2009_Book_FoodChemistry

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.
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:
1.4.4 Chemical Reactions
The chemical modification of protein is of
importance for a number of reasons. It provides
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.
(1.100)
This reaction is used analytically to assess
the amount of biologically available ε-amino
groups and for measuring protein digestibility.
Derivatization with imido esters is also possible.
The reactant is readily accessible from the corresponding
nitriles:
1.4.4.1 Lysine Residue
Reactions involving the lysine residue can be divided
into several groups, (a) reactions leading to
a positively charged derivative, (b) reactions eliminating
the positive charge, (c) derivatizations introducing
a negative charge, and (d) reversible reactions.
The latter are of particular importance.
1.4.4.1.1 Reactions Which Retain
the Positive Charge
(1.101)
Proteins can be cross-linked with the use of a bifunctional
imido ester (cf. 1.4.4.10).
Treatment of the amino acid residue with
amino acid carboxyanhydrides yields a polycondensation
reaction product:
Alkylation of the free amino group of lysine with
aldehydes and ketones is possible, with a simultaneous
reduction step:
(1.102)
(1.99)
The value n depends on reaction conditions.
The carboxyanhydrides are readily accessible
through interaction of the amino acid with