Structure and thermotropic phase behavior of a homologous series of bioactive N-acyldopamines.

N-Acyldopamines (NADAs), which are present in mammalian nervous tissues, exhibit interesting biological and pharmacological properties. In the present study, a homologous series of NADAs with varying acyl chains (n = 12-20) have been synthesized and characterized. Differential scanning calorimetric studies show that in the dry state the transition temperatures, enthalpies, and entropies of NADAs exhibit odd-even alternation with the values corresponding to the even chain length series being slightly higher. Both even and odd chain length NADAs display a linear dependence of the transition enthalpies and entropies on the chain length. However, odd-even alternation was not observed in the calorimetric properties upon hydration, although the transition enthalpies and entropies exhibit linear dependence. Linear least-squares analyses yielded incremental values contributed by each methylene group to the transition enthalpy and entropy and the corresponding end contributions. N-Lauroyldopamine (NLDA) crystallized in the monoclinic space group C2/c with eight symmetry-related molecules in the unit cell. Single-crystal X-ray diffraction studies show that NLDA molecules are organized in the bilayer form, with a head-to-head (and tail-to-tail) arrangement of the molecules. Water-mediated hydrogen bonds between the hydroxyl groups of the dopamine moieties of opposing layers and N-H···O hydrogen bonds between the amide groups of adjacent molecules in the same layer stabilize the crystal packing. These results provide a thermodynamic and structural basis for investigating the interaction of NADAs with other membrane lipids, which are expected to provide clues to understand how they function in vivo, e.g., as signaling molecules in the modulation of pain.