Formation of rice protein fibrils is highly sensitive to the different types of metal ions: Aggregation behavior and possible mechanisms.

The effects of Ca, Cu, and Fe on rice protein (RP) fibril formation were investigated in this study. Low Ca concentration (≤150 mM) moderately unfolded the conformation of RP, promoting the exposure of hydrophobic sites and RP fibril assembly. Fibril formation was especially promoted with earlier addition of Ca. Cu and Fe inhibited RP fibril formation in a dose-dependent manner, and the inhibitory effect of Fe was stronger due to higher affinity with RP. Additionally, the addition of Cu and Fe reduced α-helix and β-sheet contents of RP, respectively, hindering the formation of stacked β-sheet, the main internal structure of fibrils. These two ions also resulted in the formation of random aggregates within 15-50 nm, which further inhibited the conversion of proteins to fibrils. Moreover, Cu and Fe prevented the recruitment of nucleus into fibril-growth-sites, and formed fibrils were disrupted into fragments when these ions were added.