92168.pdf

1 1 2 Liquid-gas and liquid-liquid interfaces

Many of the proteins from which these films can be formed are

approximately spherical in the native state, with diameters of c.

5-10 nm. Since a limiting area of 1 m^2 mg~! corresponds to about

0.15 nm^2 per peptide residue, or a film only 0.8-1.0 nm thick, then

clearly some unfolding of the polypeptide chains takes place at the

surface. Proteins unfold even further at oil- water interfaces.

At low compressions, up to c. I mN m"^1 , protein films tend to be

gaseous, thus permitting relative molecular mass determination.

For an ideal gaseous film,

irAm = RT

where Am is the molar area of the film material. Therefore,

= RT

where A represents the area per unit mass and M is the molar mass of

the film material. To realise ideal gaseous behaviour experimental

data must be extrapolated to zero surface pressure -i.e.

RT

limir/t= — (4.37)

»-*o M

The relative molecular masses of a number of spread proteins have
been determined from surface-pressure measurements^54. In many
cases they compare with the relative molecular masses in bulk
solution. Relative molecular masses significantly lower than the bulk-
solution values have also been reported, which suggests surface
dissociation of the protein molecules into sub-units.

Interactions in mixed films

Mixed surface films, especially those likely to be of biological
importance, have been subject to a great deal of investigation.
Often there is evidence of interaction between stoichiometric
proportions of the components of a mixed monolayer. Evidence of
interaction can be sought by measuring partial molecular areas or by
studying the collapse of the mixed film. The partial molecular areas of
the components of a mixed film are usually different from the
molecular areas of the pure components when interaction occurs. A
mixed monolayer may collapse in one of two ways: (a) with no