Chiral Self-Assembly of Nanotubules and Ribbons from Phospholipid Mixtures Mark S. Spector,* ,† Alok Singh, † Phillip B. Messersmith, ‡ and Joel M. Schnur † Center for Bio/Molecular Science and Engineering, Code 6900, NaVal Research Laboratory, Washington, D.C. 20375-5348, and Biomedical Engineering Department, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 Received May 16, 2001; Revised Manuscript Received June 14, 2001 ABSTRACT Nanoscale structures have been found to self-assemble in a binary mixture of a long-chain, diacetylenic phospholipid and a short chain, saturated lipid. We observe unusual chiral-optical signatures of nanotubule, twisted ribbon, and microtubule morphologies that can be used to monitor their temporal and thermal stability. Circular dichroism results suggest that chiral packing of the lipids drives formation of these aggregates. The ease of formation, stability, and robust behavior of these lipid cylinders suggest that they have potential use in applications requiring high aspect ratio nanomaterials. The self-assembly of biologically based amphiphiles into potentially useful nanostructures has been the subject of intense study in recent years. Many types of amphiphilic molecules self-assemble to form cylindrical tubules and helical ribbons. 1-4 In most cases, these systems consist of a single chiral amphiphile in an aqueous environment. One system, diacetylenic phospholipids, self-assemble into hol- low, cylindrical aggregates with diameters of 0.5 μm and lengths of 50-200 μm. 5,6 These microtubules have attracted interest due to potential applications in controlled release, electroactive composites and multifunctional materials. 1 Theoretical work suggesting that the formation of helical morphologies is driven by twisting of the amphiphile bilayer due to symmetry breaking in the packing of chiral mol- ecules 7,8 has been supported by circular dichroism (CD) studies. 9,10 When molecules form chiral aggregates, nonchiral molecular absorptions can become chirally active, with differential absorption of left- and right-handed circularly polarized light. CD peaks in such chiral structures can be quite large and provide useful structural information about their molecular architecture. 11 Lipid tubules are found to have intense peaks in their CD spectra, indicating large chiral correlations in molecular packing. 10 This chiral order can no longer be maintained when the chains become disordered and the tubules melt, leading to a decrease in CD peak intensity by 4 orders of magnitude. Addition of short chain lipid spacers to diacetylenic lipid tubules has been found to significantly enhance polymeri- zation efficiency when the length of the acyl chains in the spacer lipid is matched to the number of methylenes in the upper segment (closer to the headgroup) of the diacetylenic acyl chains. 12 This is probably due to geometric consider- ations in the mixed lipid bilayer, namely an increase in the average area-to-volume ratio of the lipids. Surprisingly, a gel-like network of twisted fibers was found to form in an equimolar mixture of 1,2-bis(tricosa-10,12-diynoyl)-sn-glyc- ero-3-phosphocholine (DC 8,9 PC) and 1,2-bis(dinonanoyl)-sn- glycero-3-phosphocholine (DNPC). 13 Further studies found that a new, nanotubular morphology, consisting of cylinders with diameters around 50 nm, preceded the formation of twisted ribbons. 14 While these nanotubules were transformed into the ribbon-gel after a few hours at ambient temperature, they appeared to be stable at 4 °C. Such structures have prospective use as substrates for fabrication of electroactive composites based upon the incorporation of nanofibers. 15 There also appear to be potential applications of colloidal suspensions of nanotubules in biochemical sensing, energy transduction, and catalysis applications. However, it will be important to first learn how to prepare large-scale colloidal suspensions purely of nanotubules. In addition, surface modifications are often required in order to metallize nanotubules or to perform further functionalization for use in the above-mentioned applications. 16 In this paper, we report on the thermal behavior and stability of nanotubules as characterized by circular dichroism spectroscopy, electron microscopy, and NMR. * Corresponding author: Phone (202) 404-6051, Fax (202) 767-9594, E-mail: spector@cbmse.nrl.navy.mil. ² Naval Research Laboratory. ‡ Northwestern University. NANO LETTERS 2001 Vol. 1, No. 7 375-378 10.1021/nl015554u CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001