<P> The pI value can affect the solubility of a molecule at a given pH . Such molecules have minimum solubility in water or salt solutions at the pH that corresponds to their pI and often precipitate out of solution . Biological amphoteric molecules such as proteins contain both acidic and basic functional groups . Amino acids that make up proteins may be positive, negative, neutral, or polar in nature, and together give a protein its overall charge . At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge . Proteins can, thus, be separated by net charge in a polyacrylamide gel using either preparative gel electrophoresis, which uses a constant pH to separate proteins or isoelectric focusing, which uses a pH gradient to separate proteins . Isoelectric focusing is also the first step in 2 - D gel polyacrylamide gel electrophoresis . </P> <P> In biomolecules, proteins can be separated by ion exchange chromatography . Biological proteins are made up of zwitterionic amino acid compounds; the net charge of these proteins can be positive or negative depending on the pH of the environment . The specific pI of the target protein can be used to model the process around and the compound can then be purified from the rest of the mixture . Buffers of various pH can be used for this purification process to change the pH of the environment . When a mixture containing a target protein is loaded into an ion exchanger, the stationary matrix can be either positively - charged (for mobile anions) or negatively - charged (for mobile cations). At low pH values, the net charge of most proteins in the mixture is positive - in cation exchangers, these positively - charged proteins bind to the negatively - charged matrix . At high pH values, the net charge of most proteins is negative, where they bind to the positively - charged matrix in anion exchangers . When the environment is at a pH value equal to the protein's pI, the net charge is zero, and the protein is not bound to any exchanger, and therefore, can be eluted out . </P> <P> For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the mean of the pKas of this molecule . </P> <Dl> <Dd> p I = p K a 1 + p K a 2 2 (\ displaystyle \ mathrm (pI) = (\ frac (\ mathrm (p) K_ (\ mathrm (a1)) + \ mathrm (p) K_ (\ mathrm (a2))) (2))) </Dd> </Dl>

What is the difference between pka and pi