Cyclic Water Pentamers in L-Leucyl-L-alanine Tetrahydrate

The structure of the title compound, C₉H₁₈N₂O₃.4H₂O, contains a very unusual and elaborate system of hydrogen-bonded water molecules, which includes cyclic water pentamers with idealized hydrogen-bond cooperativity. The solvent molecules form columns in the crystal, as do hydrophobic aggregates of peptide side chains. Both these units are located between hydrophilic sheets formed by the peptide main chains. The sheets contain two head-to-tail hydrogen-bonded chains, one of which is interrupted by a bridging water molecule that is functionally different from those constituting the core of the water channels.

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