shielded from the solvent due to association (it occurs as a dimer).b-casein is a
disordered protein, having many hydrophobic residues exposed to the aqueous
phase. This would cause a negative solvation free energy, implying very small
solubility.
- Figure 7.1 shows that the part of the molecule comprising the first 47
residues differs greatly from the rest (162 residues). It follows that
First part: 53%charged, z¼15, 22%hydrophobic
Other 162: 14%charged, z¼ 2 :5, 31%hydrophobicThe molecule thus has a hydrophilic ‘‘head’’ and a long and flexible, rather
hydrophobic, ‘‘tail.’’ It resembles a huge soap molecule. Such molecules tend to form
micelles above a fairly well-defined concentration (Figure 2.8), in accordance with
the shape of the curves in Figure 7.15. In such aggregates, the polar heads are at the
outside, and the more apolar tails at the inside. The driving force for micellization is
hydrophobic interaction between these tails, and this interaction strongly decreases
with decreasing temperature in the range considered (Figure 3.4). At smaller ionic
strength, mutual repulsion between the negatively charged heads will be sensed over
a longer distance, which would counteract the tendency to form micelles. This is in
qualitative agreement with the trends observed.
7.4 RECAPITULATION
Description. Proteins are polyelectrolytes, but they occur in such a
bewildering variety of composition, structure, and properties that
physicochemical polymer theory is of limited use for understanding them.
The properties ultimately depend on the primary structure of a protein, i.e.,
what amino acid residues occur and in what sequence. The 20 amino acid
building blocks differ in several respects. The most important general
properties may be their charge, which determines the charge of the protein
as a function of pH; and the hydrophobicity, which is of prime importance
for conformation and solubility.
Theconformationis the total three-dimensional folding of the peptide
chain. Some levels of structure can be distinguished. The secondary
structure involves fairly regular orderings of amino acid residues, especially
thea-helix and theb-sheet. These are strongly hydrogen bonded, primarily
via 55 O and NH of the peptide bonds. The tertiary structure involves the
further folding of the peptide chain, including the secondary structure
elements. Three types of tertiary structure can be distinguished, viz., (a)
globular, which implies a tightly compacted chain forming a roughly
spherical mass, with most of the very hydrophobic residues at the inside and
nearly all charged residues at the outside; (b) fibrous, which implies