The amino acids exist as zwitter ions, $H_3N^{+} —OHR—COO—$. 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 tothis salt like character, they interact strongly with $H_2O$. Thus, solubility of amino acids in water is higher than that of the corresponding halo acids which do not have salt like character.