Amino acids are the monomer building blocks of proteins. Roughly 20 different amino acids occur naturally, and every one shares the same basic skeleton: a central (alpha-) carbon bonded to an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a variable side chain (the 'R group') that gives each amino acid its distinct chemical identity. Because an amino acid carries both an acidic group and a basic group, it is amphoteric - able to act as an acid or a base - and typically exists as a zwitterion (a dipolar ion with both a positive and a negative charge, and zero net charge) at its isoelectric point. Based on the chemical nature of the R group, amino acids are classified as acidic, basic, polar (uncharged) or non-polar; among the basic amino acids, arginine's guanidinium side chain gives it the highest s …