Amino Acids pH and Charge

Isoelectric Point (pI)

• The isoelectric point (pI) is the pH at which an amino acid has no net charge, and the concentration of positive and negative charge species is equal.
• For the simplest amino acid, glycine, pKa1 = 2.34, pKa2 = 9.6, and pI = 5.97.

Amino Acids with Acidic Side Chains

• The pI will be at a lower pH because the acidic side chain introduces an "extra" negative charge.
• The neutral form exists under more acidic conditions when the extra -ve has been neutralized.
• For example, aspartic acid has a pI = 2.77, which is halfway between pKa1 and pKa3.

Amino Acids with Basic Side Chains

• The pI will be at a higher pH because the basic side chain introduces an "extra" positive charge.
• The neutral form exists under more basic conditions when the extra +ve has been neutralized.
• For example, histidine has a pI = 7.59, which is halfway between pKa2 and pKa3.

Electrophoresis

• Electrophoresis is a technique used to show the distribution of charged species in a sample by observing the movement of solute molecules in an electric field.
• An ionic buffer solution is incorporated in a solid matrix layer, composed of paper or a crosslinked gelatin-like substance.

Titration Curve

• The titration curve for alanine demonstrates the relationship between pH and charge state.
• At a pH lower than 2, the alanine molecule has a net positive charge.
• At a pH greater than 10, the alanine molecule has a net negative charge.
• 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 concentrations.

Calculating pI

• The pI is given by the average of the pKas that involve the zwitterion, i.e., that give the boundaries to its existence.
• For neutral side chains, pI = 1/2 (pKa1 + pKa2).
• For acidic side chains, pI = 1/2 (pKa1 + pKa3).
• For basic side chains, pI = 1/2 (pKa2 + pKa3).

Classification of Amino Acids

• Amino acids are classified into 4 groups based on the general chemical characteristics of their R groups.
• The four groups are: non-polar, polar with no charge, polar with a positive charge, and polar with a negative charge.
• Non-polar amino acids are hydrophobic (water hating) and have no charge on the 'R' group.
• Examples of non-polar amino acids include alanine, leucine, isoleucine, valine, methionine, phenylalanine, tryptophan, and proline.

Understanding how amino acids change charge at different pH levels, including pKa1 and pKa2 values, and isoelectric point (pI).