Rubisco at interfaces I: Conformational flexibility enhances air-water interface and foam stabilization

Summary

Rubisco exists in all green leaves and is the most abundant protein species on earth. Extraction methods can affect the molecular conformation of Rubisco, while its influence in the behavior of Rubisco at the air-water interface and foam stabilization is largely unknown. This work focuses on elucidating the role of the Rubisco molecular structure in stabilizing the air-water interface and the multiphase system of foam. Rubisco was extracted from spinach using ultrafiltration (RU) and acid precipitation-alkaline redispersion (RA), respectively. Protein molecular properties were evaluated using SDS-PAGE, DSC, fluorescence spectrometry, and size and zeta-potential measurements. Surface adsorption behavior was measured in the millisecond and longtime regime. Surface mechanical properties were assessed with large amplitude oscillatory dilatation (LAOD) and large amplitude oscillatory shear (LAOS), and LAOD results were analyzed by stress decomposition. Interfacial structures were characterized by imaging Langmuir-Blodgett films with atomic force microscopy. Protein foaming properties were evaluated by whipping. We found that RU was primarily native and more flexible than RA, which was fully denatured and aggregated. Consequently, RU diffused faster to air-water interface (576 ms) than RA (926 ms), and formed stiffer soft solid-like air-water interfaces (G_i’ = 56.7 mN/m; E_d’ = 98.2 mN/m) than RA (G_i’ = 39.6 mN/m; E_d’ = 84.3 mN/m). RU also formed denser and thicker surfaces with higher network connectivity than RA. These advantages endow RU with much higher foam stability (152 min half-life time) than RA (94 min). Our study reveals a clear relationship between the molecular conformation of Rubisco and its air-water interface and foam stabilization properties. It might also guide the development of improved extraction/treatment methods to obtain ideal molecular structures of proteins for foam stabilization.