Lysine is an essential amino acid characterized by multiple ionizable groups, meaning it does not possess a single pKb value, but rather a unique pKb for each of its titratable functional groups. These groups include an alpha-carboxyl group, an alpha-amino group, and a side-chain epsilon-amino group. Each pKb value indicates the strength of a specific basic functional group within the lysine molecule.
The relationship between pKa (acid dissociation constant) and pKb (base dissociation constant) is fundamental in acid-base chemistry, given by the equation:
pKa + pKb = pKw
At 25 °C, the ion product of water (Kw) is 1.0 x 10^-14, making pKw equal to 14.00. Therefore, if the pKa of a conjugate acid is known, the pKb of its conjugate base can be precisely calculated as pKb = 14.00 - pKa.
pKb Values for Lysine's Ionizable Groups
Based on its chemical structure, lysine has three distinct pKa values, each corresponding to the deprotonation of a specific acidic group. From these, the corresponding pKb values for their conjugate bases can be determined.
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Alpha-Carboxyl Group (–COOH)
- pKa: For the alpha-carboxyl group, lysine has a pKa value of 2.16. This value represents the acidity of the carboxylic acid group, which readily donates a proton.
- pKb (for the carboxylate group, –COO⁻): The pKb for the conjugate base (carboxylate) can be calculated as 14.00 - 2.16 = 11.84. This high pKb value indicates that the carboxylate group is a very weak base.
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Alpha-Amino Group (–NH₃⁺)
- pKa: The alpha-amino group typically has a pKa value of approximately 9.06. This pKa reflects the acidity of the protonated alpha-amino group.
- pKb (for the alpha-amino group, –NH₂): The basicity of the alpha-amino group (when it accepts a proton) is calculated as 14.00 - 9.06 = 4.94.
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Side-Chain Epsilon-Amino Group (–(CH₂)₄–NH₃⁺)
- pKa: Lysine's unique characteristic is its long side chain with a terminal epsilon-amino group, which has a pKa value of approximately 10.54. This makes it a significant contributor to lysine's overall basicity.
- pKb (for the side-chain epsilon-amino group, –(CH₂)₄–NH₂): The basicity of this side-chain amino group is calculated as 14.00 - 10.54 = 3.46. This relatively low pKb indicates that the epsilon-amino group is the strongest base among lysine's basic functional groups.
Summary of Lysine's pKa and pKb Values
The following table summarizes the typical pKa values for lysine's ionizable groups and their corresponding pKb values at 25 °C:
| Ionizable Group | pKa (Acidic Form) | pKb (Conjugate Base Form) |
|---|---|---|
| Alpha-Carboxyl | 2.16 | 11.84 |
| Alpha-Amino | ~9.06 | ~4.94 |
| Epsilon-Amino (Side Chain) | ~10.54 | ~3.46 |
Note: The pKa of 2.16 for the alpha-carboxyl group is a specific value for lysine. The other pKa values are generally accepted standard values for the respective groups in lysine, reflecting slight variations across different sources.
Understanding these pKb values is essential for predicting lysine's ionization state and behavior in various biological environments and pH conditions, playing a critical role in protein structure and function.
