Reinvestigation of tricyclic acyclovir: characterization of a 'proton-wire' model

The crystal structure of a dihydrate of the title compound, 3-[(2-hydroxyethoxy)methyl]-6-methyl-3H-imidazolo[1,2-a]-purin-9(5H)-one, C11H13N5O3·2H2O, has been reinvestigated. The asymmetric unit contains two molecules of tricyclic acyclovir and four molecules of water. The structure consists of a three-dimensional network of strong hydrogen bonds that integrates all of the components. While the crystal structure and the formation of an (H2O)8 solvent water molecule cluster through a disordered strong hydrogen bond [O···O = 2.807 (3) Å] between two water molecules across an inversion centre has been described previously [Suwińska, Golankiewicz & Zielenkiewicz (2001). Acta Cryst. C57, 767-769], the disorder was incompletely modelled. In this work, the disorder model is extended and includes disorder of one tricyclic acyclovir hydroxy group across another inversion centre [O···O = 2.644 (4) Å]. The resulting infinite O-H···O hydrogen-bonded water-hydroxy chains, analogous to the `proton wires' found in the membrane protein gramicidin A, are discussed and an unusual disorder model involving infinite concerted chains of O-H···O hydrogen bonds is provided.