Synthesis and characterization of a new generation of cryptand-based triple-tailed amphiphiles: spontaneous formation of vesicles and X-ray crystallographic studies

Abstract

A cryptand incorporating three secondary amino groups has been derivatized by reacting with acid chlorides of different alkyl chain lengths (between C₄ and C₁₅) to get a new generation of six cryptand-based triple-tailed neutral amphiphiles. The cavity of the cryptand headgroup accommodates a Cu(II) ion giving another set of six amphiphiles. These amphiphiles can aggregate spontaneously as vesicles in 4% ethanolic water medium. They form vesicles by the extrusion method as well. In both methods, vesicles formed are mostly unilamellar. No distinguishable change can be seen in the shape of the vesicular structures between free and complexed amphiphiles. Spontaneously formed vesicles are less stable compared to those formed by the extrusion method. Stability of the vesicles decreases from weeks to hours with the lowering of the hydrophobic chain lengths. The amphiphile with three octanoyl groups attached (L₄) crystallizes in the monoclinic space group P2₁/c with a = 10.013(9) Å, b = 30.923(7) Å, c = 18.298(4) Å, β = 92.67(4)°, Z = 4, R_f = 0.092, R_wf = 0.120, and GOF = 4.572. The solid-state structure of L₄ shows the hydrophobic tails pack in the lattice by a mixed interdigitizing and noninterdigitizing manner, which is rare. We have also found in this study that spontaneous vesicular aggregates are possible with cryptand-based amphiphiles even when each hydrophobic tail has only four carbon atoms.