Overview
This lecture covers the titration of amino acids, focusing on their amphoteric nature, the relationship between pH and pKa, zwitterion formation, titration curves, and calculation of isoelectric point (pI) for different amino acid types.
Amino Acids: Basics and Properties
- Amino acids have a basic amino group and an acidic carboxyl group; both can accept or donate protons (amphoteric).
- The environmental pH determines if amino acid groups are protonated (acidic conditions) or deprotonated (basic conditions).
- Amino acids are represented by a full name, three-letter, and one-letter abbreviations.
- Each ionizable group (carboxyl, amino, and sometimes R group) has its own pKa value.
pH vs. pKa: Rules and Concepts
- pKa is the pH at which half the molecules are protonated and half are deprotonated (1:1 ratio).
- If pKa > pH, the protonated form predominates.
- If pKa < pH, the deprotonated form predominates.
- pKa is constant for each group, while pH is variable depending on the medium.
Forms of Amino Acids at Different pH
- At very low pH (acidic), both groups are protonated (net positive charge).
- At intermediate pH (between pKa values), amino acids exist as zwitterions (neutral, with positive and negative charges).
- At high pH (basic), both groups are deprotonated (net negative charge).
Titration Curves and Buffering
- Titration is the gradual addition of acid or base to monitor pH changes.
- Plateaus (horizontal lines) on titration curves indicate buffering regions where pH resists change.
- A buffer occurs near the pKa of each ionizable group.
Calculating the Isoelectric Point (pI)
- The isoelectric point (pI) is the pH where the amino acid is electrically neutral (zwitterion).
- For neutral amino acids, pI = (pKa1 + pKa2)/2.
- For acidic amino acids, use the two carboxyl pKas; for basic amino acids, use the two amino pKas.
- Lower pI indicates acidic amino acids; higher pI indicates basic amino acids.
Slope and Titration Curve Interpretation
- Slope = change in pH (vertical) over change in added base (horizontal).
- Greater slope: steeper/vertical line, large pH change.
- Lower slope: horizontal line, little to no pH change (buffer region).
Examples: Glycine, Glutamic Acid, Lysine
- Glycine: Neutral, pI ≈ 6 (average of amino and carboxyl pKas).
- Glutamic acid: Acidic, pI ≈ 3.25 (average of the two carboxyl pKas).
- Lysine: Basic, pI ≈ 9.75 (average of two amino pKas).
Key Terms & Definitions
- Amphoteric — molecule that can act as both acid and base.
- pKa — the pH at which a group is 50% protonated and 50% deprotonated.
- Zwitterion — molecule with both positive and negative charges but overall neutral.
- Isoelectric Point (pI) — the pH at which a molecule carries no net charge.
- Buffer — region where addition of acid/base causes minimal pH change.
Action Items / Next Steps
- Review titration curves for glycine, glutamic acid, and lysine.
- Practice calculating pI for different amino acids.
- Read about the Henderson-Hasselbalch equation and its application to acid-base balance.