logD

logD is logP's realistic, pH-aware sibling. logP only counts the neutral form of a molecule, but many drugs carry an acidic or basic group that becomes charged depending on the surrounding pH. logD asks a more honest question: at a given pH, counting every form of the molecule (neutral and charged together), how does it actually distribute between oil and water?

Charged species strongly prefer water, so for an ionizable drug logD at physiological pH is usually lower than its logP. The two values coincide only when the molecule is essentially un-ionized at that pH. By convention, logD is reported with the pH attached, most commonly logD at pH 7.4, the pH of blood.

Because it reflects the molecule as it really exists in the body, logD often predicts permeability, plasma protein binding, and clearance better than logP for acids and bases. Medicinal chemists frequently tune logD by adjusting a compound's pKa as well as its lipophilicity.