Glossary

Acid Dissociation Constant (pKa)

Chemistry

Acid Dissociation Constant (pKa) — A quantitative measure of the strength of an acid in solution, determining the protonation state of ionizable amino acid side chains at a given pH.

What Is the Acid Dissociation Constant?

The acid dissociation constant (Ka) and its negative logarithm (pKa) quantify the tendency of an acid to donate a proton in aqueous solution. For peptides, pKa values determine the protonation state of ionizable amino acid side chains, the N-terminal α-amino group (pKa ~8.0), and the C-terminal α-carboxyl group (pKa ~3.1) at any given pH. These protonation states directly control peptide charge, solubility, receptor binding, and chromatographic behavior.

Key pKa Values in Peptide Chemistry

Asp (β-COOH): pKa 3.65. Negatively charged above pH 4
Glu (γ-COOH): pKa 4.25. Negatively charged above pH 5
His (imidazole): pKa 6.0. Protonation state changes near physiological pH, making His a critical catalytic residue
Cys (thiol): pKa 8.3. Thiolate form required for disulfide bond formation
Lys (ε-NH₂): pKa 10.5. Positively charged at physiological pH
Arg (guanidinium): pKa 12.5. Permanently protonated under all physiological conditions

Applications in Peptide Research

pKa values determine the isoelectric point (pI), which predicts solubility minimum and optimal ion-exchange chromatography conditions. They guide buffer selection for reconstitution (peptides are most soluble when charged, i.e., at pH values away from pI). In HPLC, mobile phase pH relative to analyte pKa values controls retention and peak shape.

Context Effects

pKa values in peptides can shift significantly from free amino acid values due to electrostatic interactions with neighboring charged residues, hydrogen bonding, and burial within folded structures. Histidine pKa, for example, can range from 5.0 to 8.0 depending on local environment, directly affecting catalytic activity in enzyme active sites and peptide-receptor interactions.

Frequently Asked Questions

What is Acid Dissociation Constant (pKa)?

A quantitative measure of the strength of an acid in solution, determining the protonation state of ionizable amino acid side chains at a given pH.

Why is Acid Dissociation Constant (pKa) important in peptide research?

Acid Dissociation Constant (pKa) is a fundamental concept in chemistry as it relates to peptide science. It directly influences experimental design, compound characterization, and the reliability of research outcomes across biochemistry and molecular biology disciplines.

Authority Sources

← Back to Glossary