1R9L:
reference: Cation-pi interactions as determinants for binding of the compatible solutes glycine betaine and proline betaine by the periplasmic ligand-binding protein ProX from Escherichia coli., Schiefner A, Breed J, Bosser L, Kneip S, Gade J, Holtmann G, Diederichs K, Welte W, Bremer E, J Biol Chem 2004 Feb 13;279(7):5588-96. Epub 2003 Nov 11. PMID: 14612446

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