Both acidic (carboxyl)
as well as basic (amino) groups are present in the same molecule of
amino acids. In aqueous solutions, the carboxyl group can lose a
proton and the amino group can accept a proton, thus giving rise to a
dipolar ion known as a zwitter ion.
Due to this dipolar
behaviour, they have strong electrostatic interactions within them
and with water. But halo-acids do not exhibit such dipolar behaviour.
For this reason, the
melting points and the solubility of amino acids in water is higher
than those of the corresponding halo-acids.