Self-assembling structures of long-chain sugar-based amphiphiles influenced by the introduction of double bonds

Abstract

Nine phenyl glucoside or galactoside amphiphiles possessing a saturated or unsaturated long alkyl-chain group as the self-assembling unit of a highly organized, molecular architecture were synthesized. Their self-assembly properties were investigated by using energy-filtering TEM (EF-TEM), SEM, CD, XRD, and FT-IR techniques. Compound 2, possessing one cis double bond in the lipophilic portion, exhibited twisted helical fibers, which formed a bilayered structure with a 3.59 nm period, while 3 exhibited helical ribbons and left-handed nanotubular structures with 150-200 nm inner diameters and a wall thickness of approximately 20 nm. Very interestingly, 4, possessing three cis double bonds, exhibited a nanotubular structure with an inner diameter of approximately 70 nm and a d spacing value of 4.62 nm. On the other hand, 7, possessing two trans double bonds in the lipophilic region, exhibited crystal- or plate-like structures, which formed a bilayer structure with a d spacing value of 3.93 nm. These results indicate that the self-assembly properties are strongly dependent on the type of double bond. Furthermore, 8 and 9, with the galactopyranose moiety, revealed helical ribbon and well-defined double helical fiber structures, respectively. These findings support the view that the orientation of the intermolecular hydrogen-bonding interaction between the sugar moieties plays a critical role in producing the nanotubular structures. According to CD and powder XRD experiments, the relatively strong intermolecular hydrogen-bonding interaction of the glucopyranoside moiety in 3 and 4 provided a highly ordered chiral packing structure. Even though these compounds formed a weak hydrophobic interaction between lipophilic groups, it led to the formation of the nanotubular structure.