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Acta Crystallogr Sect E Struct Rep Online. 2009 Nov 7;65(Pt 12):o3027-8. doi: 10.1107/S1600536809042512.

Redetermination of 2-[4-(2-hydroxy-ethyl)piperazin-1-ium-1-yl]ethanesul-fonate at 100 K.

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University of Virginia, Department of Molecular Physiology & Biological Physics, 1340 Jefferson Park Avenue, Charlottesville, VA 22908, USA.


The crystal structure of the title compound (common name HEPES), C(8)H(18)N(2)O(4)S, has been redetermined at 100 K in order to properly elucidate the protonation state of the HEPES molecule. The piperazine ring has a chair conformation and one of the N atoms in the ring is protonated, which was not previously reported [Gao, Yin, Yang, & Xue (2004). Acta Cryst. E60, o1328-o1329]. The change of protonation state of the nitrogen atom significantly affects the intermolecular interactions in the HEPES crystal. The structure is stabilized by N-H⋯O and O-H⋯O hydrogen bonds and ionic inter-actions, as the title compound in solid state is a zwitterion. HEPES mol-ecules pack in layers that are held together by ionic and weak inter-actions, while a hydrogen-bonded network connects the layers.

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