2009_Book_FoodChemistry

68 1 Amino Acids, Peptides, Proteins
reaction with 5,5 ′ -dithio-bis-(2-nitro-benzoic
acid). These mixed disulfides are then cleaved by
cyanide at pH 7.
Electrophilic cleavage occurs with Ag + and Hg +
or Hg 2+ as follows:
The introduction of methyl groups is possible
with methyl iodide or methyl isourea, and the introduction
of methylthio groups with methylthiosulfonylmethane:
(1.127)
(1.128)
(1.123)
Electrophilic cleavage with H + is possible only in
strong acids (e. g. 10 mol/L HCl). The sulfenium
cation which is formed can catalyze a disulfide
exchange reaction:
(1.129)
Maleic acid anhydride and methyl-p-nitrobenzene
sulfonate are also alkylating agents:
(1.124)
In neutral and alkaline solutions a disulfide exchange
reaction is catalyzed by the thiolate anion:
(1.130)
1.4.4.5 Cysteine Residue
(cf. also Section 1.2.4.3.5)
(1.125)
A number of alkylating agents yield derivatives
which are stable under the conditions for acidic
hydrolysis of proteins. The reaction with ethylene
imine giving an S-aminoethyl derivative and,
hence, an additional linkage position in the protein
for hydrolysis by trypsin, was mentioned in
Section 1.4.1.3. Iodoacetic acid, depending on the
pH, can react with cysteine, methionine, lysine
and histidine residues:
(1.131)
A number of reagents make it possible to measure
the thiol group content spectrophotometrically.
The molar absorption coefficient, ε, for the
derivative of azobenzene-2-sulfenylbromide,
ε 353 ,is16,700 M −1 cm −1 at pH 1:
(1.126)
(1.132)
5,5 ′ -Dithiobis-(2-nitrobenzoic acid) has a somewhat
lower ε 412 of 13,600 at pH 8 for its product,