Formation of square lamellae by self-assembly of long-chain bolaphospholipids in water

Abstract

The self-assembly process of the symmetric single-chain polymethylene-1,ω-bis(phosphocholine) (PC-C34-PC) with a chain length of 34 carbon atoms and two polar phosphocholine headgroups when put into water is exclusively driven by hydrophobic interactions of the long alkyl chains. This process leads to the formation of a dense network of helical nanofibers and the formation of a hydrogel (Meister et al. J. Phys. Chem. B, 2008, 112, 4506). In contrast, the single-chain bolalipids tetra- and hexatriacontane-1,ω-diyl-bis[2-(dimethylammonio)ethylphosphate] (Me2PE-Cn-Me2PE, n = 34, 36) and the partly deuterated analogue Me2PE-C11-(CD2)12-C11-Me2PE (dMe2PE-C34-Me2PE) form a different type of stable aggregate. In a first step, the self-assembly of these long-chain bolalipids in water at room temperature leads to the formation of a dense network of nanofibers which eventually form a hydrogel. Within one day, the nanofibers transform into square lamellae that grow up to an edge length of about 100 nm. Nanofibers are linked to one or two (opposite) corners of the squares leading to the appearance of a kite-like structure. After one week, all fibers have been transformed into square lamellae which are apparently stacked and form a gel cake. Within several weeks, a more compact cake is formed by syneresis, i.e. the expulsion of water.