Food Chemistry

36 1 Amino Acids, Peptides, Proteins

acids:

(1.74)

The first three letters of the amino acids
are used as symbols to simplify desig-
nation 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 prefixD-. 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 il-
lustration along with its corresponding disulfide,
oxidized glutathione:

(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 PhysicalProperties

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 1.13. 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 SensoryProperties

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

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

Peptidea Taste

Quality Intensityb

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

aL-Configuration if not otherwise designated.

bRecognition threshold value in mmol/l.