Cysteine's R-group contains a sulfhydryl group (-SH), which can participate in the formation of a disulfide bridge in a protein's tertiary and/or quaternary structure. Cysteine is the only amino acid to contain a sulfur atom.

The isoelectric point is the average of the two pKa values around which the molecule has an overall neutral charge. In the case of glutamate, the amino acid is in its neutral (zwitterion) form in between the lower two pKa values. Taking the average of the two values gives the pI.

Cysteine is an unusual amino acid. Although each of the normal biological amino acids have the L configuration (with the exception of glycine, which is achiral), meaning they can be drawn with the Fischer projection as shown:

the fact that cysteine contains a sulfur group makes the side chain a higher priority than the carboxylic acid. Therefore, the absolute configuration of cysteine will be R, and not S like the other amino acids.

The isoelectric point is the average of the two pKa values around which the molecule has an overall neutral charge. In the case of lysine, the amino acid is in its neutral (zwitterion) form in between the higher two pKa values. Taking the average of the two values gives the pI.

Treatment of a peptide with Edman's reagent () will remove the N-terminal amino acid. The PTH derivative will look as follows:

In the molecule in the question, a sec-butyl replaces the R group. This is the amino acid isoleucine.

First step: Deprotonation of alpha hydrogen and formation of the malonic ester enolate.

Second step: bimolecular nucleophilic substitution, resulting in addition of the isopropyl group to the alpha carbon.

Third step: hydrolysis of amide and ester bonds, followed by decarboxylation

The synthesis forms valine, because the alkyl halide that is attacked is an isopropyl group (Valine's side chain is an isopropyl group). A more detailed mechanism is shown below.

The nine essential amino acids are valine, leucine, isoleucine, histidine, phenylalanine, tryptophan, methionine, threonine, and lysine. These amino acids must be consumed in the diet, since they cannot be synthesized by adult humans.

The only achiral amino acid is Glycene. Glycene's side chain is simply a hydrogen. Because a hydrogen already exists on the fundamental structure of an amino acid backbone, a side chain of a single hydrogen atom causes glycene to be achiral.

Lysine's R-group includes an amine (-NH2), which can be ionized by picking up a hydrogen. This ability classifies lysine as basic.