Amino acids are dipolar in nature (+NH3-CHR-COO-) and have strong dipolar interaction. Amino acids have amino (basic) group and carboxylic (acidic) group. A proton which comes from carboxylic group is accepted by amino group. Thus amino acid exists in the form of Zwitter ion.
Due to this ionic form, amino acids become crystalline solids and due to this salt like structure, show higher melting point.