Amino acids and proteins

Compounds that contain both an amino group –NH₂ and a carboxyl group –COOH are amino acids; the general formula is H₂N–CH(R)–COOH. The simplest are glycine H₂N–CH₂–COOH (aminoethanoic acid) and alanine CH₃–CH(NH₂)–COOH. Having two groups, they are amphoteric: they react with acids (H₂N–CH₂–COOH + HCl → ClH₃N–CH₂–COOH) and with alkalis (H₂N–CH₂–COOH + NaOH → H₂N–CH₂–COONa + H₂O) and form esters with alcohols. When two amino acids join, water is lost between the –COOH of one and the –NH₂ of the other, forming a peptide bond –CO–NH–: glycine + alanine → dipeptide + H₂O; this is polycondensation. Many amino acid residues make up proteins; natural proteins are built from about 20 α-amino acids, some of which the body cannot synthesize and must obtain from food (essential amino acids). Proteins act as enzymes, building material, antibodies and more. Hydrolysis breaks a protein down into amino acids. Heating, strong acid, alkali and heavy-metal salts denature a protein. Qualitative tests: the biuret test (violet colour with alkali and a little copper(II) sulfate) and the xanthoprotein test (yellow with concentrated HNO₃).