NH3CHRCOO

NH3CHRCOO is a shorthand chemical representation used to depict the zwitterionic form of an amino acid. In this schematic, the amino group is protonated (often written as NH3+) and the carboxyl group is deprotonated (COO−), attached to a central alpha carbon that bears a side chain denoted by R. The R group can be any of the standard amino acid side chains, such as hydrogen (glycine), methyl (alanine), or more complex substituents.

In solution, amino acids typically exist as zwitterions at physiologic pH. The common form is NH3+-CHR-COO−, reflecting protonation states: the amino group has a pKa around 9–10 and the carboxyl group a pKa around 2. At low pH both groups tend to be protonated (NH3+-CH-COOH), while at high pH both are deprotonated (NH2-CHR-COO−). The pH-dependent balance gives rise to the isoelectric point, where the molecule carries no net charge.

The zwitterionic form is central to amino acid chemistry, influencing solubility, reactivity, and interactions in proteins. It facilitates electrostatic interactions and hydrogen bonding that underpin protein folding and stability. The generic NH3CHRCOO representation is widely used in biochemical notation and reaction schemes to illustrate amino acids without specifying a particular side chain. It is often contrasted with the non-ionic form H2N-CHR-COOH found under non-aqueous or extreme conditions.