Differential scanning calorimetric and powder X-ray diffraction studies on a homologous series of N-acyl-L-alanine esters with matched chains (n = 9-18)

Abstract

A homologous series of two chain derivatives of L-alanine, namely N-acyl L-alanine alkyl esters (NAAEs), bearing matched, saturated, acyl and alkyl chains (n= 9-18) have been synthesized. The thermotropic phase transitions and supramolecular structure of NAAEs were investigated by differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). Results obtained from DSC studies indicate that the transition temperatures (T t), enthalpies (ΔH t) and entropies (ΔS t) exhibit odd-even alternation with compounds bearing odd acyl and alkyl chains showing higher values of T t, ΔH t and ΔS t as compared to NAAEs with even acyl and alkyl chains. However, the transition enthalpies and entropies of the odd- and even chain length series independently exhibit a linear dependence on the chain length. The d-spacings obtained from PXRD increase linearly with chain length with an increment of 1.76 Å/CH 2, suggesting that NAAEs adopt either a tilted bilayer structure or a bent structure. The present results provide a thermodynamic and structural basis for investigating the interaction of NAAEs with other membrane lipids, which in turn can shed light in understanding how they can enhance the transdermal permeability of stratum corneum.