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Overview of Amino Acids

• Amino acids are the building blocks of proteins.

• They have both acidic and basic groups, functioning as amphoteric molecules.

Ionizable Functional Groups

• Key ionizable groups in amino acids:

  • Amino group (-NH3+)

  • Carboxyl group (-COO-)

Protonation and Deprotonation

• pH Dependency: The state of these groups changes with the pH environment:

  • At low pH:

    • Carboxyl group is protonated (-COOH)

    • Amino group remains protonated (-NH3+)

  • As pH increases:

    • Carboxyl loses proton becoming negatively charged (-COO-)

    • Amino group loses a proton, forming a neutral state (-NH2)

pKa Values

• pKa of Carboxyl Group: Typically around 2.

• pKa of Amino Group: Generally between 9-10.

Ionizable Side Chains

• Some amino acids possess ionizable side chains which affect their acidic or basic properties:

  • Acidic Amino Acids (e.g., Aspartic acid, Glutamic acid)

    • Side chain pKa around 4.

    • Can donate protons, carry negative charge at physiological pH.

  • Basic Amino Acids (e.g., Lysine, Arginine, Histidine)

    • Side chain pKa: Lysine ~10.5, Arginine ~12.5, Histidine ~6.0

    • Remain positively charged at lower pH levels.

• Histidine is particularly significant due to its pKa near physiological pH (7.4), allowing it to easily gain or lose protons.

Titration of Amino Acids

• Titration curves display distinct pKa values for functional groups.

• Isoelectric Point (pI):

  • The pH at which the amino acid has no net charge.

  • Calculated as the average of pKa values surrounding the zwitterionic form.

• For amino acids lacking ionizable side chains, pI is typically around 5-6.

• For acidic amino acids, pI is lower; for basic amino acids, it is higher due to additional charge effects.

Buffering Capacity

• Buffering regions are observed in titration curves around pKa values where pH changes gradually upon acid/base addition.

• Amino acids play a crucial role in maintaining physiological pH within biological systems, notably in blood plasma and cells.

Application of Henderson-Hasselbalch Equation

• It describes the relationship between pH, pKa, and the ratio of protonated to deprotonated forms:

  • Equation:

    [ pH = pKa + \log \left( \frac{[A^-]}{[HA]} \right) ]

• Used to determine the ionization state of amino acids at specific pH values.

• Helps identify buffering regions and calculate pI.

Practical Instructions for Amino Acid Titrations

• Objective: Explore acid-base properties of Histidine and Alanine through titration.

Step-by-Step Process:

1. Mix 10 mL of 0.1 M Alanine with 40 mL distilled water in a beaker.

2. Stir the mixture using a magnetic stirrer.

3. Rinse and place a pH electrode in the solution.

4. Measure and stabilize the pH.

5. Fill a burette with 30 mL of 0.1 M HCl or NaOH.

6. Gradually add 0.5 mL of HCl or NaOH, record pH after stabilization.

7. Repeat until you record 25 pH values.

8. Conduct steps 1-7 for both Histidine and water.

Data Table Format

• Track volume of HCl/NaOH and corresponding pH for Alanine, Histidine, and water through coded entries.

Conclusion

• Understanding the acid-base characteristics of amino acids is crucial for their application in biological and biochemical contexts, including protein structure and enzymatic reactions.