2009_Book_FoodChemistry

36 1 Amino Acids, Peptides, Proteins
acids:
(1.74)
The first three letters of the amino acids
are used as symbols to simplify designation
of peptides (cf. Table 1.1). Thus,
the peptide shown above can also be given
as:
Ala Ser Gly or ASG (1.75)
One-letter symbols (cf. Table 1.1) are used
for amino acid sequences of long peptide
chains.
D-Amino acids are denoted by the prefix D-. In
compounds in which a functional group of the
side chain is involved, the bond is indicated by
a perpendicular line. The tripeptide glutathione
(γ-glutamyl-cysteinyl-glycine) is given as an illustration
along with its corresponding disulfide,
oxidized glutathione:
Table 1.13. Dissociation constants and isoelectric
points of various peptides (25 ◦ C)
Peptide pK 1 pK 2 pK 3 pK 4 pK 5 pl
Gly-Gly 3.12 8.17 5.65
Gly-Gly-Gly 3.26 7.91 5.59
Ala-Ala 3.30 8.14 5.72
Gly-Asp 2.81 4.45 8.60 3.63
Asp-Gly 2.10 4.53 9.07 3.31
Asp-Asp 2.70 3.40 4.70 8.26 3.04
Lys-Ala 3.22 7.62 10.70 9.16
Ala-Lys-Ala 3.15 7.65 10.30 8.98
Lys-Lys 3.01 7.53 10.05 11.01 10.53
Lys-Lys-Lys 3.08 7.34 9.80 10.54 11.32 10.93
Lys-Glu 2.93 4.47 7.75 10.50 6.10
His-His 2.25 5.60 6.80 7.80 7.30
1.3.3 Sensory Properties
While the taste quality of amino acids does
depend on configuration, peptides, except for
the sweet dipeptide esters of aspartic acid (see
below), are neutral or bitter in taste with no
relationship to configuration (Table 1.14). As
(1.76)
By convention, the amino acid residue with the
free amino group is always placed on the left.
The amino acids of the chain ends are denoted as
N-terminal and C-terminal amino acid residues.
The peptide linkage direction in cyclic peptides is
indicated by an arrow, i. e., CO → NH-.
1.3.2 Physical Properties
1.3.2.1 Dissociation
The pK values and isoelectric points for some
peptides are listed in Table 1.13. The acidity of
the free carboxyl groups and the basicity of the
free amino groups are lower in peptides than in
the corresponding free amino acids. The amino
acid sequence also has an influence (e. g., Gly-
Asp/Asp-Gly).
Table 1.14. Taste threshold values of various peptides:
effect of configuration and amino acid sequence (tested
in aqueous solution at pH 6–7); bi – bitter
Peptide a
Taste
Quality
Intensity b
Gly-Leu bi 19–23
Gly-D-Leu bi 20–23
Gly-Phe bi 15–17
Gly-D-Phe bi 15–17
Leu-Leu bi 4–5
Leu-D-Leu bi 5–6
D-Leu-D-Leu bi 5–6
Ala-Leu bi 18–22
Leu-Ala bi 18–21
Gly-Leu bi 19–23
Leu-Gly bi 18–21
Ala-Val bi 60–80
Val-Ala bi 65–75
Phe-Gly bi 16–18
Gly-Phe bi 15–17
Phe-Gly-Phe-Gly bi 1.0–1.5
Phe-Gly-Gly-Phe bi 1.0–1.5
a L-Configuration if not otherwise designated.
b Recognition threshold value in mmol/l .