Self-assembly of helical ribbons

Abstract

The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 ؎ 2°, and low pitch, with a pitch angle of 11 ؎ 2°. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47°, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.Interest in molecular self-assembly of helical structures is driven by both technological and medical applications. Helices are often precursors in the growth of tubules (1-4), which can be used as a controlled release system for drug delivery in medicine and as templates for microelectronics and magnetic applications (5). The morphology of the tubules must be rationally optimized for each application. Therefore it is important to understand the role of the various constituent molecules in the formation of these structures.Helical ribbons have been observed in a variety of systems composed of chiral amphiphiles. Although the diameters and lengths of the helical structures varied from system to system, the pitch angle observed in all systems was either 45°(6-8) or Ϸ60°(9-12). In almost all cases, helical ribbons in enantiomerically pure systems were either right-or left-handed (6,8,13). Recently, however, Thomas et al. studied an enantiomerically pure phosphonate analogue of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine and related compounds, which self-assembled into a mixture of right-and left-handed helical ribbons (10, 11).A biologically important system in which helical ribbons form is model bile, consisting of a mixture of three types of chiral molecules in water: a bile salt, a phosphatidylcholine, and cholesterol (4, 14-21). Helical ribbons are metastable intermediates in the process of cholesterol crystallization in bile (2,19,20), which precedes cholesterol gallstone formation (4,(18)(19)(20)(21)(22)(23). In contrast to all other systems studied, two pitch types of helical ribbons are observed in bile: high pitch, with a pitch angle of 54 Ϯ 2°, and low pitch, with a pitch angle of 11 Ϯ 2°. To date, the production of these two helical pitch types has been thought to be a property unique to model biles. Indeed, previous work showed that all three components of model bile are required for helical ribbons to form. In the absence of the phosphatidylcholine, only needle-like crystals form (4, 21), whereas without the bile salt only...

Microtubes self-assembled from a cholesterol-modified nucleoside

“…It should also be noted that the main gel-to-liquid transition of DOPC takes place at temperatures well below 0 °C; 13 therefore, the mechanism of tube formation cannot be related to the phase transition of the phospholipid , as is the case for diacetylenic phosphocholines and several other tube-forming lipids. 4,14 Another difference to previously described cholesterol-based systems is the quick formation and the long-term stability of the tubules made from 1 and DOPC. Once formed, the 1 : DOPC microtubes are stable at least for six months.…”

“…3 In particular, cholesterol and other natural sterols (also chiral molecules) have been reported to form helical ribbons and microtubes in multicomponent mixtures with surfactants and phospholipids. 4 These ribbons and tubules are single crystals that represent metastable intermediates in a crystallization pathway proceeding from filaments to plate-like crystals. 4 Nucleobase -derived amphiphiles can also form helical and tubular structures, as described for phospholipid -nucleoside conjugates 5 as well as for different single-and double-chain amphiphilic nucleobase derivatives prepared either via covalent modification 6 or by electrostatic complexation.…”

“…4 These ribbons and tubules are single crystals that represent metastable intermediates in a crystallization pathway proceeding from filaments to plate-like crystals. 4 Nucleobase -derived amphiphiles can also form helical and tubular structures, as described for phospholipid -nucleoside conjugates 5 as well as for different single-and double-chain amphiphilic nucleobase derivatives prepared either via covalent modification 6 or by electrostatic complexation. 7 The formation of these supramolecular structures is attributed to the combination of the aggregation properties of the lipid part (hydrophobic effect) with the hydrogen bonding and π-π stacking of the nucleobase moieties.…”

“…In contrast, cholesterol-based tubes are transient intermediates that take several days to form and then convert into non-tubular crystalline structures over a short time. 4 The surface structure and the morphology of the tubules were studied using electron microscopy . A scanning electron microscopy (SEM) image of a microtubule (Fig.…”

Helical polymer in cylindrical confining geometries

“…In amphiphilic systems the formation of helical ribbons has been observed in multicomponent mixtures of a bile salt or some other nonionic surfactant, phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol [1,2]. The ribbons have typical diameters in the range of 5 -20 m and pitch angles between 10°and 60°.…”