2009_Book_FoodChemistry

1.4 Proteins 65
phosgene:
Deamination can be accomplished with nitrous
acid:
(1.107)
(1.103)
This reaction involves α- and ε-amino groups
as well as tryptophan, tyrosine, cysteine and
methionine residues.
1.4.4.1.2 Reactions Resulting in a Loss
of Positive Charge
Acetic anhydride reacts with lysine, cysteine,
histidine, serine, threonine and tyrosine residues.
Subsequent treatment of the protein with hydroxylamine
(1 M, 2 h, pH 9, 0 ◦ C) leaves only the
acetylated amino groups intact:
1.4.4.1.3 Reactions Resulting
in a Negative Charge
Acylation with dicarboxylic acid anhydrides, e. g.
succinic acid anhydride, introduces a carboxyl
group into the protein:
(1.104)
Carbamoylation with cyanate attacks α- and
ε-amino groups as well as cysteine and tyrosine
residues. However, their derivatization is
reversible under alkaline conditions:
(1.105)
Arylation with 1-fluoro-2,4-dinitrobenzene
(Sanger’s reagent; FDNB) and trinitrobenzene
sulfonic acid was outlined in Section 1.2.4.2.2.
FDNB also reacts with cysteine, histidine and
tyrosine.
4-Fluoro-3-nitrobenzene sulfonic acid, a reactant
which has good solubility in water, is also of interest
for derivatization of proteins:
(1.108)
Introduction of a fluorescent acid group is possible
by interaction of the protein with pyridoxal
phosphate followed by reduction of the intermediary
Schiff base:
(1.106)
(1.109)