08.05.2020 Views

2009_Book_FoodChemistry

food chemistry

food chemistry

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

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)

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!