At isoelectric point, the molecule doesn´t move in an electric field (for it´s net charge 0). Net charge of proteins is dependent of pH. Here´s the order of protonation/deprotonation depending in which direction the pH moves: This is the case at pI, where the negative (carboxyl group) and positive (amino group) charge cancel out. There´s an in-between-state, where there is still one proton attached to the amino group and already one proton missing at the carboxylgroup. => The amino acid is negatively charged in alkaline solutions.If you put the same amino acid in an alkaline environment, protons will be taken away from the amino acid: => The amino acid becomes positively charged in low pH.-COOH is already fully protonated and does change.If you put amino acids (for easier understanding assume a neutral side chain) in an acidic environment (coming from neutral pH), an additional proton will be added to the amino acid: pI = 6,0), but some other proteins have a pI of larger than 10 (histone proteins) or below 1 (Pepsin). The pI is for some amino acids around the physiological pH of around 7 (for Glycin e.g. To find the isoelectric point, you add the two pK values and divide them by two.Īt the pH that equals the isoelectric point (pI), the amino acids are so-called “Zwitterions”, meaning, they are electrically neutral with groups in the form of -NH 3 + and –COO. The amino group can accept a proton and acts as base, while the carboxylic group can donate one and acts as acid (this depends on the pH value).Įach of those groups has its own pK value. In biochemistry are widely spread example of amphoteric molecules are proteinogenic amino acids, since they have at least an acidic carboxylic group and an alkaline amino group. Thus: All amphiprotic (proton needed!) substances are amphoteric (substances must act as acid/base – no proton required), but not vice versa! Definition of “amphoprotic”: An „amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base”.Amphoteric substances are called ampholytes. The word roots in greek language meaning “both”. Definition of “amphoteric”: An „amphoteric species is a molecule or ion that can react as an acid as well as a base”.1.4 Examples of biochemical important ampholytesĪmpholytes, Isoelectric Point and Biochemical Examples Amphoteric and Amphoprotic.1 Ampholytes, Isoelectric Point and Biochemical Examples.

Calculate pi of lysine