Food Chemistry

1.4 Proteins 69

a thionitrobenzoate anion:

(1.133)

The derivative of p-hydroxymercuribenzoate has
anε 250 of 7500 at pH 7, while the derivative
of N-ethylmaleic imide has anε 300 of 620 at
pH 7:

(1.134)

(1.135)

Especially suitable for the specific isolation of
cysteine-containing peptides of great sensitivity
is N-dimethylaminoazobenzenemaleic acid imide
(DABMA).

(1.136)

1.4.4.6 MethionineResidue

Methionine residues are oxidized to sul-

foxides with hydrogen peroxide. The sul-

foxide can be reduced, regenerating me-

thionine, using an excess of thiol reagent

(cf. 1.2.4.3.6). α-Halogen carboxylic acids,

β-propiolactone and alkyl halogenides con-

vert methionine into sulfonium derivatives,

from which methionine can be regenerated in

an alkaline medium with an excess of thiol

reagent:

(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 HistidineResidue

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 inac-

tivates chymotrypsin) by N-alkylation of the

histidine residue:

(1.138)