They are known as 2-, alpha-, or Î±-amino acids (generic formula H 2 NCHRCOOH in most cases, where R is an organic substituent known as a  side-chain ); often the term amino acid is used to refer specifically to these.mino acids are usually classified by the properties of their side-chain into four groups. The side-chain can make an amino acid a weak acid or a weak base, and a hydrophile if the side-chain is polar or a hydrophobe if it is nonpolar.