Natural Product Biomolecules - Organic Chemistry Concepts: An EFL Approach - page 161

largely a combination of the reactions of the two individual functional groups.
For example, the carboxylic group can be esterified by a mechanism discussed
in Section 7.8.2. The amino group reacts with acyl components to give amides.
This is the same as explained in Section 7.8.3. However, the physical properties of
amino acids are more like salts rather than uncharged molecules. See Figure 8.17 .
FIGURE 8.17
General α -amino acid zwitterion structure.
Amino acids are colorless solids with melting points >200 °C. They are usually
soluble in aqueous medium, especially acidic or basic solutions. However, they
show a lower acidity than simple carboxylic acids and a lower basicity than sim-
ple amines. In acidic medium the cation is the major species. In basic medium
the anion is the major species.
For every amino acid there is an intermediate pH at which there is only the zwitterion.
The zwitterion is a form in which there is both a cation and an anion. These charges
cancel so that the zwitterion is overall neutral. This isoelectric point, pI, is a physical
property of each amino acid and is the point at which the amino acid is least soluble.
Certain amino acids are essential components of all living cells. Peptides usually have
less than 50 amino acids. Proteins are large polypeptides that have more than 50
amino acids. Both peptides and proteins can be seen as polymers of amino acids. In
these, the amino acid units are joined by peptide amide bonds as seen in Figure 8.18 .
FIGURE 8.18
Typical peptide ( amide ) bonds between amino acids.
Amino acids can have a chirality center and stereoisomers can occur. However,
all natural examples belong to the same stereochemical series and are in an opti-
cally pure state as only one of the enantiomers.
The peptide bond is the strongest and most important bond in proteins. The