However, a more exact treatment of this requires advanced acid/ base knowledge and calculations. Isoelectric focusing is also the first step in preforming 2-D gel polyacrylamide gel electrophoresis.įor an amino acid with only one amine and one carboxyl group, the pI can be calculated from the pKas of this molecule. This technique utilises a pH gradient to separate proteins. Proteins can be separated according to their isoelectric point (overall charge) on a polyacrylamide gel using a technique called isoelectric focusing. Above their pI they carry a net negative charge.
At a pH below their pI, proteins carry a net positive charge. Amino acids which make up proteins may be positive, negative, neutral or polar in nature, and together give a protein its overall charge. Biological amphoteric molecules such as proteins contain both acidic and basic functional groups. Such molecules have minimum solubility in water or salt solutions at the pH which corresponds to their pI and are often seen to precipitate out of solution. A molecule's pI can affect its solubility at a certain pH. 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 +). Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the molecule. The isoelectric point (pI) is the pH at which a particular molecule or surface carries no net electrical charge. 3 Isoelectric Point versus Point of Zero Charge.2.1 Examples of isoelectric points for various materials.2 Isoelectric Point of Ceramic Materials.List of terms related to Isoelectric point Risk calculators and risk factors for Isoelectric pointĬauses & Risk Factors for Isoelectric point US National Guidelines Clearinghouse on Isoelectric pointĭirections to Hospitals Treating Isoelectric point Ongoing Trials on Isoelectric point at Clinical Ĭlinical Trials on Isoelectric point at Google For arginine, the similar acids are the guanidinium species on the side-chain (pK a = 12.5) and the alpha-ammonium function (pK a = 9.0), so the calculated pI = (12.5 + 9.0)/2 = 10.Most recent articles on Isoelectric pointĪrticles on Isoelectric point in N Eng J Med, Lancet, BMJĬochrane Collaboration on Isoelectric point In the case of aspartic acid, the similar acids are the alpha-carboxyl function (pK a = 2.1) and the side-chain carboxyl function (pK a = 3.9), so pI = (2.1 + 3.9)/2 = 3.0. The second includes acids that are positively charged in their protonated state (e.g. The first consists of acids that are neutral in their protonated form (e.g. To assist in determining similarity we define two classes of acids. If additional acidic or basic groups are present as side-chain functions, the pI is the average of the pK a's of the two most similar acids. Thus, the pI for alanine is calculated to be: (2.34 + 9.69)/2 = 6.02, the experimentally determined value. For simple amino acids such as alanine, the pI is an average of the pK a's of the carboxyl (2.34) and ammonium (9.69) groups. In other words, the positively charged groups are exactly balanced by the negatively charged groups. The isoelectric point, pI, is the pH of an aqueous solution of an amino acid (or peptide) at which the molecules on average have no net charge. The very high pH required to remove the last acidic proton from arginine reflects the exceptionally high basicity of the guanidine moiety at the end of the side chain. Formulas for these species are written to the right of the titration curves, together with the pH at which each is expected to predominate. For each of these compounds four possible charged species are possible, one of which has no overall charge.
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