten is a very elastic mass with a high concentration of proteins in water and is hardly processable except using high shear harsh extrusions. The wheat proteins are responsible for the viscoelastic properties of standard gluten and dough and are among the most complex proteins families, with extremely broad polymorphisms and polydispersities. They are moreover insoluble in water, rendering rational studies difficult. Thanks to a novel protocol for the gluten proteins extraction that we have recently developed, stable ethanolic suspensions of gluten proteins are obtained for a wide range of protein concentrations. In this talk, we will present the viscoelasticity of those suspensions and show that they exhibit a spontaneous and concentration-dependent gelation, which we find to be driven by the slow formation of hydrogen bonds. We successfully rationalize our data using percolation models and relate the viscoelasticity of the gels to their fractal dimension measured by scattering techniques. The novel gluten gels display self-healing properties and their elastic plateaus cover several decades, from 10-2 to 104 Pa. In particular very soft gels as compared to standard hydrated gluten, suitable for processing, can be produced.

he supramolecular organization of wheat gluten proteins is largely unknown due to the intrinsic complexity of this family of proteins and their insolubility in water. We fractionate gluten in a water/ethanol (50/50 v/v) and obtain a protein extract which is depleted in gliadin, the monomeric part of wheat gluten proteins, and enriched in glutenin, the polymeric part of wheat gluten proteins. We investigate the structure of the proteins in the solvent used for extraction over a wide range of concentration, by combining X-ray scattering and multi-angle static and dynamic light scattering. Our data show that, in the ethanol/water mixture, the proteins display features characteristic of flexible polymer chains in a good solvent. In the dilute regime, the protein form very loose structures of characteristic size 150 nm, with an internal dynamics which is quantitatively similar to that of branched polymer coils. In more concentrated regimes, data highlight a hierarchical structure with one characteristic length scale of the order of a few nm, which displays the scaling with concentration expected for a semi-dilute polymer in good solvent, and a fractal arrangement at much larger length scale. This structure is strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten proteins and their assemblies.

Liquid sheets are formed by the collision of a liquid drop on a small solid target. Upon impact, the drop flattens into a radial sheet expanding in the air bounded by a thicker rim. A pure water sheet spreads out radially until it reaches a maximum diameter and then retracts due to the effect of surface tension. The destabilization mechanism is drastically modified when a dilute oil in water emulsion is used. The liquid sheet spreads out radially but now holes perforate the sheet before the retraction, as already observed for some surfactant solutions [1]. The holes do not perturb significantly the velocity field of the liquid sheet; they growth with a constant velocity given by the Culicks’s law until they merge together and form a web of ligaments, which are then destabilized into droplets. We use a simple experimental optical method we have developed recently to get time and space resolved measurements of the thickness field of the liquid sheet [2]. We show that each perforation’s event (hole) is preceded by a hole’s precursor (thinning zone of the liquid sheet) whose thickness profile and growth’s velocity have been measured . Interestingly each rupture event (transition from holes’s precursor to true hole) of the sheet is clearly evidenced by a discontinuity of the growth’s velocity of the instability. These experiments are appropriate to gain an understanding on the physical mechanisms governing the perforation of thick films of emulsion.