Classification of Amino Acids

Amino acids are classified according to the chemical nature of their R group (side chain) into four broad categories: acidic (side chain carries an extra carboxyl group, giving the amino acid a net negative charge at physiological pH), basic (side chain carries an extra amino or similar group, giving a net positive charge), polar (side chain is uncharged but contains electronegative atoms that make it hydrophilic) and non-polar (side chain is a simple hydrocarbon group, making it hydrophobic). This R-group classification predicts much of how an amino acid behaves within a folded protein - for instance, non-polar amino acids tend to cluster in a protein's hydrophobic interior, away from water. Two amino acids join together when the amino group of one reacts with the carboxyl group of another, releasing a water molecule and forming a peptide bond; the resulting two-amino-acid unit is called a dipeptide. Long chains of many amino acids linked by pe …