Formation of Polymerizable Phospholipid Nanotubules and Their Transformation into a Network Gel

Phospholipids containing diacetylene moieties along their hydrocarbon chains have been extensively studied for their ability to self-assemble into tubule structures from aqueous and aqueous/alcohol solvents. 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) has previously been shown to form hollow microtubules of length 50-200 μm and average diameter of approximately 0.5 μm. In this paper we report the formation of phospholipid tubules with outer diameters approximately an order of magnitude less than those of pure DC8,9PC microtubules, from a mixture of DC8,9PC and the short chain saturated lipid 1,2-bis(dinonanoyl)-sn-glycero-3-phosphocholine (DNPC). Optical and electron microscopy of equimolar aqueous mixtures of DC8,9PC and DNPC revealed the formation of nanotubules of diameter 50-60 nm and length up to 100 μm, after incubation at ambient temperature for several hours. Continued incubation of the nanotubule suspension at ambient temperature caused a gradual transformation of the nanotubules into a lipid gel phase consisting of interconnected helical ribbons. The rate of nanotubule to helical ribbon transformation was lipid concentration dependent; suspensions of higher lipid concentration transformed more rapidly than less concentrated solutions. However, the transformation to helical ribbons could be prevented altogether by storing the nanotubule suspension at 4 °C. Differential scanning calorimetry of the nanotubule and helical ribbon phases revealed endothermic phase transitions at 25.4 and 36.5 °C, respectively. Both phases were polymerized at 10 °C upon exposure to short wave UV (254 nm), as indicated by visible light spectroscopy. NMR analyses suggest that the molecular composition of both phases is close to the expected 1:1 ratio of DC8,9PC:DNPC. The nanotubules described herein may have use in technological applications where small diameter, high aspect ratio structures are desired.