66 1 Amino Acids, Peptides, Proteins
1.4.4.1.4 ReversibleReactions.....................................
N-Maleyl derivatives of proteins are obtained at
alkaline pH by reaction with maleic acid anhy-
dride. The acylated product is cleaved at pH<5,
regenerating the protein:
(1.110)The half-life (τ)ofε-N-maleyl lysine is 11 h
at pH 3.5 and 37◦C. More rapid cleavage is
observed with the 2-methyl-maleyl deriva-
tive (τ<3min at pH 3.5 and 20◦C) and
the 2,2,3,3-tetrafluoro-succinyl derivative (τ
very low at pH 9.5and0◦C). Cysteine binds
maleic anhydride through an addition reac-
tion. The S-succinyl derivative is quite stable.
This side reaction is, however, avoided when
protein derivatization is done with exo-cis-
3,6-end-oxo-1,2,3,6- tetrahydrophthalic acid
anhydride:
(1.111)Forε-N-acylated lysine,τ= 4 −5 h at pH 3 and
25 ◦C.
Acetoacetyl derivatives are obtained with
diketene:
(1.112)This is type of reaction also occurs with cysteine
and tyrosine residues. The acyl group is readily
split from tyrosine at pH 9.5. Complete release
of protein from its derivatized form is possible
by treatment with phenylhydrazine or hydroxy-
lamine at pH 7.
1.4.4.2 ArginineResidue........................................
The arginine residue of proteins reacts withα-or
β-dicarbonyl compounds to form cyclic deriva-
tives:(1.113)(1.114)(1.115)The nitropyrimidine derivative absorbs at
335 nm. The arginyl bond of this derivative is
not cleaved by trypsin but it is cleaved in its
tetrahydro form, obtained by reduction with
NaBH 4 (cf. Reaction 1.113). In the reaction
with benzil, an iminoimidazolidone derivative
is obtained after a benzilic acid rearrangement
(cf. Reaction 1.114).