2009_Book_FoodChemistry

1.2 Amino Acids 17
N-acetyl amino acids are being considered as
ingredients in chemically-restricted diets and
for fortifying plant proteins to increase their
biological value. Addition of free amino acids to
food which must be heat treated is not problem
free. For example, methionine in the presence
of a reducing sugar can form methional by
a Strecker degradation mechanism, imparting
an off-flavor to food. Other essential amino
acids, e. g., lysine or threonine, can lose their
biological value through similar reactions.
Feeding tests with rats have shown that N-
acetyl-L-methionine and N-acetyl-L-threonine
have nutritional values equal to those of the
free amino acids (this is true also for humans
with acetylated methionine). The growth rate
of rats is also increased significantly by the
α- or ε-acetyl or α,ε-diacetyl derivatives of
lysine.
Some readily cleavable acyl residues are of importance
as temporary protective groups in peptide
synthesis.
The trifluoroacetyl residue is readily removed by
mild base-catalyzed hydrolysis:
The benzyloxycarbonyl group can be readily removed
by catalytic hydrogenation or by hydrolysis
with HBr/glacial acetic acid:
(1.20)
(1.21)
The tert-alkoxycarbonyl residues, e. g., the tertbutyloxycarbonyl
groups, are cleaved under acidcatalyzed
conditions:
(1.18)
The phthalyl residue can be readily cleaved by
hydrazinolysis:
(1.22)
N-acyl derivatives of amino acids are transformed
into oxazolinones (azlactones) by elimination of
water:
(1.19)
(1.23)