Number of hydrogen bond acceptors

The number of hydrogen bond acceptors in the compound. Here we use the simplified definition in Lipinski’s Rule of Five (Lipinski et al. Adv. Drug Deliv. Rev. (1997) 23 pp. 3-25), being the count of Nitrogen and Oxygen atoms, although other, more formal, definitions are possible.

A large number of hydrogen bond acceptors has been associated with poor membrane permeability due to the additional desolvation energy required to break hydrogen bonds when moving from an aqueous environment to the lipid membrane.

This is defined by the following SMARTS pattern: [N,n,O,o]