2009_Book_FoodChemistry

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).
1.4.4.4 Cystine Residue
(cf. also Section 1.2.4.3.5)
Cleavage of cystine is possible by a nucleophilic
attack:
1.4.4.3 Glutamic and Aspartic Acid Residues
These amino acid residues are usually esterified
with methanolic HCl. There can be side reactions,
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 carbodiimide:
(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 (nitriles)
is of interest since the thiocyanate formed
in the reaction is cyclized to a 2-iminothiazolidine
derivative with cleavage of the N-acyl bond:
(1.118)
Amidation is also possible by activating the
carboxyl group with an isooxazolium salt (Woodward
reagent) to an enolester and its conversion
with an amine.
(1.122)
(1.119)
This reaction can be utilized for the selective
cleavage of peptide chains. Initially, all the disulfide
bridges are reduced with dithiothreitol, and
then are converted to mixed disulfides through