2009_Book_FoodChemistry

60 1 Amino Acids, Peptides, Proteins
1.4.3.2 Optical Activity
Fig. 1.34. Titration curves for β-lactoglobulin at various
ionic strengths ω. (according to Edsall and
Wyman, 1958)
where QpI is the sum of deviations of the isoelectric
points of all participating amino acids from
the neutral point:
Q pI =
4.2 · nAsp+ 3.8m Glu
3.8q Arg+ 2.6r Lys+ 0.5s His
(1.94)
The formula fails when acid or alkaline groups
occur in masked form.
The optical activity of proteins is due not only
to asymmetry of amino acids but also to the
chirality resulting from the arrangement of
the peptide chain. Information on the conformation
of proteins can be obtained from
a recording of the optical rotatory dispersion
(ORD) or the circular dichroism (CD), especially
in the range of peptide bond absorption
wavelengths (190–200 nm). The Cotton effect
occurs in this range and reveals quantitative
information on secondary structure. An α-
helix or a β-structure gives a negative Cotton
effect, with absorption maxima at 199 and
205 nm respectively, while a randomly coiled
conformation shifts the maximum to shorter
wavelengths, i. e. results in a positive Cotton
effect (Fig. 1.35).
1.4.3.3 Solubility, Hydration and Swelling
Power
Protein solubility is variable and is influenced
by the number of polar and apolar groups and
their arrangement along the molecule. Generally,
proteins are soluble only in strongly
Fig. 1.35. Cotton effect. a Polylysine α-helix (1, pH 11–11.5) β-sheet structure (2, pH 11–11.3 and heated above
50 ◦ C) and random coiled (3, pH 5–7). b Ribonuclease with 20% α-helix, 40% β-sheet structure and 40% random
coiled region. (according to Luebke, Schroeder, andKloss, 1975)