Exploring the mineralization of hydrophobins at a liquid interface

Abstract

Hydrophobins, a class of highly surface active amphiphilic proteins, can stabilize various interfaces. When used for emulsion stabilization, it has been recently shown that they are able to induce mineralization, resulting in hollow capsules. We found that not all types of hydrophobins trigger mineralization, and that the morphology of the mineral changes depending on the selected oil. We investigated the formation of hydrophobin films at interfaces by the use of CD spectroscopy. In order to elucidate the structural features that enhance the mineralization property and give a possible explanation for this behavior, we performed MD-simulations of two representative hydrophobins (EAS for class I and HFBII for class II) at a hexane–water interface, in the presence as well as in the absence of ions. Our studies showed that the class II hydrophobin HFBII, which did not induce mineralization, only slightly changes its structure during adsorption at the oil–water interface or upon addition of Ca²⁺ and HPO₄²⁻ ions. In contrast to that, the class I hydrophobin EAS changed its conformation to a large extent during the adsorption and interacted strongly with added ions. We revealed that EAS preorganizes the ions at short distances matching the lattice dimensions of hydroxyapatite. The latter finding yields a straightforward explanation for the observed differences in mineralization behavior and allows us to search for other hydrophobins that could assist mineralization, despite their different functions in nature.