X‑ray\nReflectivity Probing the Structural Evolution\nof Sunflower Proteins Adsorbed at the Air–Water Interface

The study delves into the adsorption of sunflower proteins\nat the\nair/water interface using specular X-ray reflection. The research\ninvolved fitting models of the protein films to the reflectivity data,\nresulting in detailed images of the X-ray scattering length density\nprofiles perpendicular to the air/water interface. The sunflower protein\nisolate that is examined consists of multiple components, and the\nstudy proposes a transition from a 1-slab model to a 4-slab model\nto represent the changing layer structure over time. This transition\nis significant as it reflects the increasing complexity of the protein\nfilm as more proteins adsorb at the interface. Initially, sunflower\nproteins form a monolayer at the air/water boundary, consisting of\na protein-rich, hydrophobic portion closest to the interface and a\nmore diffuse, hydrophilic portion extending into the bulk aqueous\nphase. The structural changes at the interface over time depend on\nthe bulk protein concentration in the solution. For solutions at relatively\nlow concentrations (<i>C</i> ≤ 0.5 g/L), a lower\namount of adsorption results in a larger, more extensive interface\narea for each species and a thinner protein adsorption layer. The\noverall thickness of a saturated monolayer is approximately 100 Å,\nwhich is close to the maximum dimension of sunflower globulins, with\nthe thickness of the corresponding hydrophobic portion being about\n20 Å. For solutions at relatively high concentrations (<i>C</i> ≥ 1.0 g/L), even after forming a saturated monolayer,\nstructural evolution continues within the experimental time frame,\noccurring on both hydrophilic and hydrophobic sides. Additional proteins\nfrom the bulk diffuse toward the interface, forming an extra layer\nin the water phase and causing an increase in the overall thickness.\nFurthermore, a distinct sublayer develops next to the air phase, indicating\na further structuration of the hydrophobic portion.