Models of Micellar Structure Tested by SANS and SAXS (from a Kratky Camera) in Cesium Dodecyl Sulfate Solution

Multi-Step Unfolding and Rearrangement of α-Lactalbumin by SDS Revealed by Stopped-Flow SAXS

Interactions between proteins and surfactants are both of fundamental interest and relevant for applications in food, cosmetics and detergency. The anionic surfactant sodium dodecyl sulfate (SDS) denatures essentially all proteins. Denaturation typically involves a number of distinct steps where growing numbers of SDS molecules bind to the protein, as seen in multidisciplinary approaches combining several complementary techniques. We adopt this approach to study the SDS-induced unfolding of Ca2+-depleted α-lactalbumin (aLA), a protein particularly sensitive toward denaturation by surfactants. By combining stopped-flow mixing of protein and surfactant solutions with stopped-flow synchrotron small-angle X-ray scattering (SAXS), circular dichroism (CD) and Trp fluorescence, together with information from previous calorimetric studies, we construct a detailed picture of the unfolding process at the level of both protein and surfactant. A protein-surfactant complex is formed within the dead time of mixing (2.5 ms). Initially a cluster of SDS molecules binds asymmetrically, i.e., to one side of the protein, after which aLA redistributes around the SDS cluster. This occurs in two kinetic steps where the complex grows in number of both SDS and protein molecules, concomitant with protein unfolding. During these steps, the core-shell complex undergoes changes in shell thickness as well as core shape and radius. The entire process is very sensitive to SDS concentration and completes within 10 s at an SDS:aLA ratio of 9, decreasing to 0.2 s at 60 SDS:aLA. The number of aLA molecules per SDS complex drops from 1.9 to 1.0 over this range of ratios. While both CD and Trp kinetics reveal a fast and a slow conformational transition, only the slow transition is observed by SAXS, indicating that the protein-SDS complex (which is monitored by SAXS) adjusts to the presence of the unfolded protein. We attribute the rapid unfolding of aLA to its predominantly α-helical structure, which persists in SDS (albeit as isolated helices), enabling aLA to unfold without undergoing major secondary structural changes unlike β-sheet rich proteins. Nevertheless, the overall unfolding steps are broadly similar to those of the more β-rich protein β-lactoglobulin, suggesting that this unfolding model is representative of the general process of SDS-unfolding of proteins.

Intermicellar Interactions and the Viscoelasticity of Surfactant Solutions: Complementary Use of SANS and SAXS

In ionic surfactant micelles, basic interactions among distinct parts of surfactant monomers, their counterion, and additives are fundamental to tuning molecular self-assembly and enhancing viscoelasticity. Here, we investigate the addition of sodium salicylate (NaSal) to hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB) and 1-hexadecylpyridinium chloride and bromide (CPyCl and CPyBr), which have distinct counterions and headgroup structures but the same hydrophobic tail. Different contrasts are obtained from small-angle neutron scattering (SANS), which probes differences between the nucleus of atoms, and X-rays SAXS, which probes differences in electron density. If combined, this contrast allows us to define specific intramicellar length scales and intermicellar interactions. SANS signals are sensitive to the contrast between the solvent (D₂O) and the hydrocarbonic tails in the micellar core (hydrogen), and SAXS can access the inner structure of the polar shell because the headgroups, counterions, and penetrated salt have higher electron densities compared to the solvent and to the micellar core. The number density, intermicellar distances, aggregation number, and inter/intramicellar repulsions are discussed on the basis of the dependence of the structure factor and form factor on the micellar aggregate morphology. Therefore, we confirm that micellar growth can be tuned by variations in the flexibility and size of the the headgroup as well as the ionic dissociation rate of its counterion. Additionally, we show that the counterion binding is even more significant to the development of viscoelasticity than the headgroup structure of a surfactant molecule. This is a surprising finding, showing the importance of electrostatic charges in the self-assembly process of ionic surfactant molecules.

Almost fooled again: new insights into cesium dodecyl sulfate micelle structures

Replacing sodium with cesium as the counterion for dodecyl sulfate in aqueous solution results in stronger complexation and charge shielding, which should lead to larger micelles and ultimately to a cylindrical structure (cf. spheres for sodium dodecyl sulfate), but small angle X-ray scattering (SAXS) and small angle neutron scattering patterns previously have been interpreted with ellipsoidal micelle models. We directly image CsDS micelles via cryo-transmission electron microscopy and report large core-shell spherical micelles at low concentrations (≤2 wt %) and cylindrical micelles at higher concentrations (5.0 and 8.1 wt %). These structures are shown to be consistent with SAXS patterns modeled using established form factors. These findings highlight the importance of combining real and reciprocal space imaging techniques in the characterization of self-assembled soft materials.

The role of decorated SDS micelles in sub-CMC protein denaturation and association

We have combined spectroscopy, chromatography, calorimetry, and small-angle X-ray scattering (SAXS) to provide a comprehensive structural and stoichiometric description of the sodium dodecyl sulfate (SDS)-induced denaturation of the 86-residue alpha-helical bovine acyl-coenzyme-A-binding protein (ACBP). Denaturation is a multistep process. Initial weak binding of 1-3 SDS molecules per protein molecule below 1.3 mM does not perturb the tertiary structure. Subsequent binding of approximately 13 SDS molecules per ACBP molecule leads to the formation of SDS aggregates on the protein and changes in both tertiary and secondary structures. SAXS data show that, at this stage, a decorated micelle links two ACBP molecules together, leaving about half of the polypeptide chain as a disordered region protruding into the solvent. Further titration with SDS leads to the additional uptake of 26 SDS molecules, which, according to SAXS, forms a larger decorated micelle bound to a single ACBP molecule. At the critical micelle concentration, we conclude from reduced mobility and increased fluorescence anisotropy that each ACBP molecule becomes associated with more than one micelle. At this point, 56-60 SDS molecules are bound per ACBP molecule. Our data provide key structural insights into decorated micelle complexes with proteins, revealing a remarkable diversity in the different conformations they can stabilize. The data highlight that a minimum decorated micelle size, which may be a key driving force for intermolecular protein association, exists. This may also provide a structural basis for the known ability of submicellar surfactant concentrations to induce protein aggregation and fibrillation.

Ambiguity in determining the shape of alkali alkyl sulfate micelles from small-angle scattering data

Model fitting to small-angle scattering patterns from a series of dilute sodium- and cesium alkyl sulfate micellar solutions results in two significantly different sets of best-fit parameters for each solution. One of the sets defines nearly monodisperse prolate ellipsoids; the other defines slightly, but significantly, polydisperse oblate ellipsoids. In the prolate and oblate minimum locations, the mean form and structure factors as well as the mean core volumes are equal within the experimental error such that the axial ratios are approximately the reciprocals of each other. The experimental finding is numerically generalized: it is demonstrated that, in a Q range, the upper limit of which depends on the axial ratio, the squared mean and the mean square of the scattering amplitude from homogeneous ellipsoids with equatorial radii and axial ratios, respectively (r,eta) and (reta2/3,1/eta), are indistinguishable in practice. In dilute solutions without added salt, neither the best-fit values of the model parameters nor the available thermodynamic models provide direct evidence for the conformation, although the prolate ellipsoidal shape is indirectly supported by experiment. The elongated conformation of ionic micelles in dense and/or salinated systems seems realistic.