The<unk>  <unk>  is the pH at which a particular molecule or surface carries no net electrical charge. Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the molecule. They are affected by pH of their surrounding environment and can become more positively or negatively charged due to the loss or gain of protons (H+). A molecule's<unk>  can affect its solubility at a certain pH. Such molecules have minimum solubility in water or salt solutions at the pH which corresponds to their<unk>  and are often seen to precipitate out of solution.  Biological amphoteric molecules such as proteins contain both acidic and basic functional groups. Amino acids which 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<unk> , proteins carry a net positive charge. Above their<unk>  they carry a net negative charge. Proteins can be separated according to their<unk>  (overall charge) on a polyacrylamide gel using a technique called<unk>  focusing. This technique utilises a pH gradient to separate proteins.<unk>  focusing is also the first step in preforming 2-D gel polyacrylamide gel electrophoresis.
For an amino acid with only one amine and one carboxyl group, the<unk>  can be calculated from the pKas of this molecule.
For amino acids with more than two ionizable groups, such as lysine, the same formula is used, but this time the two pKa's used are those of the two groups that lose and gain a charge from the neutral form of the amino acid.  Lysine has a single carboxylic pKa and two amine pKa values (one of which is on the R-group), so fully protonated lysine has a +2 net charge. To get a neutral charge, we must deprotonate the lysine twice , and therefore use the R-group and amine pKa values (found at List of standard amino acids).
However, a more exact treatment of this requires advanced acid/base knowledge and calculations.
Proteins can be separated according to their<unk>  in a process known as<unk>  focusing.
At a pH below the<unk> , proteins carry a net positive charge.  Above the<unk>  they carry a net negative charge.  This has implications for running electrophoretic gels (see Agarose gel electrophoresis).  The pH of an electrophoretic gel is determined by the buffer used for that gel.  If the pH of the buffer is above the<unk>  of the protein being run, the protein will migrate to the positive pole (negative charge is attracted to a positive pole).  If the pH of the buffer is below the<unk>  of the protein being run, the protein will migrate to the negative pole of the gel (positive charge is attracted to the negative pole).  If the protein is run with a buffer pH that is equal to the<unk> , it will not migrate at all. This is also true for individual amino acids.
Isoelectric point
Extraction ratio
Electrochemical gradient
Polarization density
Mass versus weight
Resting potential
Molecular mass
Spin quantum number