Characterization of the molecular packing, thermotropic phase behaviour and critical micellar concentration of a homologous series of N-acyltaurines (n = 9–18). PXRD, DSC and fluorescence spectroscopic studies

Abstract

N-acyltaurines (NATs) are amides of fatty acids that can be structurally related to endocannabinoids. They show interesting physiological and pharmacological properties. We have synthesized a homologous series of NATs with saturated acyl chains (n = 9–18) and investigated their supramolecular structure and thermotropic phase transitions by powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC). The d-spacings obtained from PXRD increase linearly with chain length with an increment of ∼0.847 Å per additional CH2 moiety suggesting that NATs adopt a tilted bilayer structure with similar packing in crystal lattice. Results obtained from DSC studies indicate that the endothermic transition temperature (Tt) of NATs showed a gradually increasing trend with increasing acyl chain length. The enthalpy (ΔHt) and entropy (ΔSt) of transition show odd-even alternations with odd-chain compounds having higher values than the even-chain compounds. The critical micellar concentration (CMC) of NATs was determined in water at room temperature by fluorescence spectroscopy by monitoring the spectral changes of 8-anilinonaphthalene-1-sulfonic acid (ANS). The CMCs of NATs were found to decrease with increase in acyl chain length. The present results provide a thermodynamic and structural basis for investigating the interaction of NATs with other membrane lipids and proteins, which in turn can shed light in understanding how they function in vivo (in biological membranes).