5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
87
connective tissue. Serine builds cell membranes as a phospholipid, i.e. phosphatidylserine. Porphyrins (e.g. heme, a cofactor of haemoglobin) are formed, inter alia,
from glycine [19].
Although the view of amino acid function is generally dominated by their role
in humans (or higher animals), it should be stressed that they are also important molecules in bacteria, plants, and animals. their role as signalling molecules (or precursors thereof) seems to be most interesting, especially in an evolutionary context.
An emerging and interesting field of research is focused on the d-amino acids, as
the role of these molecules is now being recognised. In bacteria and some invertebrates, an essential role is played by d-alanine [20–22]. In these organisms, alanine
racemases catalyse the interconversion of d- and L-alanine. In bacteria, d-alanine
is incorporated into bacterial cell walls [20, 21], while in sea invertebrates, it is one
of the major compounds responsible for intracellular osmotic regulation [22]. on
the other hand, fish species eating crustaceans and molluscs containing d-amino acids use oxidases to catalyse the decomposition of these compounds. Free d-alanine
has also been found in the mammalian brain, liver, kidney, blood, and urine [23].
An increase in d-alanine in the brain had been suggested to be associated with the
progression of Alzheimer’s disease [24, 25] and in plasma with renal disease [26].
It has been demonstrated that, in addition to d-alanine, d-aspartic acid, d-serine,
d-leucine, and d-proline are also present in the mouse brain and their degradation
by d-amino acid oxidase has been investigated [27].
Another recognised d-amino acid is d-aspartic acid, present in invertebrate
and vertebrate neuroendocrine tissues, where it carries out important physiological
functions and is implicated in nervous system development [28]. It was recently
shown that d-aspartic acid is a novel endogenous neurotransmitter in distantly related animals, and is present in high concentrations in the synaptic vesicles of axon
terminals [28, 29]. the synthesis of d-aspartic acid occurs in neurons by conversion of L- to d-aspartic acid via d-aspartate racemase, which is the main source of
neuronal d-aspartic acid [30]. Another example of an important d-amino acid is
d-serine, which modulates neurotransmission [31].
For some other important facts about the role of free amino acids in organisms
the reader is referred to the literature and biochemical textbooks [18, 32, 33].
5.3 Amino Acids in Aqueous Solutions
5.3.1 Acid Base Equilibria
A model α-amino acid molecule is composed of an aliphatic skeleton with one carboxy and one amino group which may dissociate in water. Such a simple case means
that the neutral, zwitterion, and anion or cation of an amino acid may coexist, depending on the acidity of the surrounding media and the acid-base characteristics of a given
amino acid (Scheme 1). the equilibria become more and more complicated with every additional functional group present in the molecule. An additional terminal acidic
87
connective tissue. Serine builds cell membranes as a phospholipid, i.e. phosphatidylserine. Porphyrins (e.g. heme, a cofactor of haemoglobin) are formed, inter alia,
from glycine [19].
Although the view of amino acid function is generally dominated by their role
in humans (or higher animals), it should be stressed that they are also important molecules in bacteria, plants, and animals. their role as signalling molecules (or precursors thereof) seems to be most interesting, especially in an evolutionary context.
An emerging and interesting field of research is focused on the d-amino acids, as
the role of these molecules is now being recognised. In bacteria and some invertebrates, an essential role is played by d-alanine [20–22]. In these organisms, alanine
racemases catalyse the interconversion of d- and L-alanine. In bacteria, d-alanine
is incorporated into bacterial cell walls [20, 21], while in sea invertebrates, it is one
of the major compounds responsible for intracellular osmotic regulation [22]. on
the other hand, fish species eating crustaceans and molluscs containing d-amino acids use oxidases to catalyse the decomposition of these compounds. Free d-alanine
has also been found in the mammalian brain, liver, kidney, blood, and urine [23].
An increase in d-alanine in the brain had been suggested to be associated with the
progression of Alzheimer’s disease [24, 25] and in plasma with renal disease [26].
It has been demonstrated that, in addition to d-alanine, d-aspartic acid, d-serine,
d-leucine, and d-proline are also present in the mouse brain and their degradation
by d-amino acid oxidase has been investigated [27].
Another recognised d-amino acid is d-aspartic acid, present in invertebrate
and vertebrate neuroendocrine tissues, where it carries out important physiological
functions and is implicated in nervous system development [28]. It was recently
shown that d-aspartic acid is a novel endogenous neurotransmitter in distantly related animals, and is present in high concentrations in the synaptic vesicles of axon
terminals [28, 29]. the synthesis of d-aspartic acid occurs in neurons by conversion of L- to d-aspartic acid via d-aspartate racemase, which is the main source of
neuronal d-aspartic acid [30]. Another example of an important d-amino acid is
d-serine, which modulates neurotransmission [31].
For some other important facts about the role of free amino acids in organisms
the reader is referred to the literature and biochemical textbooks [18, 32, 33].
5.3 Amino Acids in Aqueous Solutions
5.3.1 Acid Base Equilibria
A model α-amino acid molecule is composed of an aliphatic skeleton with one carboxy and one amino group which may dissociate in water. Such a simple case means
that the neutral, zwitterion, and anion or cation of an amino acid may coexist, depending on the acidity of the surrounding media and the acid-base characteristics of a given
amino acid (Scheme 1). the equilibria become more and more complicated with every additional functional group present in the molecule. An additional terminal acidic