Isoelectric point
The isoelectric point (pI) is the pH value at which the molecule carries no electrical charge. The concept is particularly important for zwitterionic molecules such as amino acids, peptides, and proteins. For an amino acid, the isoelectric point is the average of pKa values for the amine and the carboxyl group. In the case of amino acids with multiple groups ionizable in solution (e.g., lysine with two amino groups or aspartic acid with two acid groups), the isoelectric point is given by the average of the two pKa of the acid and base that lose/gain a proton from the neutral form of the amino acid. This can be extended to the definition of pI of peptides and proteins. The pI value can be used to indicate the global basic or acidic character of a zwitterionic molecule, and compounds with pI > 7 can be considered basic, and those with pI < 7 can be considered acidic. For complex molecules such as proteins, the isoelectric point is useful in the description of acidic or basic character, where individual pKa values are not relevant.
