Ionization constants of alpha-amino acids: Typical pK values are approximately what for the alpha-carboxyl group and the alpha-amino group, respectively?

Introduction / Context:The acid–base behavior of amino acids underlies protein charge, buffering, and electrophoresis. Knowing approximate pK values for the alpha-carboxyl and alpha-amino groups helps predict net charge across pH and interpret titration curves.

Given Data / Assumptions:

• We consider typical, average pK values for free amino acids in water.
• Side-chain pK values vary (e.g., Lys ~10.5, Asp ~3.9), but the backbone groups have characteristic ranges.

Concept / Approach:Alpha-carboxyl groups are relatively acidic with pK near ~2.0–2.5; alpha-amino groups are weak bases with pK near ~9.0–10.0. These values explain zwitterion formation near neutral pH.

Step-by-Step Solution:Estimate carboxyl pK: ~2.2 (deprotonated above this pH).Estimate amino pK: ~9.4 (protonated below this pH).Match to the provided options → 2.2 and 9.4.

Verification / Alternative check:Textbook titration curves for glycine show inflection points consistent with pK1 ~2.3 and pK2 ~9.6, aligning with the selected values.

Why Other Options Are Wrong:

• 1.1/12.1: unrealistically extreme for alpha groups.
• 6.5/8.0 and 3.3/10.5: do not reflect typical paired values for free alpha groups.

Common Pitfalls:Confusing side-chain pK values with backbone pK values; forgetting environmental effects (ionic strength, temperature) shift precise numbers slightly.

Final Answer:2.2 and 9.4