Foam fractionation of protein: Correlation of protein adsorption onto bubbles with a pH-induced conformational transition

A foam fractionation apparatus was prepared to aid protein separation at the gas–liquid interface. Using lysozyme as a model protein, we investigated the alteration of enzymatic and optical activities through foaming. The lysozyme transferred to the gaseous nitrogen phase after 5 min of bubbling with no exogenous detergent. The bacteriolytic and optical activities of lysozyme from the foamate were nearly equivalent to those of the original lysozyme. This result indicated that lysozyme did not irreversibly denature during foam fractionation. We then performed protein separation using binary mixtures of lysozyme and α-amylase. When the two proteins were dissolved in bulk solution of pH 10.5, which is close to the isoelectric point (pI) of lysozyme (10.7), selective fractionation of lysozyme from the foam was observed. Indeed, this fractionation was identical to that from a single component solution of lysozyme. Similarly, selective fractionation of α-amylase was achieved in pH 3.0 buffer. Furthermore, circular dichroism (CD) and subsequent model fitting revealed that the protein had a reduced or nearly complete absence of α-helical content, whereas the amount of β-sheet structure and random coil was elevated in the buffer conditions that promoted protein adsorption. These results indicate that a pH-induced conformational transition might correlate with protein foaming.