Lecture on Isoelectric Point (pI) of Amino Acids

Definition and Importance

• Isoelectric Point (pI):
  • The point on the pH scale where a molecule, specifically an amino acid, exists in a neutral form with zero net charge.
  • "Isoelectric" means equal charge distribution, resulting in no overall charge.
  • Knowing the pI helps predict the charge state of an amino acid at different pH levels.

Amino Acid Structure

• Generic Structure:
  • Composed of an amino group, carboxylic acid group, and an R group (side chain, initially ignored).
• Functional Groups:
  • Amino Group:
    • Contains nitrogen, acts as a proton acceptor.
    • Is basic and usually protonated, carrying a positive charge in its protonated form.
  • Carboxylic Acid Group:
    • Proton donor, thus considered acidic.
    • When deprotonated, carries a negative charge.

Zwitterion

• Definition:
  • A molecule with both positive and negative charges but overall neutral.
  • For amino acids, zwitterion forms at specific pH levels.

pH Impact on Amino Acids

• Low pH (Acidic, e.g., pH 1):
  • Excess protons cause the amino acid to be fully protonated.
  • Amino group remains positively charged; carboxylic group gains a proton, losing its negative charge.
  • Net charge: +1.
• High pH (Basic, e.g., pH 12):
  • Excess hydroxide ions lead to deprotonation.
  • Amino group becomes neutral; carboxylic acid becomes negatively charged again.
  • Net charge: -1.

Calculating the Isoelectric Point (pI)

• Determining pI:
  • pI is the pH at which the amino acid exists as a zwitterion.
  • Calculated by averaging the pKa values of the functional groups.
• pKa Values:
  • Amino group pKa: ~9.
  • Carboxylic acid group pKa: ~2.
  • Isoelectric point (pI) for a generic amino acid: (9 + 2) / 2 = 5.5.

Influence of Side Chains

• Effect of R groups:
  • If the R group is a functional group, its pKa must also be considered.

Summary

• The isoelectric point is crucial for understanding the neutral form of amino acids.
• Calculation involves averaging the pKa of all functional groups.
• Essential for predicting amino acid behavior in different pH environments.