34 1 Amino Acids, Peptides, Proteins
reduced to an aldehyde group. Reaction with
phenylhydrazine produces an indole derivative.
Lastly, hydantoin is saponified with alkali:
(1.70)L-Tryptophan is also produced through enzymatic
synthesis from indole and serine with the help of
tryptophan synthase:
(1.71)1.2.6 SensoryProperties
Free amino acids can contribute to the flavor of
protein-rich foods in which hydrolytic processes
occur (e. g. meat, fish or cheese).
Table 1.12 provides data on taste quality and
taste intensity of amino acids. Taste quality
is influenced by the molecular configuration:
sweet amino acids are primarily found among
members of theD-series, whereas bitter amino
acids are generally within theL-series. Conse-
quently amino acids with a cyclic side chain
(1-aminocycloalkane-1-carboxylic acids) are
sweet and bitter.
The taste intensity of a compound is reflected
in its recognition threshold value. The recogni-
tion threshold value is the lowest concentration
needed to recognize the compound reliably, as as-
sessed by a taste panel. Table 1.12 shows that the
taste intensity of amino acids is dependent on the
hydrophobicity of the side chain.
L-Tryptophan and L-tyrosine are the most
bitter amino acids with a threshold value of
ct bitter = 4 −6 mmol/l. D-Tryptophan, with
ct sweet= 0. 2 − 0 .4 mmol/l, is the sweetest amino
acid. A comparison of these threshold values
with those of caffeine (ctbi= 1 − 1 .2 mmole/l)
and sucrose (ctsw= 10 −12 mmol/l) shows that
caffeine is about 5 times as bitter asL-tryptophan
and thatD-tryptophan is about 37 times as sweet
as sucrose.
L-Glutamic acid has an exceptional position. In
higher concentrations it has a meat broth flavor,
while in lower concentrations it enhances the
characteristic flavor of a given food (flavor en-
hancer, cf. 8.6.1).L-Methionine has a sulfur-like
flavor.
The bitter taste of theL-amino acids can interfere
with the utilization of these acids, e. g., in chemi-
cally defined diets.1.3 Peptides
1.3.1 GeneralRemarks,Nomenclature...........................
Peptides are formed by binding amino acids to-
gether through an amide linkage.
On the other hand peptide hydrolysis results in
free amino acids:(1.72)Functional groups not involved in the peptide
synthesis reaction should be blocked. The
protecting or blocking groups must be removed
after synthesis under conditions which retain
the stability of the newly formed peptide
bonds: