Differential scanning calorimetric studies on the thermotropic phase transitions of N-acylethanolamines of odd chainlengths

Abstract

N-acylethanolamines (NAEs) exhibit a wide spectrum of properties that are biologically important. Recently, the authors investigated the thermotropic phase transitions of a homologous series of NAEs containing even number of C-atoms by differential scanning calorimetry. These studies have been extended now to the NAEs with odd number of C-atoms (n = 9 - 19) in the acyl chain. All the NAEs show a major sharp endothermic phase transition that coincides with the melting point for the dry NAEs, while for the hydrated samples it occurs at considerably lower temperatures. The NAEs in the longer chainlength range also display an additional, minor transition before the chain-melting transition. The transition enthalpy (ΔH(t)) and transition entropy (ΔS(t)) have been found to depend linearly on the chainlength, n and a least squares analysis yielded the incremental values, ΔH(inc) and ΔS(inc) contributed by each CH2 unit to ΔH(t) and ΔS(t), respectively. The incremental values obtained for the dry NAEs are smaller than those obtained for the hydrated samples if only the main chain-melting phase transitions are considered, however, if the enthalpy changes of the major as well as the minor transitions are combined then the incremental values obtained in the two cases are comparable. Further, the values of ΔH(inc) and ΔS(inc) obtained for odd chainlength series are comparable to those obtained for the even chainlength series of NAEs, but are considerably higher than those obtained for the diacyl phospholipids on a per chain basis. An alternation has been observed in the ΔH(t), ΔS(t) and transition temperatures obtained for the odd chainlength series and the even chainlength series, similar to that observed in long chain hydrocarbons and fatty acids.