Chemistry Reference
In-Depth Information
requires for the
lmw
surfactant to have a strong tendency to adsorb at
the interface, but not necessarily to interact with the protein.
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Properties of protein-stabilised interfaces
Another function that proteins impart on the oil-water interfaces they
adsorb onto is a certain degree of viscoelasticity (Bos and van Vliet,
2001). Once at the interface, proteins, unlike
lmw
surfactants, can in-
crease interfacial viscosity via non-covalent intermolecular interactions
or covalent intermolecular disulphide cross-linking. The viscoelastic
properties of the protein-stabilised interface depend on the type of pro-
tein, with globular protein films displaying two to three orders of mag-
nitude larger viscosities than that of films stabilised by flexible proteins
(Bos and van Vliet, 2001). The difference in viscoelastic properties,
displayed by interfaces stabilised by globular or flexible proteins, has
been attributed to the larger flexibility of the disordered/flexible protein
molecules (Beverung
et al.
, 1999).
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Protein denaturation
Once proteins adsorb at the oil-water interface, they may undergo
considerable changes in their structure/conformation, a process usu-
ally referred to as 'surface denaturation'. In aqueous solutions, protein
molecules adopt a characteristic 'folded' structure which allows them to
expose their hydrophilic segments to the aqueous environment, while,
at the same time, 'hide' their hydrophobic segments within the core of
the molecular arrangement. Upon adsorption at the oil-water interface,
the protein's molecular 'surroundings' change, as the structure is now
exposed to both an aqueous and an oil environment. It is this change of
environment that drives 'surface denaturation' as the molecule unfolds,
exposing its hydrophobic residues, in order to maximise the number
of favourable interactions and minimise the number of unfavourable
interactions (Wilde, 2000b).
The dynamics of protein adsorption and conformational rearrange-
ment depend, to a great extent, on the structure of the 'native' protein
molecule (Freer
et al.,
2004). Proteins with a flexible/random native
configuration (e.g. β-casein) undergo relatively rapid conformational
changes, with time scales of minutes to a few hours. On the other hand,
for globular proteins (e.g. lysozyme), changes in conformation at the
oil-water interface are extremely slow, often found to occur over time
scales of days at both the air-water and oil-water interfaces (Freer
et al.,
2004).
Denaturation of proteins can also occur prior to or post adsorp-
tion at the oil-water interface due to processing conditions (e.g. heat,
