Investigation of the thermotropic behavior of isomer mixtures of diacyl arginine-based surfactants. Comparison of polarized light microscopy, DSC, and SAXS observations

Abstract

Dilauroylglycerol arginine conjugates constitute a novel class of specific biocompatible surfactants, which can be considered analogues of partial glycerides and phospholipids. During their preparation, spontaneous intramolecular acyl-migration reactions are observed and both possible regioisomers are obtained: 1,2-dilauroyl-rac-glycero-3-(Nα-acetyl-l-arginine) (1212RAc) and 1,3-dilauroylglycero-2-(Nα-acetyl-l-arginine) (12RAc12). To study the influence of the presence of both regioisomers on the physicochemical properties, the phase behavior in the dry state of pure 1,2-dilauroyl-rac-glycero-3-(Nα-acetyl-l-arginine) and two mixtures of both regioisomers were studied by differential scanning calorimetry and small-angle X-ray diffraction complemented by polarized light microscopy. The experiments were performed for a sequence of heating, cooling and reheating scans. The results have shown that all samples investigated display thermotropic liquid crystallinity. The transition temperatures have been observed to be independent of the particular technique. The first heating scan is different from the cooling and reheating scans with respect to both thermal and structural parameters. The SAXS curves have shown the coexistence of two orders of lamellar spacing in the first heating scan. This coexistence is lost in subsequent scans. This suggests the presence of microphase separation of isomers in the lyophilized samples with miscibility after melting. The lamellar distance obtained for the third scan was used to study the molecular conformation of 1212RAc and 12RAc12 compounds.