2009_Book_FoodChemistry

1.4 Proteins 69
a thionitrobenzoate anion:
(1.133)
The derivative of p-hydroxymercuribenzoate has
an ε 250 of 7500 at pH 7, while the derivative
of N-ethylmaleic imide has an ε 300 of 620 at
pH 7:
1.4.4.6 Methionine Residue
Methionine residues are oxidized to sulfoxides
with hydrogen peroxide. The sulfoxide
can be reduced, regenerating methionine,
using an excess of thiol reagent
(cf. 1.2.4.3.6). α-Halogen carboxylic acids,
β-propiolactone and alkyl halogenides convert
methionine into sulfonium derivatives,
from which methionine can be regenerated in
an alkaline medium with an excess of thiol
reagent:
(1.134)
(1.137)
Reaction with cyanogen bromide (BrCN), which
splits the peptide bond on the carboxyl side
of the methionine molecule, was outlined in
Section 1.4.1.3.
1.4.4.7 Histidine Residue
(1.135)
Especially suitable for the specific isolation of
cysteine-containing peptides of great sensitivity
is N-dimethylaminoazobenzenemaleic acid imide
(DABMA).
Selective modification of histidine residues
present on active sites of serine proteinases
is possible. Substrate analogues such as
halogenated methyl ketones inactivate such
enzymes (for example, 1-chloro-3-tosylamido-
7-aminoheptan-2-one inactivates trypsin and 1-
chloro-3-tosylamido-4-phenylbutan-2-one inactivates
chymotrypsin) by N-alkylation of the
histidine residue:
(1.136)
(1.138)