Food Chemistry

72 1 Amino Acids, Peptides, Proteins

Table 1.29. Ornithine is formed during cleavage
of arginine (Reaction 1.54).

(1.154)

Formation of D-amino acids occurs through
abstraction of a proton via a C2-carbanion.
The reaction with L-isoleucine is particu-
larly interesting. L-Isoleucine is isomerized to
D-alloisoleucine which, unlike other D-amino
acids, is a diastereoisomer and so has a retention
time different from L-isoleucine, making its
determination possible directly from an amino
acid chromatogram:

(1.155)

Heating proteins in a dry state at neutral pH
results in the formation of isopeptide bonds
between theε-amino groups of lysine residues
and theβ-orγ-carboxamide groups of asparagine
and glutamine residues:

(1.156)

Table 1.29.Formation of unusual amino acids by alkali

treatment of protein

Name Formula

3-N^6 -Lysinoalanine COOH COOH

(R = H) ||

3-N^6 -Lysino-3-methyl- CHNH 2 CHNH 2

alanine(R = CH 3 ) ||

CHR—NH—(CH 2 ) 4

3-N^5 -Ornithinoalanine COOH COOH

(R = H) ||

3-N^5 -Ornithino-3- CHNH 2 CHNH 2

methylalanine (R = CH 3 ) ||

CHR—NH—(CH 2 ) 3

Lanthionine (R = H) COOH COOH

3-Methyllanthionine ||

(R = CH 3 ) CHNH 2 CHNH 2

||

CHR——S——CH 2

3-Aminoalanine (R =H) COOH

2,3-Diamino |

butyric acid (R = CH 3 ) CHNH 2

|

CHRNH 2

These isopeptide bonds are cleaved during acidic

hydrolysis of protein and, therefore, do not

contribute to the occurrence of unusual amino

acids. A more intensive heat treatment of proteins

in the presence of water leads to a more extensive

degradation.

Oxidative changes in proteins primarily involve

methionine, which relatively readily forms me-

thionine sulfoxide:

(1.157)

The formation of methionine sulfoxide was ob-

served in connection with lipid peroxidation, phe-

nol oxidation and light exposure in the presence

of oxygen and sensitizers such as riboflavin.

Afterin vivoreduction to methionine, protein-

bound methionine sulfoxide is apparently biolog-

ically available.

Figure 1.37 shows the effect of alkaline treatment

of a protein isolate of sunflower seeds. Serine,

threonine, arginine and isoleucine concentrations