Amino acids react with each other in a typical acid-base neutralization
reaction to form a salt.
The reaction is simply the transfer of the -H (positive
ion) from the acid to the amine and the attraction of the positive
and negative charges. The acid group becomes negative, and
the amine nitrogen becomes positive because of the positive hydrogen
For example in the graphic on the left - top, glycine (gly)
and alanine (ala) may just interact in the zwitterion form by
an attraction of the positive (amine) of the alanine and negative
(carboxyl acid) charges to form the salt.
Salt formation of Side Chains:
A more important interaction for protein tertiary structure
is the interaction of the acid and base "side chains".
If the amino acid has an extra acid or amine on the "side
are used in the salt formation.
For example in the left -bottom graphic, Aspartic acid (asp)
has a side chain that forms a salt with the amine on the lysine
(lys) side chain. The hydrogen ion (red) moves to the amine nitrogen
resulting in the salt with the attraction of the positive and