Molecular simulation of hydrophobin adsorption at an oil-water interface.

Department of Chemistry and Centre for Scientific Computing, University of Warwick, Coventry CV4 7AL, UK.
Langmuir (Impact Factor: 4.38). 05/2012; 28(23):8730-6. DOI: 10.1021/la300777q
Source: PubMed

ABSTRACT Hydrophobins are small, amphiphilic proteins expressed by strains of filamentous fungi. They fulfill a number of biological functions, often related to adsorption at hydrophobic interfaces, and have been investigated for a number of applications in materials science and biotechnology. In order to understand the biological function and applications of these proteins, a microscopic picture of the adsorption of these proteins at interfaces is needed. Using molecular dynamics simulations with a chemically detailed coarse-grained potential, the behavior of typical hydrophobins at the water-octane interface is studied. Calculation of the interfacial adsorption strengths indicates that the adsorption is essentially irreversible, with adsorption strengths of the order of 100 k(B)T (comparable to values determined for synthetic nanoparticles but significantly larger than small molecule surfactants and biomolecules). The protein structure at the interface is unchanged at the interface, which is consistent with the biological function of these proteins. Comparison of native proteins with pseudoproteins that consist of uniform particles shows that the surface structure of these proteins has a large effect on the interfacial adsorption strengths, as does the flexibility of the protein.

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    ABSTRACT: Class II hydrophobin (HFBII) is a very promising ingredient for improving foam stability. Pure HFBII-stabilized bubbles exhibited exceptional stability to disproportionation (dissolution) but were not stable to bubble coalescence induced by a pressure drop. Bubbles stabilized by mixtures of HFBII + sodium caseinate (SC) or -lactoglobulin (BL) showed decreased shrinkage rates compared to pure SC or BL and improved the stability to pressure drop induced coalescence. Higher bubble stability was more closely correlated with higher surface shear viscosity than the surface dilatational elasticity of the mixed protein systems. Brewster angle microscopy observations and the high shear strength of adsorbed films including HFBII, even in the presence of hydrophobic and hydrogen bond breaking agents, confirm that intermolecular attractive cross-links are unlikely to be the origin of the high strength of HFBII films. Possibly the HFBII molecules form a tightly interlocking monolayer of Janus-like particles at the air-water interface.
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    ABSTRACT: Here, we investigate the surface shear rheology of class II HFBII hydrophobin layers at the oil/water interface. Experiments in two different dynamic regimes, at a fixed rate of strain and oscillations, have been carried out with a rotational rheometer. The rheological data obtained in both regimes comply with the same viscoelastic thixotropic model, which is used to determine the surface shear elasticity and viscosity, Esh and ηsh. Their values for HFBII at oil/water interfaces are somewhat lower than those at the air/water interface. Moreover, Esh and ηsh depend on the nature of oil, being smaller for hexadecane in comparison with soybean-oil. It is remarkable that Esh is independent of the rate of strain in the whole investigated range of shear rates. For oil/water interfaces, Esh and ηsh determined for HFBII layers are considerably greater than for other proteins, like lysozyme and β-casein. It is confirmed that the hydrophobin forms the most rigid surface layers among all investigated proteins not only for the air/water, but also for the oil/water interface. The wide applicability of the used viscoelastic thixotropic model is confirmed by analyzing data for adsorption layers at oil/water interfaces from lysozyme and β-casein - both native and cross-linked by enzyme, as well as for films from asphaltene. This model turns out to be a versatile tool for determining the surface shear elasticity and viscosity, Esh and ηsh, from experimental data for the surface storage and loss moduli, G' and G''.
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