X-ray diffraction evidence for aromatic π hydrogen bonding to water

Abstract

THEinteraction of water with aromatic moieties is of importance in biological systems, as most encounter an aqueous environment during their normal functions. For example, common constituents of globular proteins such as phenylalanine, tryptophan and tyrosine possess aromatic side-chains¹ that may encounter water molecules inside the protein structure². As a model for hydrogen-bonding interactions with aromatics, we have performed X-ray diffraction studies on crystalline Na₄[calix[4]arene sulphonate].13.5H₂O. Calixarene molecules^3,4contain hydrophobic cavities comprised of aromatic groups, rimmed, in the case of the water-soluble sulphonates (R = -SO₃Na), by hydrophilic groups^5,6. In the absence of a hydrophobic guest, the cavity invariably contains a water molecule. The low-temperature X-ray crystal structure of this compound (see Table 1 and Fig. 1) shows direct evidence for hydrogen bonding between water and the aromatic π electrons in the solid state.