Polar surface area

The Polar Surface Area (PSA) is defined as the surface sum over all polar atoms, (usually oxygen and nitrogen), including also attached hydrogens.

PSA is a commonly used medicinal chemistry metric for the optimisation of cell permeability. Molecules with a polar surface area of greater than 140 angstroms squared are usually believed to be poor at permeating cell membranes. For molecules to penetrate the blood-brain barrier, PSA should be less than 60 angstroms squared.