Adsorption of Macromolecules at Oil
Water Interfaces during Emulsification - Food Colloids: Self-Assembly and Material Science - page 444

The simplest interpretation would be that the most hydrophobic protein should
adsorb selectively. However, these proteins are all water soluble and they do
not display any significant differences in overall hydrophobicity. A second
possible explanation is that a higher molecular weight would give more
potential sticking possibilities (per molecule), which should favour adsorption.
However, the results indicate favoured adsorption for one high-molecular-
weight and one low-molecular-weight species, depending on pH, seemingly
excluding the possibility that the molecular weight has an important overall
influence on the adsorptive competitiveness in the system. A third alternative
explanation could involve the kinetics. In this system with comparatively small
molecules adsorbing, we might expect diffusion to determine the adsorption
rate. However, this hypothesis would also imply that the competitiveness
should follow the molecular weight, when it does not. The fourth possibility
would be that the competitiveness is controlled by the distribution of hydro-
philic and hydrophobic domains in the protein molecule. Both YGP40 and
serum albumin contain long contiguous stretches of low hydrophilicity, as can
be seen from the Kyte Doolittle plots 34 in Figure 10. Therefore, a coherent
hydrophobic block in the amino acid sequence might be an important property
in the adsorption selectivity, as proteins once adsorbed can unfold and spread
out at the interface. 35-37
Figure 10 Kyte Doolittle plots of four of the five most abundant proteins in the egg yolk
livetin fraction with a window of 15 amino acids. 34 The positive direction of the
y-axis implies increasing hydrophobicity. No plot is shown for immunoglobulin
G due to the large sequence variation. IgG tends to consist of alternating
hydrophobic and hydrophilic stretches, as it is the case for ovotransferrin. To
facilitate comparison, a horizontal dashed line has been added at -0.4, repre-
senting the glycine residue in the Kyte Doolittle algorithm
(Redrawn from Ref. 12).