Computational and experimental model studies establish the importance of donor atom-pi bonding on the recognition and function of flavins. This noncovalent interaction arises from electrostatic attraction between electron-rich (donor) atoms and electron-deficient aromatic systems. The electrostatic origin of this bonding was verified through comparison of binding of model receptors to flavin in both oxidized (Fl(ox)) and radical anion (Fl(rad)(-)) forms: Interaction was strongest with the electron-poor Fl(ox) and was greatly attenuated with the comparatively electron-rich Fl(rad)(-). Other factors determining the magnitudes of these donor-pi interaction are the size and polarizability of the donor atom.