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Journal of Lipid Research, Vol. 47, 1424-1433, July 2006
Copyright © 2006 by American Society for Biochemistry and Molecular Biology

Molecular packing and intermolecular interactions in two structural polymorphs of N-palmitoylethanolamine, a type 2 cannabinoid receptor agonist

Ravi Kanth Kamlekar and Musti J. Swamy¹

School of Chemistry, University of Hyderabad, Hyderabad 500 046, India

Published, JLR Papers in Press, April 11, 2006.

¹ To whom correspondence should be addressed. e-mail: mjssc{at}uohyd.ernet.in

The molecular structure, packing properties, and intermolecular interactions of two structural polymorphs of N-palmitoylethanolamine (NPEA) have been determined by single-crystal X-ray diffraction. Polymorphs and ß crystallized in monoclinic space group P2₁/c and orthorhombic space group Pbca, respectively. In both polymorphs, NPEA molecules are organized in a tail-to-tail manner, resembling a bilayer membrane. Although the molecular packing in polymorph is similar to that in N-myristoylethanolamine and N-stearoylethanolamine, polymorph ß is a new form. The acyl chains in both polymorphs are tilted by 35° with respect to the bilayer normal, with their hydrocarbon moieties packed in an orthorhombic subcell. In both structures, the hydroxy group of NPEA forms two hydrogen bonds with the hydroxy groups of molecules in the opposite leaflet, resulting in extended, zig-zag type H-bonded networks along the b-axis in polymorph and along the a-axis in polymorph ß. Additionally, the amide N-H and carbonyl groups of adjacent molecules are involved in N-H···O hydrogen bonds that connect adjacent molecules along the b-axis and a-axis, respectively, in and ß. Whereas in polymorph the L-shaped NPEA molecules in opposite layers are arranged to yield a Z-like organization, in polymorph ß one of the two NPEA molecules is rotated 180°, leading to a W-like arrangement. Lattice energy calculations indicate that polymorph is more stable than polymorph ß by 2.65 kcal/mol.

Supplementary key words N-acylethanolamine • hydrogen bonding • single-crystal X-ray diffraction • chain packing • bilayer membrane

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