64 1 Amino Acids, Peptides, Proteins
boundary layer and the polar “head”, which
projects into the aqueous medium, prevents
coalescence.
The solubility and emulsifying capacity of some
proteins can be improved by limited enzymatic
hydrolysis.
1.4.4 ChemicalReactions
The chemical modification of protein is of
importance for a number of reasons. It pro-
vides derivatives suitable for sequence analysis,
identifies the reactive groups in catalytically
active sites of an enzyme, enables the binding of
protein to a carrier (protein immobilization) and
provides changes in protein properties which are
important in food processing. In contrast to free
amino acids and except for the relatively small
number of functional groups on the terminal
amino acids, only the functional groups on
protein side chains are available for chemical
reactions.
1.4.4.1 LysineResidue
Reactions involving the lysine residue can be di-
vided into several groups, (a) reactions leading to
a positively charged derivative, (b) reactions elim-
inating the positive charge, (c) derivatizations in-
troducing a negative charge, and (d) reversible re-
actions. The latter are of particular importance.
1.4.4.1.1 ReactionsWhichRetainthePositiveCharge.................
Alkylation of the free amino group of lysine with
aldehydes and ketones is possible, with a simul-
taneous reduction step:
(1.99)A dimethyl derivative [Prot−N(CH 3 ) 2 ] can
be obtained with formaldehyde (R=R 1 =H)
(cf. 1.2.4.2.2).
Guanidination can be accomplished by using
O-methylisourea as a reactant.α-Amino groups
react at a much slower rate than ε-amino
groups:(1.100)This reaction is used analytically to assess
the amount of biologically available ε-amino
groups and for measuring protein digesti-
bility.
Derivatization with imido esters is also possible.
The reactant is readily accessible from the corres-
ponding nitriles:(1.101)Proteins can be cross-linked with the use of a bi-
functional imido ester (cf. 1.4.4.10).
Treatment of the amino acid residue with
amino acid carboxyanhydrides yields a poly-
condensation reaction product:(1.102)The value n depends on reaction conditions.
The carboxyanhydrides are readily accessible
through interaction of the amino acid with