J. C. Dobrowolsk et al.
88
Cooh group is found in aspartic and glutamic acids, an additional terminal amide
group occurs in asparagine and glutamine, an extra terminal amine group is present in
lysine, additional aliphatic oh groups appear in serine and threonine, while phenolic
oh is present in tyrosine. moreover, an additional Nh group may be incorporated
into a bicyclic aromatic system, such as in tryptophan, or in an apparently simple fivemembered imidazole ring (allowing for additional tautomeric equilibria), as in histidine. obviously, the Sh, SCh 3 , and aromatic ring groups, as in cysteine, methionine,
and phenylalanine, respectively, change the complexity of equilibria in water as well.
Furthermore, some parts of amino acid molecules are hydrophobic and force specific
conformations to be populated in water. Last but not least, amino acid molecules are
flexible molecules and usually exhibit a fascinatingly rich conformational landscape.
All these complex problems must be taken into account when one aims to measure
the (chiroptical) vibrational spectra of single, free amino acid molecules in aqueous
solution and/or to model them by quantum chemical methods.
Let us now provide some basic facts about α-amino acid molecules in water,
their acid-base equilibria, hydrophobic-hydrophilic character and solubility. First,
note that acetic acid and ammonia, model molecules for the two functional groups
of amino acids, exhibit dissociation constants of similar values:
Scheme 1 Scheme of the simplest acid base equilibria occuring for an α amino acid
molecule in aqueous solution. I—isoelectric point, R side chain
This makes α-amino acid molecules in water capable of forming a dipolar ion by
the transfer of a proton from the carboxy to the amino group. then, the Coo
−
and
Nh 3
+
ionic moieties are simultaneously present in the molecule, forming a zwitO
OH
R
NH 2
O
O
R
NH 3 +
O
OH
R
NH 3 +
O
O
R
NH 2
aq
H +
OH -
OH -
H +
K 1
K 2
I
]
[
]
][
[
1
+
+
= A
H
Z
K
]
[
]
][
[
2
Z
H
A
K
+
−
=
2
2
1
pK
pK
pI
+
=
CH 3 COOH + H 2 O ↔ CH 3 COO - + H 3 O
+
K a ≈ 1.8∙10
-5
NH 3 +H 2 O ↔ NH 4
+ + OH -
K b ≈ 1.8∙10
-5
88
Cooh group is found in aspartic and glutamic acids, an additional terminal amide
group occurs in asparagine and glutamine, an extra terminal amine group is present in
lysine, additional aliphatic oh groups appear in serine and threonine, while phenolic
oh is present in tyrosine. moreover, an additional Nh group may be incorporated
into a bicyclic aromatic system, such as in tryptophan, or in an apparently simple fivemembered imidazole ring (allowing for additional tautomeric equilibria), as in histidine. obviously, the Sh, SCh 3 , and aromatic ring groups, as in cysteine, methionine,
and phenylalanine, respectively, change the complexity of equilibria in water as well.
Furthermore, some parts of amino acid molecules are hydrophobic and force specific
conformations to be populated in water. Last but not least, amino acid molecules are
flexible molecules and usually exhibit a fascinatingly rich conformational landscape.
All these complex problems must be taken into account when one aims to measure
the (chiroptical) vibrational spectra of single, free amino acid molecules in aqueous
solution and/or to model them by quantum chemical methods.
Let us now provide some basic facts about α-amino acid molecules in water,
their acid-base equilibria, hydrophobic-hydrophilic character and solubility. First,
note that acetic acid and ammonia, model molecules for the two functional groups
of amino acids, exhibit dissociation constants of similar values:
Scheme 1 Scheme of the simplest acid base equilibria occuring for an α amino acid
molecule in aqueous solution. I—isoelectric point, R side chain
This makes α-amino acid molecules in water capable of forming a dipolar ion by
the transfer of a proton from the carboxy to the amino group. then, the Coo
−
and
Nh 3
+
ionic moieties are simultaneously present in the molecule, forming a zwitO
OH
R
NH 2
O
O
R
NH 3 +
O
OH
R
NH 3 +
O
O
R
NH 2
aq
H +
OH -
OH -
H +
K 1
K 2
I
]
[
]
][
[
1
+
+
= A
H
Z
K
]
[
]
][
[
2
Z
H
A
K
+
−
=
2
2
1
pK
pK
pI
+
=
CH 3 COOH + H 2 O ↔ CH 3 COO - + H 3 O
+
K a ≈ 1.8∙10
-5
NH 3 +H 2 O ↔ NH 4
+ + OH -
K b ≈ 1.8∙10
-5