In an amino acid the -COOH group is acidic and the -NH2 group is basic. Within one molecule, the carboxylic acid group can donate H+ to the amine group:H2N-CHR-COOH → +H3N-CHR-COO− 
The product is a zwitterion: a species with both a positive and a negative charge in the same molecule, but no overall charge.In the solid state, and in aqueous solution at a particular pH called the isoelectric point, amino acids exist mainly as zwitterions. Each amino acid has its own isoelectric point, which depends on its side chain; for amino acids with neutral side chains it is close to pH 6.Zwitterions explain the unusual physical properties of amino acids:
• High melting points: the solid is held together by strong ionic attractions between the -NH3+ of one zwitterion and the -COO− of its neighbours, rather than by weaker hydrogen bonds. Glycine (Mr 75) melts, with decomposition, at over 200 °C, whereas ethanoic acid (Mr 60) boils at 118 °C.
• Solubility: amino acids dissolve well in water, a polar solvent, but are insoluble in non-polar solvents such as hexane.Worked example. Draw the zwitterion of alanine, CH3CH(NH2)COOH. Move one H+ from the -COOH group to the -NH2 group: CH3CH(NH3+)COO−.
The product is a zwitterion: a species with both a positive and a negative charge in the same molecule, but no overall charge.In the solid state, and in aqueous solution at a particular pH called the isoelectric point, amino acids exist mainly as zwitterions. Each amino acid has its own isoelectric point, which depends on its side chain; for amino acids with neutral side chains it is close to pH 6.Zwitterions explain the unusual physical properties of amino acids:• High melting points: the solid is held together by strong ionic attractions between the -NH3+ of one zwitterion and the -COO− of its neighbours, rather than by weaker hydrogen bonds. Glycine (Mr 75) melts, with decomposition, at over 200 °C, whereas ethanoic acid (Mr 60) boils at 118 °C.
• Solubility: amino acids dissolve well in water, a polar solvent, but are insoluble in non-polar solvents such as hexane.Worked example. Draw the zwitterion of alanine, CH3CH(NH2)COOH. Move one H+ from the -COOH group to the -NH2 group: CH3CH(NH3+)COO−.