Solubility, Permeability, and Polar Surface Area

Two opposite demands for absorption

Picture a swallowed tablet. First the molecule must let go of its solid form and spread through the watery contents of the gut — that is aqueous solubility. Then those dissolved molecules must pass through the wall of the intestine, whose cells are wrapped in oily membranes — that is permeability. Absorption needs both: a molecule that won't dissolve never presents itself for crossing, and a molecule that dissolves but can't cross just washes away.

Here is the tension at the heart of this guide: solubility loves polar, water-friendly molecules, while membrane crossing loves greasy, water-shedding ones. The same change that helps one usually hurts the other. Good oral drugs are the ones that thread this needle — soluble enough to dissolve, greasy enough to cross. This is the everyday meaning of the lipophilicity sweet spot from earlier.

Polar surface area: a ruler for membrane crossing

Polar surface area (PSA, often the calculated "TPSA") is the total surface taken up by polar atoms — mainly the nitrogens and oxygens, plus the H-bond donor hydrogens attached to them. Why does it predict permeability so well? Because every polar atom carries a coat of water, and stripping off that water to dive into a membrane costs energy. The more polar surface, the higher the toll, and the worse the passive permeability.

Handy PSA guideposts for passive absorption:

  PSA < 60 Å²   -> usually good gut absorption
  PSA < 90 Å²   -> often fine for oral drugs
  PSA > 140 Å²  -> poor passive permeability
  PSA < 70-90 Å² (and few donors)
                -> a common target for crossing into the brain

PSA is dominated by N and O atoms (and their attached H's).
Each extra amide, hydroxyl, or carboxyl pushes PSA up.

PSA thresholds are rules of thumb, not laws — but they flag permeability risk before any synthesis.

PSA is so useful because it is closely tied to the count of hydrogen-bond donors and acceptors — the very features that the rule of five also counts. Donors are especially costly to membrane crossing because their water coat is hard to shed. This is one reason burying or capping a free NH (turning a donor into a non-donor) is a classic trick to rescue permeability without losing the polar atom entirely.

Measuring it in the lab

Permeability is not left to calculation alone. A workhorse assay is Caco-2 permeability, which grows a single layer of human gut-like cells and measures how fast a compound crosses from one side to the other. It captures both passive diffusion and the action of efflux transporters that can pump a molecule back out. Solubility is measured directly too — kinetic solubility for fast triage, and thermodynamic solubility on the real solid form for development decisions.