2009_Book_FoodChemistry

1.4 Proteins 59
Table 1.28. pK values of protein side chains
Group pK Group pK
(25 ◦ C) (25 ◦ C)
α-Carboxyl- 3–4 Imidazolium- 4–8
β,γ-Carboxyl- 3–5 Hydroxyα-Ammonium-
7–8 (aromatic) 9–12
ε-Ammonium- 9–11 Thiol 8–11
Guanidinium- 12–13
of 6–6.5, while the pK of the β-carboxyl group of
Asp 66 is 1.5–2, of Asp 52 is 3–4.6 and of Asp 101
is 4.2–4.7.
The total charge of a protein, which is the absolute
sum of all positive and negative charges,
is differentiated from the so-called net charge
which, depending on the pH, may be positive,
zero or negative. By definition the net charge is
zero and the total charge is maximal at the isoelectric
point. Lowering or raising the pH tends
to increase the net charge towards its maximum,
while the total charge always becomes less than
at the isolectric point.
Since proteins interact not only with protons but
also with other ions, there is a further differentiation
between an isoionic and an isoelectric point.
The isoionic point is defined as the pH of a protein
solution at infinite dilution, with no other
ions present except for H + and HO − . Such a protein
solution can be acquired by extensive dialy-
sis (or, better, electrodialysis) against water. The
isoionic point is constant for a given substance
while the isoelectric point is variable depending
on the ions present and their concentration. In
the presence of salts, i. e. when binding of anions
is stronger than that of cations, the isoelectric
point is lower than the isoionic point. The
reverse is true when cationic binding is dominant.
Figure 1.33 shows the shift in pH of an
isoionic serum albumin solution after addition
of various salts. The shift in pH is consistently
positive, i. e. the protein binds more anions than
cations.
The titration curve of β-lactoglobulin at various
ionic strengths (Fig. 1.34) shows that the isoelectric
point of this protein, at pH 5.18, is independent
of the salts present. The titration curves are,
however, steeper with increasing ionic strength,
which indicates greater suppression of the electrostatic
interaction between protein molecules.
At its isoelectric point a protein is the least soluble
and the most likely to precipitate (“isoelectric
precipitation”) and is at its maximal crystallization
capacity. The viscosity of solubilized proteins
and the swelling power of insoluble proteins
are at a minimum at the isoelectric point.
When the amino acid composition of a protein is
known, the isoelectric point can be estimated according
to the following formula:
(1.93)
Fig. 1.33. pH-shift of isoionic serum albumin solutions by added salts. (according to Edsall and Wymann, 1958)