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:
(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:
(1.124)In neutral and alkaline solutions a disulfide ex-
change reaction is catalyzed by the thiolate anion:
(1.125)1.4.4.5 CysteineResidue(cf.alsoSection1.2.4.3.5).................
A number of alkylating agents yield derivatives
which are stable under the conditions for acidic
hydrolysis of proteins. The reaction with ethy-
lene imine giving an S-aminoethyl derivative and,
hence, an additional linkage position in the pro-
tein 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:
(1.126)The introduction of methyl groups is possible
with methyl iodide or methyl isourea, and the in-
troduction of methylthio groups with methylthio-
sulfonylmethane:(1.127)(1.128)(1.129)Maleic acid anhydride and methyl-p-nitro-
benzene sulfonate are also alkylating agents:(1.130)(1.131)A number of reagents make it possible to measure
the thiol group content spectrophotometrically.
The molar absorption coefficient, ε,forthe
derivative of azobenzene-2-sulfenylbromide,
ε 353 ,is16,700 M−^1 cm−^1 at pH 1:(1.132)5 , 5 ′-Dithiobis-(2-nitrobenzoic acid) has a some-
what lowerε 412 of 13,600 at pH 8 for its product,