Using in Situ X‑ray Reflectivity to Study Protein Adsorption on Hydrophilic and Hydrophobic Surfaces: Benefits and Limitations

We have employed in situ X-ray reflectivity (IXRR) to study the adsorption of a variety of proteins (lysozyme, cytochrome c, myoglobin, hemoglobin, serum albumin, and immunoglobulin G) on model hydrophilic (silicon oxide) and hydrophobic surfaces (octadecyltrichlorosilane self-assembled monolayers), evaluating this recently developed technique for its applicability in the area of biomolecular studies. We report herein the highest resolution depiction of adsorbed protein films, greatly improving on the precision of previous neutron reflectivity (NR) results and previous IXRR studies. We were able to perform complete scans in 5 min or less with the maximum momentum transfer of at least 0.52 Å^–1, allowing for some time-resolved information about the evolution of the protein film structure. The three smallest proteins (lysozyme, cytochrome c, and myoglobin) were seen to deposit as fully hydrated, nondenatured molecules onto hydrophilic surfaces, with indications of particular preferential orientations. Time evolution was observed for both lysozyme and myoglobin films. The larger proteins were not observed to deposit on the hydrophilic substrates, perhaps because of contrast limitations. On hydrophobic surfaces, all proteins were seen to denature extensively in a qualitatively similar way but with a rough trend that the larger proteins resulted in lower coverage. We have generated high-resolution electron density profiles of these denatured films, including capturing the growth of a lysozyme film. Because the solution interface of these denatured films is diffuse, IXRR cannot unambiguously determine the film extent and coverage, a drawback compared to NR. X-ray radiation damage was systematically evaluated, including the controlled exposure of protein films to high-intensity X-rays and exposure of the hydrophobic surface to X-rays before adsorption. Our analysis showed that standard measuring procedures used for XRR studies may lead to altered protein films; therefore, we used modified procedures to limit the influence of X-ray damage

Unraveling the Single-Nanometer Thickness of Shells of Vesicle-Templated Polymer Nanocapsules

Vesicle-templated nanocapsules have emerged as a viable platform for diverse applications. Shell thickness is a critical structural parameter of nanocapsules, where the shell plays a crucial role providing mechanical stability and control of permeability. Here we used small-angle neutron scattering (SANS) to determine the thickness of freestanding and surfactant-stabilized nanocapsules. Despite being at the edge of detectability, we were able to show the polymer shell thickness to be typically 1.0 ± 0.1 nm, which places vesicle-templated nanocapsules among the thinnest materials ever created. The extreme thinness of the shells has implications for several areas: mass-transport through nanopores is relatively unimpeded; pore-forming molecules are not limited to those spanning the entire bilayer; the internal volume of the capsules is maximized; and insight has been gained on how polymerization occurs in the confined geometry of a bilayer scaffold, being predominantly located at the phase-separated layer of monomers and cross-linkers between the surfactant leaflets