pKa values

Meaning: pKa = −log Ka, so a lower pKa means a stronger acid. At pH = pKa, a group is half protonated and half deprotonated. Below its pKa it is mostly protonated, and above its pKa it is mostly deprotonated. Each ionizable group in an amino acid has its own approximate pKa: α-carboxyl about 2, α-amino about 9 to 10, and side chains such as Asp and Glu about 4, His about 6 and Lys about 10.5.

Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are unitless and the concentrations are in mol/L.

On the MCAT: Expect titration curves, where buffering plateaus sit at each pKa, and questions that ask for the net charge of an amino acid or peptide at a given pH. You may also be asked for the pI, which is the average of the two pKa values on either side of the neutral form. His, with a side-chain pKa near 6, is a useful buffer in the neighborhood of physiological pH.

Common confusion: A pKa is not the pH at which a group is neutral. At the pKa the group is 50% ionized, and whether the protonated form is charged depends on the group. A protonated carboxylic acid is neutral, but a protonated amino group (NH₃⁺) carries a positive charge. Also, the pKa of a group can shift with its local environment, such as inside a protein.