isoelectric point

Examples of isoelectric point in the following topics:

• The Effect of pH on Solubility

  • The pH at which the net charge is neutral is called the isoelectric point, or pI (sometimes abbreviated to IEP).
  • Proteins can therefore be separated according to their isoelectric point.
  • In a method called isoelectric focusing, proteins are run through a gel that has a pH gradient.
  • For example, a protein that is in a pH region below its isoelectric point will be positively charged and so will migrate towards the cathode (negative charge).
  • At this point, it has no net charge, and so it stops moving in the gel.

• α-Amino Acids

  • All three compounds are soluble in organic solvents (e.g. ether) and have relatively low melting points.
  • These differences all point to internal salt formation by a proton transfer from the acidic carboxyl function to the basic amino group.
  • At intermediate pH's the zwitterion concentration increases, and at a characteristic pH, called the isoelectric point (pI), the negatively and positively charged molecular species are present in equal concentration.
  • The isoelectric points range from 5.5 to 6.2.
  • As defined above, 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.

• Peptides & Proteins

  • As expected, the free amine and carboxylic acid functions on a peptide chain form a zwitterionic structure at their isoelectric pH.

• Boiling Point Elevation

  • The boiling point of a solvent is elevated in the presence of solutes.
  • This is referred to as boiling point elevation.
  • The extent of the boiling point elevation can be calculated.
  • In this equation, $\Delta T_b$ is the boiling point elevation, $K_b$ is the boiling point elevation constant, and m is the molality of the solution.
  • The boiling point of a pure liquid.

• Freezing Point Depression

  • Freezing point depression is the phenomena that describes why adding a solute to a solvent results in the lowering of the freezing point of the solvent.
  • The freezing point depression can also be explained in terms of vapor pressure.
  • In this equation, $\Delta T_f$ is the freezing point depression, Kf is the freezing point depression constant, and i is the van 't Hoff factor.
  • The value of 0.93 oC is the change in the freezing point.
  • Discuss the effects of a solute on the freezing point of a solvent

• Boiling & Melting Points

  • The melting points of crystalline solids cannot be categorized in as simple a fashion as boiling points.
  • Spherically shaped molecules generally have relatively high melting points, which in some cases approach the boiling point.
  • The data in the following table serves to illustrate these points.
  • Notice that the boiling points of the unbranched alkanes (pentane through decane) increase rather smoothly with molecular weight, but the melting points of the even-carbon chains increase more than those of the odd-carbon chains.
  • The last compound, an isomer of octane, is nearly spherical and has an exceptionally high melting point (only 6º below the boiling point).

• Boiling Point and Water Solubility

  • It is instructive to compare the boiling points and water solubility of amines with those of corresponding alcohols and ethers.
  • Corresponding -N-H---N- hydrogen bonding is weaker, as the lower boiling points of similarly sized amines (light green columns) demonstrate.
  • Alkanes provide reference compounds in which hydrogen bonding is not possible, and the increase in boiling point for equivalent 1º-amines is roughly half the increase observed for equivalent alcohols.
  • Indeed, 3º-amines have boiling points similar to equivalent sized ethers; and in all but the smallest compounds, corresponding ethers, 3º-amines and alkanes have similar boiling points.
  • In the examples shown here, it is further demonstrated that chain branching reduces boiling points by 10 to 15 ºC.

• Physical Properties of Carboxylic Acids

  • The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms.
  • The boiling points increased with size in a regular manner, but the melting points did not.
  • In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound.
  • The factors that influence the relative boiling points and water solubilities of various types of compounds were discussed earlier.
  • Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.

• Acid-Base Titrations

  • Before you begin the titration, you must choose a suitable pH indicator, preferably one that will experience a color change (known as the "end point") close to the reaction's equivalence point; this is the point at which equivalent amounts of the reactants and products have reacted.
  • Below are some common equivalence point indicators:
  • You can estimate the equivalence point's pH using the following rules:
  • You can determine the pH of a weak acid solution being titrated with a strong base solution at various points; these fall into four different categories: (1) initial pH; (2) pH before the equivalence point; (3) pH at the equivalence point; and (4) pH after the equivalence point.
  • The end point is reached when the indicator permanently changes color.

• Crystalline Solids

  • Most organic compounds have melting points below 200 ºC.
  • For a given compound, this temperature represents its melting point (or freezing point), and is a reproducible constant as long as the external pressure does not change.
  • If two crystalline compounds (A & B) are thoroughly mixed, the melting point of that mixture is normally depressed and broadened, relative to the characteristic sharp melting point of each pure component.
  • The lowest mixture melting point, e, is called the eutectic point.
  • The A:B complex has a melting point of 54 ºC, and the phase diagram displays two eutectic points, the first at 50 ºC, the second at 30 ºC.