The α-carbon, with its four different groups, is a chiral (stereogenic) center. Glycine, the simplest
amino acid, is an exception to this rule because its R group is a hydrogen atom. All naturally
occurring amino acids in eukaryotes—except for glycine—are optically active, and all are L-isomers.
Therefore, by convention, the Fischer projection for an amino acid is drawn with the amino group
on the left, as shown in Figure 10.2. L-amino acids have (S) configurations, except for cysteine, which
is (R) because of the change in priority caused by the sulfur in its R group.
Figure 10.2. L- and D-Amino AcidsPROPERTIES
Amino acids, with their acidic carboxyl group and basic amino group, are amphoteric molecules.
That is, they can act as both acids and bases. Amino groups can take on a positive charge by being
protonated, and carboxyl groups can take on negative charges by being deprotonated. When an
amino acid is put into solution, it will take on both of these charges, forming a dipolar ion or
zwitterion, as shown in Figure 10.3. How an amino acid acts depends on the pH of the environment.
In basic solutions, the amino acid can become fully deprotonated; in acidic solutions, it can become
fully protonated.
Figure 10.3. Amino Acids Exist as Zwitterions (Dipolar Ions) at Neutral pHBRIDGE
Amino acids are amphoteric molecules, just like water—they can act as both acids and bases.
These acid–base characteristics (and titrations of amino acids) are discussed thoroughly in