Tannin-assisted aggregation of natively unfolded proteins

Tannin-protein interactions are essentially physical: hydrophobic and hydrogen-bond$\hbox{--} $mediated. We explored the tannin-assisted protein aggregation on the case of $\beta $-casein, which is a natively unfolded protein known for its ability to form micellar aggregates. We used several tannins with specified length. Our SAXS results show that small tannins increase the number of proteins per micelle, but keeping their size constant. It leads to a tannin-assisted compactization of micelles. Larger tannins, with linear dimensions greater than the crown width of micelles, lead to the aggregation of micelles by a bridging effect. Experimental results can be understood within a model where tannins are treated as effective enhancers of hydrophobic attraction between specific sites in proteins.