Heat-induced aggregation of a globular egg-white protein in aqueous solution: Investigation by atomic force microscope imaging and surface force mapping modalities

摘要

Aqueous solution conditions have been chosen so that a globular gelling food protein, ovalbumin, is adsorbed on a molecularly smooth muscovite mica surface. Atomic force microscope (AFM) fluid-tapping and electrical double layer imaging modes and force mapping have been used to characterize the heat-induced aggregation mechanism and surface properties of the protein. At a solution concentration of 0.5 mug/mL, single globular structures, which were consistent with the dimensions of ovalbumin monomers, were observed with the fluid-tapping imaging mode. Heat treatment caused protein aggregation and produced larger globular structures. At a solution concentration of 20 mug/mL, a protein film was formed and the electrical double layer mode of imaging revealed that the heat-induced globular aggregates were more heterogeneous than the native protein. The heterogeneity of the heat-treated ovalbumin film was also apparent from the effective film thickness maps derived from the force data obtained on approach of the AFM tip toward the surface covered with protein. The interfacial spring constant for heat-treated ovalbumin was moderately lower than for the native protein. This is indicative of a reduction of the internal cohesiveness of the molecular structure upon heating. The observation that there are regular multiple adhesion events, upon retraction of the AFM tip from the surface, for heat-induced ovalbumin aggregates suggests that the globular structures were composed of noncovalently linked monomeric units of ovalbumin. [References: 41]