GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Thursday, May 19, 2016

10:00 AM
in MRDC 3515

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Xujun Zhang

" Characterization of Cerato-ulmin Hydrophobin Protein-Coated Air Bubbles and Oil Droplets in Dispersions and at Interfaces"

Committee Members:

Prof. Paul S. Russo, Advisor, MSE

Prof. Karl I. Jacob, MSE

Prof. Seung Soon Jang, MSE

Prof. Zhiqun Lin, MSE

Prof. Victor Breedveld, CHBE

Abstract:

Hydrophobins are a class of surface-active proteins produced by filamentous fungi. Their molecular weights range from 8-14 kDa. About 50 members of the hydrophobin family are known and they all have the characteristic feature of 8 conserved cysteine groups. The hydrophilic and hydrophobic patches on the hydrophobin surface induce the characteristic amphipathic feature resulting in the formation of various assemblies. All hydrophobins can be categorized into two classes depending on the solubility of their assemblies under treatment of acids or detergents. Class I hydrophobins generate insoluble aggregates that can only be dissolved in trifluoroacetic acid or similarly strong acids. Class II hydrophobin assemblies can be dissociated with ethanol or sodium dodecyl sulfate. Due to the amphipathic properties and globular structure, hydrophobins have been described as Janus particles. Assembled hydrophobins exhibit high surface activity and can modify the hydrophilic or hydrophobic surface and switch their hydrophilicity or hydrophobicity. They also have been found to stabilize food foams and emulsions by forming a layer of biofilm at air-water or oil-water interfaces. Hydrophobins have been used as natural dispersants for water-insoluble drugs or semiconducting polymers.

In this proposal, a Class II hydrophobin Cerato-ulmin is of interest. Cerato-ulmin is produced by the filamentous fungus Ophiostoma ulmi and has been implicated in Dutch elm disease. The biological role of Cerato-ulmin is not well understood. Cerato-ulmin is known to produce an especially wide variety of unusual structures in water. Dispersions of Cerato-ulmin generate cylindrical air bubbles or oil droplets under gentle agitation. The phenomenological study shows air bubbles and oil droplets are stable in terms of different physical treatments such as pressure and temperature. Dynamic light scattering results indicate air bubbles in micron size tend to float up to the air-water interface. Small bubbles (hundreds of nanometers) stay stable for at least a month. The unusual shape is stabilized by a thin and solid membrane of Cerato-ulmin. The surface activity of Cerato-ulmin at the air-water and oil-water interfaces is determined using a microtensiometer technique. The dilatational moduli of the oscillating interfaces are calculated and are shown to be higher than those of typical small surfactants. Future work will continue to characterize and understand the stability of air bubbles and oil droplets as well as Cerato-ulmin’s surface activity and interfacial behavior of hydrophobin-surfactant mixtures. Studies will focus on using Cerato-ulmin as a host to encapsulate oil-soluble polymer which undergoes alignment in confinement or to polymerize monomers in the Cerato-ulmin capsule.