pKa and Ionization: The Charge That Changes Everything

pKa is the pH where a group is half charged

Many drug molecules contain a group that can gain or shed a proton — an acid (like a carboxylic acid, which can lose H⁺ to become negative) or a base (like an amine, which can grab H⁺ to become positive). Whether that group is actually charged depends on the pH of its surroundings. The pKa is simply the pH at which the group is exactly half ionized and half neutral. It is a single number that tells you the group's tipping point.

The rule for which way a group leans is short and worth memorizing. An acid is charged (deprotonated) when the pH is above its pKa. A base is charged (protonated) when the pH is below its pKa. Everything else is just counting how far above or below you are.

Henderson–Hasselbalch, the one equation you need:

  for an acid:  pH - pKa = log10( [charged] / [neutral] )
  for a base:   pKa - pH = log10( [charged] / [neutral] )

Each unit of difference = a 10-fold shift:
  1 unit  ->  ~91% one form
  2 units ->  ~99% one form
  3 units -> ~99.9% one form

Example: a base with pKa 9.0 in blood (pH 7.4)
  pKa - pH = 9.0 - 7.4 = 1.6  ->  ~98% protonated (charged)

Why charge changes the journey

A charged molecule is dressed in a shell of water and hates leaving it. That single fact pushes its properties in opposite directions. On the good side, charge boosts aqueous solubility — ions dissolve far better than neutral grease. On the hard side, that same water shell makes it much harder to slip through the oily core of a membrane, so charge lowers passive permeability. This is exactly why logD at body pH can be much lower than logP: the ionized fraction is effectively trapped in water.

Charge also matters at the target. A charged group can form a strong ionic interaction or salt bridge with an oppositely charged residue in the binding pocket, adding real affinity. Many successful drugs deliberately place a basic amine to reach an acidic residue, or an acid to reach a basic one. So pKa is not just a liability to manage — it is also a design handle for binding.

Using pKa in practice

1. Identify the ionizable groups in your molecule and label each as acid or base.
2. Estimate each pKa (from analogues, calculation, or measurement), then apply the rule: acid charged above its pKa, base charged below its pKa.
3. Use Henderson–Hasselbalch to get the charged fraction at the pH that matters (gut ~6.5, blood 7.4), and read off the effect on logD, solubility, and permeability.
4. If a needed group is too acidic or too basic, tune its pKa with neighbouring substituents — an electron-withdrawing group lowers a base's pKa and raises an acid's acidity.