Food Chemistry

1.4 Proteins 67

Reaction of the arginine residue with 1,2-
cyclohexanedione is highly selective and
proceeds under mild conditions. Regeneration
of the arginine residue is again possible with
hydroxylamine (cf. Reaction 1.115).

1.4.4.3 GlutamicandAsparticAcidResidues

These amino acid residues are usually esterified
with methanolic HCl. There can be side reac-
tions, such as methanolysis of amide derivatives
or N,O-acyl migration in serine or threonine
residues:

(1.116)

Diazoacetamide reacts with a carboxyl group and
also with the cysteine residue:

(1.117)

Amino acid esters or other similar nucleophilic
compounds can be attached to a carboxyl
group of a protein with the help of a carbodi-
imide:

(1.118)

Amidation is also possible by activating the
carboxyl group with an isooxazolium salt (Wood-
wardreagent) to an enolester and its conversion
with an amine.

(1.119)

1.4.4.4 CystineResidue(cf.alsoSection1.2.4.3.5)..................

Cleavage of cystine is possible by a nucleophilic

attack:

(1.120)

The nucleophilic reactivity of the reagents

decreases in the series: hydride>arsenite and

phosphite > alkanethiol > aminoalkanethiol

>thiophenol and cyanide>sulfite>OH−>

p-nitrophenol > thiosulfate > thiocyanate.

Cleavage with sodium borohydride and with

thiols was covered in Section 1.2.4.3.5. Complete

cleavage with sulfite requires that oxidative

agents (e. g. Cu^2 +) be present and that the pH be

higher than 7:

(1.121)

The resultant S-sulfo derivative is quite stable in

neutral and acidic media and is fairly soluble in

water. The S-sulfo group can be eliminated with

an excess of thiol reagent.

Cleavage of cystine residues with cyanides (ni-

triles) is of interest since the thiocyanate formed

in the reaction is cyclized to a 2-iminothiazolidine

derivative with cleavage of the N-acyl bond:

(1.122)

This reaction can be utilized for the selective

cleavage of peptide chains. Initially, all the disul-

fide bridges are reduced with dithiothreitol, and

then are converted to mixed disulfides through