Acid–Base Properties of Amino Acids:
Amino acids have both + and – charge → called zwitterion
At neutral pH, they exist as:
–NH₃⁺ (positive) and –COO⁻ (negative)
They can act as:
o Acid (give H⁺)
o Base (accept H⁺)
→ so they are amphoteric
pH Effect:
Low pH (acidic): → NH₃⁺ and COOH (fully protonated)
High pH (basic): → NH₂ and COO⁻ (deprotonated)
pKa:
Shows strength of acid
Low pKa = strong acid
At pH = pKa, half molecules are protonated
Titration of Amino Acids:
Two steps:
1. COOH → COO⁻ (loses H⁺ first)
2. NH₃⁺ → NH₂ (loses H⁺ later)
Isoelectric Point (pI):
pH where net charge = 0
Formula:
pI = (pK₁ + pK₂) / 2
Example (Glycine)
pI = (2.34 + 9.60) / 2 = 5.97
Buffering:
Amino acids act as buffers near their pKa values
Important Points:
Simple amino acids → 2 pKa value
With ionizable side chain → 3 pKa values
Example:
o Glycine → pI ≈ 5.97
o Alanine → pI ≈ 5.7
1
, o Histidine → good buffer near pH 7
Abbreviations and Symbols of Amino Acids:
Each amino acid has:
o 3-letter code (e.g., Gly)
o 1-letter symbol (e.g., G)
Rules for One-Letter Symbols:
1. Unique first letter:
If only one amino acid starts with a letter, that letter is used
Example: I = Isoleucine
2. Common amino acid gets priority:
If more than one amino acid has same first letter, the most common gets it
Example: G = Glycine (not Glutamate)
3. Similar sound:
Some symbols sound like the amino acid name
Example: F = Phenylalanine, W = Tryptophan
4. Closest letter used:
For others, a letter close to the name is chosen
Example: K = Lysine
Classification of Amino Acids:
Amino acids are classified based on R group (side chain) and its behavior with water.
1. Nonpolar (Hydrophobic) Amino Acids
Do not interact with water
Found inside proteins (hydrophobic core)
Examples:
Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline
Special points:
Glycine: smallest, flexible
Proline: ring structure, causes bend
2. Aromatic Amino Acids
Contain ring structure
Mostly hydrophobic
2
Amino acids have both + and – charge → called zwitterion
At neutral pH, they exist as:
–NH₃⁺ (positive) and –COO⁻ (negative)
They can act as:
o Acid (give H⁺)
o Base (accept H⁺)
→ so they are amphoteric
pH Effect:
Low pH (acidic): → NH₃⁺ and COOH (fully protonated)
High pH (basic): → NH₂ and COO⁻ (deprotonated)
pKa:
Shows strength of acid
Low pKa = strong acid
At pH = pKa, half molecules are protonated
Titration of Amino Acids:
Two steps:
1. COOH → COO⁻ (loses H⁺ first)
2. NH₃⁺ → NH₂ (loses H⁺ later)
Isoelectric Point (pI):
pH where net charge = 0
Formula:
pI = (pK₁ + pK₂) / 2
Example (Glycine)
pI = (2.34 + 9.60) / 2 = 5.97
Buffering:
Amino acids act as buffers near their pKa values
Important Points:
Simple amino acids → 2 pKa value
With ionizable side chain → 3 pKa values
Example:
o Glycine → pI ≈ 5.97
o Alanine → pI ≈ 5.7
1
, o Histidine → good buffer near pH 7
Abbreviations and Symbols of Amino Acids:
Each amino acid has:
o 3-letter code (e.g., Gly)
o 1-letter symbol (e.g., G)
Rules for One-Letter Symbols:
1. Unique first letter:
If only one amino acid starts with a letter, that letter is used
Example: I = Isoleucine
2. Common amino acid gets priority:
If more than one amino acid has same first letter, the most common gets it
Example: G = Glycine (not Glutamate)
3. Similar sound:
Some symbols sound like the amino acid name
Example: F = Phenylalanine, W = Tryptophan
4. Closest letter used:
For others, a letter close to the name is chosen
Example: K = Lysine
Classification of Amino Acids:
Amino acids are classified based on R group (side chain) and its behavior with water.
1. Nonpolar (Hydrophobic) Amino Acids
Do not interact with water
Found inside proteins (hydrophobic core)
Examples:
Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline
Special points:
Glycine: smallest, flexible
Proline: ring structure, causes bend
2. Aromatic Amino Acids
Contain ring structure
Mostly hydrophobic
2
