Liquid water can form π-hydrogen bonds with benzene and other aromatic rings, reports a group led by Purdue University chemistry professor Dor Ben-Amotz (J. Phys. Chem. Lett., DOI: 10.1021/jz201373e). Hydrogen bonds between aromatic rings and the hydrogens of NH and OH groups have been observed in clusters at low temperature and in the interiors of folded proteins but not previously in bulk liquid water. Ben-Amotz and colleagues used Raman scattering to study benzene, phenol, and phenylalanine in water and found that liquid water does form hydrogen bonds with benzene and other aromatic rings, with the bonds forming with nearly every benzene molecule. The researchers found that a water-water hydrogen bond is stronger than a water-benzene bond. But because water molecules associated with benzene have more rotational and translational freedom than their water-water counterparts, the π-hydrogen bonds are entropically favored. The authors suggest that π-hydrogen bonding may be important for biological binding, recognition, and signaling processes involving aromatic groups.