Structure and phase behavior of O-stearoylethanolamine: a combined calorimetric, spectroscopic and X-ray diffraction study.

Recent studies show that O-acylethanolamines (OAEs), structural isomers of the putative stress-fighting lipids, namely N-acylethanolamines (NAEs), can be derived from NAEs and are present in biological membranes under physiological conditions. In view of this, we have synthesized O-stearoylethanolamine (OSEA) as a representative OAE and investigated its phase behavior and crystal structure. The thermotropic phase transitions of OSEA dispersed in water and in 150mM NaCl were characterized using calorimetric, spectroscopic, turbidimetric and X-ray diffraction studies. These studies have revealed that when dispersed in water OSEA undergoes a cooperative phase transition centered at 53.8 degrees C from an ordered gel phase to a micellar structure whereas in presence of 150mM NaCl the transition temperature increases to 55.8 degrees C and most likely the bilayer structure is retained above the phase transition. O-Stearoylethanolamine crystallized in the orthorhombic space group P2(1)2(1)2(1) with four symmetry-related molecules in the unit cell. Single-crystal X-ray diffraction studies show that OSEA molecules adopt a linear structure with all-trans conformation in the acyl chain region. The molecules are organized in a tail-to-tail fashion, similar to the arrangement in a bilayer membrane. These studies are relevant to understanding the role of salt on the phase properties of this new class of lipids.