New Dicholesteryl-Based Gelators:  Chirality and Spacer Length Effect

Abstract

Eight new diacid amides of dicholesteryl L(D)-alaninates were designed and prepared. The compounds with spacers containing three, four, five, or six carbon atoms and L-alanine residues are denoted as 1a, 2a, 3a, and 4a, respectively, and those containing D-alanine residues are denoted as 1b, 2b, 3b, and 4b, respectively. A gelation test revealed that a subtle change in the length of the spacer and an inverse in the chirality of the amino acid residue can produce a dramatic change in the gelation behavior of the compounds and the microstructures of the gels, as revealed by SEM, XRD, and CD measurements. Importantly, for the compounds 1 and 2, those containing d-alanine residues (1b, 2b) are more efficient gelators than their analogues with opposite chirality (1a, 2a). For the compounds of longer spacers (3, 4), however, those containing l-alanine residues (3a, 4a) are superior to the corresponding ones with d-alanine residues (3b, 4b). Very interestingly, of the 139 gel systems studied, at least 11 of them gel spontaneously at room temperature. Studies of the rheological properties of the example systems of these gels demonstrated that change in the spacer lengths of the gelators has a great effect upon the mechanical properties of the corresponding gels, and the studies also revealed the thixotropic properties of the gels. Furthermore, it was observed that 4a forms water-in-oil gel emulsions with some organic solvents by simple agitating the systems at room temperature.

Preparation of dicholesteryl-derivatives: The effect of spatial configuration upon gelation

“…Likewise, the impact of the different inherent chirality of the gelators on the morphology was displayed by the varied morphologies of the xerogels of MaDC/DMSO and MaLC/DMSO, FaDC/cyclohexane and FaLC/cyclohexane, as well as FaDC/benzene and FaLC/ benzene. These findings indicate clearly that the slight change in the spatial configuration of the linkers between the two cholesteryl moieties of each compound shows a significant effect on the morphologies [22,37]. This conclusion is also supported by the results from polarized optical microscopy (POM) studies of the gels of MaDC/DMSO, FaDC/DMSO and MaLC/DMSO (Figure 2).…”

“…It is a little bit surprising that the direction of the spectral shifts of MaLC system is different from other gel systems (Table 3), indicating that a different aggregation mode might be adopted by this system [48]. Similar phenomena were reported by our group before [22].…”

“…The G′ values of FaDC/ n-nonane systems at different concentrations are shown in Figure 5 as functions of shear stress amplitude. From the Figure 5, it can be seen that in the linear viscoelastic region, the value of G′ increases from only 1851 to 12750 Pa along with increasing the gelator concentration from 1.0% to 3.0% (w/v), and correspondingly, the yield stress changes from only 9 to 141 Pa, indicating that both the mechanical stability of the gel network and the elastic property of the gel depend well upon the concentration of the gelator [22].…”

“…Upon further examination of the CD spectra in Figure 3(a), it can be observed that the CD signal of FaDC gel is negative, whereas that of FaLC gel is positive, suggesting that the chiralities of the assemblies in the two kinds of gels are opposite. Obviously, this difference can be attributed to the difference in the chirality of the amino acid residues in the gelators [22].…”

“…Our group reported the gelation of eight new diacid amides of dicholesteryl L(D)-alaninates. We found that an inverse in the chirality of the amino acid residue can produce a dramatic change in the gelation behavior of the compounds and the microstructures of the gels [22].…”