Structure and Thermotropic Phase Behavior of a Homologous Series of N-Acylglycines: Neuroactive and Antinociceptive Constituents of Biomembranes

Abstract

N-Acylglycines (NAGs) with different acyl chains have been found in the mammalian brain and other tissues. They exhibit significant biological and pharmacological properties and appear to play important roles in communication and signaling pathways within and between cells. In view of this, a homologous series of NAGs have been synthesized and characterized in the present study. Differential scanning calorimetric (DSC) studies show that the transition enthalpies and entropies of dry as well as hydrated NAGs exhibit a linear dependence on the acyl chain length. Most of the NAGs show a minor transition below the chain-melting phase transition, suggesting the presence of polymorphism in the solid state. Structures of N-myristoylglycine (NMG) and N-palmitoylglycine (NPG) were solved in monoclinic system with C2/c and P21 space groups, respectively. Analysis of the crystal structures show that NAGs are organized in a bilayer fashion, with head-to-head (and tail-to-tail) arrangement of molecules. The acyl chains in both structures are essentially perpendicular to the bilayer plane, which is consistent with a lack of odd–even alternation in the thermodynamic properties. The bilayer is stabilized by strong hydrogen bonding interactions between −COOH groups of the molecules from opposite leaflets as well as N–H···O hydrogen bonds between the amide groups of adjacent molecules in the same leaflet and dispersion interactions among the acyl chains. Powder X-ray diffraction data show that the d-spacings for the NAGs with different acyl chains (n = 8–20) exhibit a linear dependence on the chain length, suggesting that all the NAGs investigated here adopt a similar packing arrangement in the crystal lattice. These observations are relevant for understanding the role of N-acylglycines in biological membranes.