The amino acids exist as zwitter ions, H3N+—OHR—COO—. Because of this dipolar salt like character they have strong dipole-dipole attractions. So, their melting points are higher than halo acids which do not have sail like character. Moreover, due to this salt like character, they interact strongly with H2O. Thus, solubility of amino acids in water is higher than that of the corresponding halo acids which do not have salt like character.