J. C. Dobrowolsk et al.
90
5.3.3 Hydrophilicity vs. Hydrophobicity
the other factor accounting for both the solubility of amino acids and the complexity of the acid-base equilibria of amino acids in water solution is the hydrophobicity
vs. hydrophilicity of the side chain. this property of amino acids is extremely important in biology. hydrophobic forces drive the formation of secondary, tertiary, and
quaternary protein structures. the character of side chains plays an important role
in protein location at interfaces or in the transmembrane space. It also influences the
behaviour of individual amino acids in solution as well as the structure of water [39].
According to the IuPAC gold Book definition [40], hydrophobicity is the association of non-polar groups or molecules in an aqueous environment which arises
from the tendency of water to exclude non-polar molecules. the origin of the effect
lies in the disruption of the hydrogen bond network between molecules of water
Table 5.1 α-Amino acids: dissociation constants and isoelectric points at 25 °C and solubility in
water at different temperatures (g/100 g). [34]
Amino
dissociation constants
Solubility in water, temp.
°
C
Acid
pK 1
pK 2
pK 3
pK 4
pI
0
25
50
75
100
Alanine
2.34 9.69
6.0
12.73 16.51
21.79 28.51 37.30
Arginine
2.18 9.09
12.60
10.8
18.1 [35]
Asparagine 2.02 8.80
5.4
2.51 [35]
Aspartic
acid
1.88 3.65
9.60
2.8
0.209 0.500
1.199 2.875 6.893
Cysteine
1.71 8.35
10.66
5.0
vg [35]
Cystine
1.04 2.10
8.02 8.71 5.1
0.011 [35]
glutamine 2.17 9.13
5.7
4.2 [35]
glutamic
acid
2.19 4.25
9.67
3.2
0.341 0.843
2.186 5.532 14.00
glycine
2.34 9.60
6.0
14.18 24.99
39.10 54.39 67.17
histidine
1.80 5.99
9.07
7.5
–
4.29
–
–
–
4-hydroxyproline
1.82 9.65
5.7
28.86 36.11
45.18 51.67 –
Isoleucine 2.36 9.68
6.0
3.791 4.117
4.818 6.076 8.255
Leucine
2.36 9.60
6.0
2.270 2.19
2.66 3.823 5.638
Lysine
2.20 8.90
10.28
9.6
0.58 [35]
methionine 2.28 9.21
5.7
1.818 3.381
6.070 10.52 17.60
Phenylalanine
1.83 9.13
5.5
1.983 2.965
4.431 6.624 9.900
Proline
1.99 10.60
6.3
127.4 162.3
206.7 239.0 –
Serine
2.21 9.15
5.7
2.204 5.023
10.34 19.21 32.24
threonine 2.15 9.12
5.6
9.81 [35]
tryptophan 2.38 9.39
5.9
0.823 1.136
1.706 2.795 4.987
tyrosine
2.20 9.11
10.07
5.7
0.020 0.045
0.105 0.244 0.565
valine
2.32 9.62
6.0
8.34
8.85
9.62 10.24 –
vg very good
90
5.3.3 Hydrophilicity vs. Hydrophobicity
the other factor accounting for both the solubility of amino acids and the complexity of the acid-base equilibria of amino acids in water solution is the hydrophobicity
vs. hydrophilicity of the side chain. this property of amino acids is extremely important in biology. hydrophobic forces drive the formation of secondary, tertiary, and
quaternary protein structures. the character of side chains plays an important role
in protein location at interfaces or in the transmembrane space. It also influences the
behaviour of individual amino acids in solution as well as the structure of water [39].
According to the IuPAC gold Book definition [40], hydrophobicity is the association of non-polar groups or molecules in an aqueous environment which arises
from the tendency of water to exclude non-polar molecules. the origin of the effect
lies in the disruption of the hydrogen bond network between molecules of water
Table 5.1 α-Amino acids: dissociation constants and isoelectric points at 25 °C and solubility in
water at different temperatures (g/100 g). [34]
Amino
dissociation constants
Solubility in water, temp.
°
C
Acid
pK 1
pK 2
pK 3
pK 4
pI
0
25
50
75
100
Alanine
2.34 9.69
6.0
12.73 16.51
21.79 28.51 37.30
Arginine
2.18 9.09
12.60
10.8
18.1 [35]
Asparagine 2.02 8.80
5.4
2.51 [35]
Aspartic
acid
1.88 3.65
9.60
2.8
0.209 0.500
1.199 2.875 6.893
Cysteine
1.71 8.35
10.66
5.0
vg [35]
Cystine
1.04 2.10
8.02 8.71 5.1
0.011 [35]
glutamine 2.17 9.13
5.7
4.2 [35]
glutamic
acid
2.19 4.25
9.67
3.2
0.341 0.843
2.186 5.532 14.00
glycine
2.34 9.60
6.0
14.18 24.99
39.10 54.39 67.17
histidine
1.80 5.99
9.07
7.5
–
4.29
–
–
–
4-hydroxyproline
1.82 9.65
5.7
28.86 36.11
45.18 51.67 –
Isoleucine 2.36 9.68
6.0
3.791 4.117
4.818 6.076 8.255
Leucine
2.36 9.60
6.0
2.270 2.19
2.66 3.823 5.638
Lysine
2.20 8.90
10.28
9.6
0.58 [35]
methionine 2.28 9.21
5.7
1.818 3.381
6.070 10.52 17.60
Phenylalanine
1.83 9.13
5.5
1.983 2.965
4.431 6.624 9.900
Proline
1.99 10.60
6.3
127.4 162.3
206.7 239.0 –
Serine
2.21 9.15
5.7
2.204 5.023
10.34 19.21 32.24
threonine 2.15 9.12
5.6
9.81 [35]
tryptophan 2.38 9.39
5.9
0.823 1.136
1.706 2.795 4.987
tyrosine
2.20 9.11
10.07
5.7
0.020 0.045
0.105 0.244 0.565
valine
2.32 9.62
6.0
8.34
8.85
9.62 10.24 –
vg very good