2009_Book_FoodChemistry

70 1 Amino Acids, Peptides, Proteins
1.4.4.8 Tryptophan Residue
N-Bromosuccinimide oxidizes the tryptophan
side chain and also tyrosine, histidine and
cysteine:
1.4.4.10 Bifunctional Reagents
Bifunctional reagents enable intra- and intermolecular
cross-linking of proteins. Examples are
bifunctional imidoester, fluoronitrobenzene, isocyanate
derivatives and maleic acid imides:
(1.143)
(1.139)
The reaction is used for the selective cleavage of
peptide chains and the spectrophotometric determination
of tryptophan.
(1.144)
1.4.4.9 Tyrosine Residue
Selective acylation of tyrosine can occur with
acetylimidazole as a reagent:
(1.145)
(1.146)
(1.140)
Diazotized arsanilic acid reacts with tyrosine and
with histidine, lysine, tryptophan and arginine:
(1.141)
Tetranitromethane introduces a nitro group into
the ortho position:
(1.142)
1.4.4.11 Reactions Involved in Food
Processing
The nature and extent of the chemical changes induced
in proteins by food processing depend on
a number of parameters, for example, composition
of the food and processing conditions, such
as temperature, pH or the presence of oxygen. As
a consequence of these reactions, the biological
value of proteins may be decreased:
• Destruction of essential amino acids
• Conversion of essential amino acids into
derivatives which are not metabolizable
• Decrease in the digestibility of protein as a result
of intra- or interchain cross-linking.
Formation of toxic degradation products is also
possible. The nutritional/physiological and toxicological
assessment of changes induced by processing
of food is a subject of some controversy
and opposing opinions.
The Maillard reaction of the ε-amino group of
lysine prevails in the presence of reducing sugars,