At the National Synchrotron Radiation Research Center, a small/wide‐angle X‐ray scattering (SAXS/WAXS) instrument has been installed at the BL23A beamline with a superconducting wiggler insertion device. This beamline is equipped with double Si(111) crystal and double Mo/B4C multilayer monochromators, and an Si‐based plane mirror that can selectively deflect the beam downwards for grazing‐incidence SAXS (GISAXS) studies of air–liquid or liquid–liquid interfaces. The SAXS/WAXS instrument, situated in an experimental hutch, comprises collimation, sample and post‐sample stages. Pinholes and slits have been incorporated into the beam collimation system spanning a distance of ∼5 m. The sample stage can accommodate various sample geometries for air–liquid interfaces, thin films, and solution and solid samples. The post‐sample section consists of a 1 m WAXS section with two linear gas detectors, a vacuum bellows (1–4 m), a two‐beamstop system and the SAXS detector system, all situated on a motorized optical bench for motion in six degrees of freedom. In particular, the vacuum bellows of a large inner diameter (260 mm) provides continuous changes of the sample‐to‐detector distance under vacuum. Synchronized SAXS and WAXS measurements are realized via a data‐acquisition protocol that can integrate the two linear gas detectors for WAXS and the area detector for SAXS (gas type or Mar165 CCD); the protocol also incorporates sample changing and temperature control for programmable data collection. The performance of the instrument is illustrated via several different measurements, including (1) simultaneous SAXS/WAXS and differential scanning calorimetry for polymer crystallization, (2) structural evolution with a large ordering spacing of ∼250 nm in a supramolecular complex, (3) SAXS for polymer blends under in situ drawing, (4) SAXS and anomalous SAXS for unilamellar lipid vesicles and metalloprotein solutions, (5) anomalous GISAXS for oriented membranes of Br‐labeled lipids embedded with peptides, and (6) GISAXS for silicate films formed in situ at the air–water interface.

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Using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) with contrast variation, we have studied the complex aggregation of Alzheimer amyloid-b(1-40) (Ab) peptides with sodium dodecyl sulfate (SDS). With the addition of 0.115 mM Ab peptide into an aqueous solution containing 6 mM SDS, time-dependent SAXS indicates the formation of a globular SDS-Ab complex with a core-shell structure. The emergence of the complex aggregates, however, lags significantly behind the fast transition of the secondary structure of Ab peptides from random coil to a-helical structure observed by circular dichroism (CD). With scattering contrast varied by SDS and deuterated SDS, SANS results reveal the coexistence of Ab aggregates and the SDS-Ab complex, which together form clusters of a mass fractal structure. Based on the changes of the zero-angle scattering intensity with the contrast variation, a molecular association ratio, $30 : 1, of SDS to Ab is extracted for the globular complex micelle. With a concentration (20 mM) above the critical micelle concentration (CMC) of SDS, time-dependent SAXS and CD reveal a better synchronization between the formation of the SDS-Ab complex and the changes in Ab secondary structure. Using an ellipsoid model with a core-shell structure in the SAXS data analysis, we have extracted detailed structural information of the prolate, core-shelled SDS-Ab complex, having size and shape resembling pure SDS micelles. The significantly larger electron density of the shell of the complex, as compared to that of pure SDS micelles, suggests that the hydrophilic parts of Ab peptides can situate well with the sulfate headgroups of SDS in the shell region. Delicate differences in the micellar structure and the formation pathway of the two types of SDS-Ab complexes, respectively formed in aqueous solutions containing SDS concentrations below and above its CMC, are discussed in terms of the dissimilar association efficiencies of Ab peptides with SDS monomers and SDS micelles. The morphology, association ratio, and clustering behavior of the SDS-Ab complex obtained in this study may have implications in interpreting the related spectroscopy and SDS-PAGE results of the protein.