Food Chemistry

1.4 Proteins 65

phosgene:

(1.103)

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 hydrox-
ylamine (1 M, 2 h, pH 9, 0◦C) leaves only the
acetylated amino groups intact:

(1.104)

Carbamoylation with cyanate attacks α-and
ε-amino groups as well as cysteine and tyro-
sine 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 in-
terest for derivatization of proteins:

(1.106)

Deamination can be accomplished with nitrous

acid:

(1.107)

This reaction involvesα-andε-amino groups

as well as tryptophan, tyrosine, cysteine and

methionine residues.

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:

(1.108)

Introduction of a fluorescent acid group is pos-

sible by interaction of the protein with pyridoxal

phosphate followed by reduction of the interme-

diarySchiffbase:

(1.109)