Hydrophobins: self‐assembly protein monolayers designed to reverse the hydrophobicity of a surface
Authors: Lo Victor , Lai Jennifer I‐Chun , Kwan Ann , Sunde Margaret 
Published: December 2016

Natural materials can inspire the production and development of materials that can be environmentally friendly, cheap and have a variety of applications. Fungal hydrophobin proteins, in particular, have several interesting traits that make them an ideal sample to investigate. These proteins are found only in the fungal kingdom and are known to play several distinct roles in the life and development of different fungi: acting as a biosurfactants to allow formation of aerial hyphae, protecting the spores from the environment, acting as an adhesive film during germination. This is all possible due to the hydrophobin protein's unique amphiphile structure and the ability to self‐assemble into an amphiphilic film at any hydrophobic:hydrophilic interface. At such an interface, the hydrophobins undergo a conformational change to form amphiphilic monolayers. With certain hydrophobins, known as class I, these monolayer films are very robust under acidic and basic conditions, and have been shown to be able to reverse the hydrophobicity of any surface.

This body of research has been investigating the structure of the amphiphilic films formed by the hydrophobin EAS
Δ15
, a truncated version of the wild type hydrophobin EAS, found on the surface of spores of
Neurospora crassa
. Additionally, we have compared the ability of EAS
Δ15
and the hydrophobin RodA, which is found on spores produced by
Aspergillus fumigatus
, to form hydrophobin protein films with the ability to alter surface hydrophobicity.

By transmission electron microscopy (TEM), it was possible to visualise the morphology of the hydrophobin films formed by EAS
Δ15
at the hydrophobic:hydrophilic interface upon a holey carbon grid. Changes in the film morphology were observed when the film was formed from protein solutions with increasing amounts of alcohol, and hence exhibiting variation in surface tension. A TEM tomogram of the hydrophobin film was constructed from 168 TEM images tilted at ±60. 3D topography information was collected from atomic force microscopy (AFM) to confirm the morphology visualised on TEM and compare the resulting models. Both imaging techniques revealed rodlet structures, which exhibit a “double track” structure and forms nanosized ridges.

Hydrophobin proteins can vary in sequence length and the amount of hydrophobic residues within the sequence; this provides a variety of functionalities for different fungi. Two different class 1 hydrophobin proteins, RodA and EAS
Δ15
, were used to compared the difference in their ability to alter the surface hydrophobicity of
a)
hydrophobic surfaces (OTS silicon wafer) and
b)
hydrophilic surfaces (StarFrost
(R)
Superclean hydrophilic glass slides). Hydrophobin proteins were solubilised in deionised water (5µg/mL) and a 50 µL droplet of the protein solution was allow dry down to form a film upon the OTS silicon and hydrophilic glass. The hydrophobin protein films were able to reduce the contact angle of water from 104±2 °C on bare OTS silicon wafer to 73±4 °C for EAS
Δ15
coated surface and 81±7 °C for RodA coated surface. For the hydrophilic glass, the contact angle for water on the bare surface was increased from 31±3 °C to 59±4 °C for the EAS
Δ15
coated surface and the 58±8 °C for RodA coated surface. AFM images indicates that even when protein films are formed from solutions with such low protein concentration, both surfaces were coated with enough hydrophobin protein to form a layer that effects the hydrophobicity of both the OTS silicon wafer and hydrophilic glass surfaces.