Final answer:
Glycine is the only one of the 20 amino acids that is not chiral; the remaining 19 amino acids are chiral, with different R groups determining their chemical properties. Most of these chiral amino acids are in the L-form with an (S) configuration, except for cysteine, which is (R).
Step-by-step explanation:
Not all of the 20 amino acids present in the human body are chiral. Except for glycine, all the other amino acids possess at least one chiral center, which is the α-carbon atom to which four different groups are attached, making them chiral or optically active.
The side chains (R groups) of these amino acids determine their specific chemical properties, such as whether they are acidic, basic, polar or nonpolar. For example, valine, methionine, and alanine are nonpolar, while serine, threonine, and cysteine are polar.
Furthermore, most proteinogenic amino acids are in the L-form and have an (S) absolute configuration at the αC, the exception being cysteine, which is (R). However, glycine does not exhibit chirality due to having two hydrogen atoms attached to its α-carbon. This uniqueness among amino acids results from the lack of four different groups being bound to its α-carbon, hence glycine is called achiral.