1.3 Peptides 37Table 1.15.Bitter taste of dipeptide A–B: dependence of recognition threshold value (mmol/l) on side chain
hydrophobicity (0: sweet or neutral taste)
A/B Asp Glu Asn Gln Ser Thr Gly Ala Lys Pro Val Leu Ile Phe Tyr Trp
0 0 0 0 0 0 0 0 85 26 21 12 11 6 5 5Gly 0a ––––––00–45752120161713
Ala0––––––00––7020––––
Pro 26 – – – – – – – – – – – 6 – – – –
Val 21 – – – – – – 65 70 – – 20 10 – – – –
Leu 12 – – – – – – 20 20 – – – 4.5– – 3. 50. 4
Ile114343333333332121234 9 5. 55 .5– – 0. 9
Phe6––––––17––2–1.4– 0. 80 .8–
Tyr5–––––––––––4––––
Trp5–28––––––––––––––
aThreshold of the amino acid (cf. Table 1.12).
with amino acids, the taste intensity is influenced
by the hydrophobicity of the side chains (Ta-
ble 1.15). The taste intensity does not appear to be
dependent on amino acid sequence (Table 1.14).
Bitter tasting peptides can occur in food after
proteolytic reactions. For example, the bitter taste
of cheese is a consequence of faulty ripening.
Therefore, the wide use of proteolytic enzymes
to achieve well-defined modifications of food
proteins, without producing a bitter taste, causes
some problems. Removal of the bitter taste of
a partially hydrolyzed protein is outlined in
the section dealing with proteins modified with
enzymes (cf. 1.4.6.3.2).
The sweet taste of aspartic acid dipeptide es-
ters (I) was discovered by chance in 1969 for
α-L-aspartyl-L-phenylalanine methyl ester (“As-
partame”, “NutraSweet”). The corresponding
peptide ester ofL-aminomalonic acid (II) is also
sweet.
A comparison of structures I, II and III reveals
a relationship between sweet dipeptides and
(1.77)sweetD-amino acids. The required configuration
of the carboxyl and amino groups and the side
chain substituent, R, is found only in peptide
types I and II.
Since the discovery of the sweetness of com-
pounds of type I, there has been a systematic
study of the structural prerequisites for a sweet
taste.
The presence ofL-aspartic acid was shown to be
essential, as was the peptide linkage through the
α-carboxyl group.
R^1 maybeanHorCH 3 group^2 , while the R^2
and R^3 groups are variable within a certain range.
Several examples are presented in Table 1.16.
The sweet taste intensity passes through a maxi-
mum with increasing length and volume of the R^2
residue (e. g., COO-fenchyl ester is 22− 23 × 103
times sweeter than sucrose). The size of the R^3
substituent is limited to a narrow range. Obvi-
ously, the R^2 substituent has the greatest influence
on taste intensity.
The following examples show that R^2 should be
relatively large and R^3 relatively small:L-Asp-
L-Phe-OMe (aspartame, R^2 —CH 2 C 6 H 5 ,R 3 =
COOMe) is almost as sweet (fsac,g( 1 )=180)
asL-Asp-D-Ala-OPr (fsac, g( 0. 6 )=170), while
L-Asp-D-Phe-OMe has a bitter taste.
In the case of acylation of the free amino group
of aspartic acid, the taste characteristics depend
on the introduced group. Thus, D-Ala-L-Asp-
L-Phe-OMe is sweet (fsac, g( 0. 6 )=170), while
L-Ala-L-Asp-L-Phe-OMe is not. It should be
noted that superaspartame is extremely sweet
(cf. 8.8.15.2).(^2) Data are not yet available for compounds with R (^1) >
CH 3.