The acidic carboxyl group and basic amine group of the amino acid can react and form an internal salt called a zwitterion. The carboxyl group donates a proton to the amine group. The zwitterion has no overall charge as the negative COO- and positive NH3+ groups cancel each other out.The isoelectric point is the pH at which there is no overall charge. At a pH more acidic than the isoelectric point, the amino acid is positive and acts as a base. At a pH more basic than the isoelectric point, the amino-acid acts as an acid and is negative. Many amino-acids have their isoelectric point around pH 6.