Langmuir 1999, 15, 6135-6138
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Easily Accessible Uniform Wide-Diameter Helical, Cylindrical, and Nested Diacetylene Superstructures That Can Be Metallized and Oriented in Magnetic Fields Guijun Wang and Rawle I. Hollingsworth* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 Received June 8, 1999
Very long lipid tubules with uniform widths, spirals, and spirals nested in tubular superstructures are formed rapidly from a lipid analogue made by acylating 1- (N,N-1-dimethylamino)ethyl-2,3dihydroxybutyramide with two molecules of pentacosa-10,12-diynoic acid. The high propensity of this molecule for tubule formation is attributed to the large carbonyl dipole in the headgroup. The tubules formed from the pure lipid are hollow and can be gold-plated by vapor deposition without collapsing. The superstructures can also be metallized by adsorption of nickel(II) or copper(II) ions followed by reduction with borohydride. The nickel-metallized tubules can be oriented in very modest magnetic fields. Tubules can be over 1000 µm in length and have a wide diameter. The unusual length, stability, and ease of preparation of these tubules should be of great utility in the construction of microstructures.
* Corresponding author: phone, 517-353-0613; fax, 517-3539334; E-mail,
[email protected].
pholipids can form such structures in the pure state11 but this is rather rare. Some naturally occurring glycolipids, which are generally much more inaccessible, have been reported to form tubules.12 The formation of tubules from most lipids is usually by a slow cooling process.13 Tubule formation at constant temperature often requires many days to months.14,15 Generally, tubules are not stable above the melting transition temperature of the chains.10 They are often thin and fragile and need to be stabilized. They have been stabilized by silica deposition16,17 and metal coating.18-20 Metal-coated tubules have additional functionality as microscale conductors. Because chirality appears to be essential to helix and tubule formation,21 the preparation of tubule-forming lipids is usually not straightforward. Therefore simple chiral lipid surrogates that easily form tubules, helices, and other uniform supersystems are highly desirable. Recently22 we described the preparation and physical characterization of a new chiral biomembrane lipid analogue (1) containing a chiral 1-(N,N-1-dimethylamino)ethyl-2,3-dihydroxybutyramide headgroup that could
(1) Browning, S. L.; Lodge, J.; Price, R. R.; Schelleng, J.; Schoen, P. E.; Zabetakis, D. J. Appl. Phys. 1998, 84, 6109-6113. (2) Kirpatrick, D. A.; Bergeron, G. L.; Czarnaski, M. A.; Hickman, J. J.; Chow, G. M.; Price, R.; Ratna, B. L.; Stockton, W. B.; Baral, S.; Ting, A. C.; Schnur, J. M. Appl. Phys. Lett. 1992, 60, 1556-1558. (3) (a) Schnur, J. M.; Price, R.; Rudolph, A. S. J. Controlled Release 1994, 28, 3-13. (b) Johnson, D. L.; Polikandritou-Lambros; Martonen, T. B. Drug Delivery 1996, 3, 9-15. (4) Schnur, J. M. Science 1993, 262, 1669-1676. (5) Dagani, R. Chem. Eng. News 1993, Aug. 9, 19-20. (6) Rudolph, A. S.; Calvert, J. M.; Schoen, P. E.; Schnur, J. M. In Biotechnological Applications of Lipid Microstructures; Gaber, B. P., Schnur, J. M., Chapman, D., Eds.; Plenum Press: New York, 1988; Vol. 238, pp 305-320. (7) Schnur, J. M.; Price, R.; Schoen, P.; Yager, P.; Calvert, J.; Georger, J.; Singh, A. Thin Solid Films 1987, 152, 181-206. (8) Stockton, W.; Lodge, J.; Rachford, F.; Orman, M.; Falco, F.; Schoen, P. J. Appl. Phys. 1991, 70, 4679-4686. (9) Krebs, J. J.; Rubinstein, M.; Lubitz, P.; Harford, M. Z.; Baral, S.; Shashidhar, R.; Ho, Y. S.; Chow, G. M.; Quadri, S. J. Appl. Phys. 1991, 70, 6404-6406. (10) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371381.
(11) Mishima, K.; Ogihara, T.; Tomita, M.; Satoh, K. Chem. Phys. Lipids 1992, 62, 87-91. (12) Kulkarni, V. S.; Anderson, W. H.; Brown, R. E. Biophys. J. 1995, 69, 1976-1986. (13) Thomas, B. N.; Safinya, C. R.; Plano, R. J.; Clark, N. A. Science 1995, 267, 1635-1638. (14) Nakashima, N.; Asakuma, S.; Kunitake, T. J. Am. Chem. Soc. 1985, 107, 509-510. (15) Georger, J. H.; Singh, A.; Price, R. R.; Schnur, J. M.; Yager, P.; Schoen, P. E. J. Am. Chem. Soc. 1987, 109, 6169-6175. (16) Calvert, J. M.; Georger, J. H.; Peckerar, M. C.; Pehrsson, P. E.; Schnur, J. M.; Schoen, P. E. Thin Solid Films 1992, 210, 359-363. (17) Barals, S.; Schoen, P. Chem. Mater. 1993, 5, 145-147. (18) Rumlik, C. J.; Menton, V. P.; Martin, C. R. J. Mater. Res. 1994, 9, 1174-1183. (19) Markowitz, M.; Baral, S.; Brandow, S.; Singh, A. Thin Solid Films 1993, 224, 242-247. (20) Archibald, D. D.; Mann, S. Nature 1993, 364, 430-433. (21) Schnur, J. M.; Ratna, B. R.; Selinger, J. V.; Singh, A.; Jyothi, G.; Easwaran, K. R. K. Science 1994, 264, 945-947. (22) Wang, G.; Hollingsworth, R. I. Langmuir, 1999, 15, 3062-3069.
Introduction Lipid tubules and helices are extremely interesting superstructures that have captured the imagination of scientists in disciplines from biology through material science to chemistry and physics. Tubules have much promise as advanced materials for use in several applications ranging from small molecular wires1,2 to drug encapsulation3-5 to the miniature electronic devices.6-9 Tubules containing polydiacetylene cores are especially interesting since they have greater stability,6,7,10 and the optical and electrooptical properties of polydiacetylenes provide very special functionality. However, only a few classes of lipids can form tubular structures under certain conditions. The difficulty in preparing optically active phospholipid variants is a major obstacle to the use of typical lipids and phospholipid analogues in the fabrication of lipid helices and tubules. Naturally occurring phos-
10.1021/la990724s CCC: $18.00 © 1999 American Chemical Society Published on Web 08/03/1999
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be fabricated in two steps from an activated acid chloride, (S)-3-hydroxy-γ-butyrolactone and 3-amino1-dimethylaminoethane. The diacetylene-containing acyl chain is derived from pentacosa-10,12-diynoic acid. The presence of the carbonyl function with its strong anisotropy in the headgroup should generate a significant twist leading to the deformation of narrow strips to form helices and eventually tubules. This lipid analogue can be obtained in very high chemical purity thus allowing for extremely good packing with a minimum of defects, this should lead to some interesting aggregate strucrtures. Its supramolecular chemistry was therefore explored.
Experimental Section Laser scanning microscopy was performed on a Zeiss 210 instrument with a 488 nm laser. Images were obtained in the bright field, dark-field, phase contrast, and polarization modes. Scanning electron microscopy (SEM) was performed on a JEOL JMS-35CF SEM instrument. Samples were coated with gold (10 nm thickness) in an Emscope Sputter Coater, model SC 500, purged with argon gas, and then examined. For investigating the conditions for tubule formation, the lipid (10 mg) was dissolved in 0.5 mL of chloroform and 9.5 mL of ethanol. Water (0.1 mL) was added to a portion of this solution (0.25 mL). Some preparations contained acetic acid, trifluoroacetic acid, and sodium bicarbonate in amounts 0.5, 1, 5, or 10 times the number of moles of lipid. Samples to which no acid nor base was added were incubated at 14, 25, 30, 37, and 50 °C. An increase in opacity indicated tubule formation. A drop of the sample which contained the lipid tubes or other aggregates was transferred to a thoroughly cleaned glass slide. After 1 or 2 h the residue water evaporated, and the sample was observed directly under the laser confocal microscope. The samples were observed after several hours up to 2-3 days. Longer tubules were obtained from longer incubation times. In the metallization experiments, samples were prepared as above method except that the water contained copper(II) or nickel(II) sulfate in amounts 5-10 equiv of the lipid. A suspension of the nickel-impregnated tubules in water was placed on a microscope slide aligned north-south in a 0.3-0.8 T magnetic field, and ∼10 equiv of sodium borohydride was added. The slides were left in the magnetic field for 2 h to 1 day.
Results and Discussion Tubule formation was observed under most conditions tested. When solutions of 1 in ethanol containing a small proportion of chloroform were treated with a small amount of water in the presence of acetic acid and the solutions left standing at room temperature, helices and tubules were observed in less than 2 days (Figure 1A). The typical length of a tubule was 50 µm for the neutral lipid water systems; for the systems formed in the presence of acid, the tubules are longer and straighter, with average length 100-400 µm. Over longer times, of the order of 1 week, tubules with lengths around 1200 µm were obtained (Figure 1B). Tubules were uniform in diameter and the typical external diameter was ∼ 2 µm. The helices were left-handed (Figure 1C). Tubules were hollow, and their
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interiors could be observed by optical cross-sectioning using scanning laser confocal fluorescence microscopy (Figure 1D). Nested superstructures in which spirals resided inside of hollow tubes were also observed (Figure 1E). Such structures were not very common and, to the best of our knowledge, have not been reported before. The tubules were examined by scanning electron microscopy after coating with gold. The structures were very uniform and robust (Figure 1F), and no flattening was observed as was reported for other tubules.23 The optimum condition for tubule formation by 1 was in the presence of acid at ∼25 °C. Both acetic acid and trifluroacetic acid in the amount of 1-6 equiv per lipid equivalent gave good results. The tubules were very straight and uniform in size and much longer than tubules formed in the absence of acid. There was also a significant increase in their average width. Few or no helices were formed under these conditions. Tubule formation was less frequent under basic conditions. Generally, tubules formed more easily at room temperature. Few were formed at 37 °C or higher. Smaller tubules were formed at lower temperatures. They were observed at temperatures as low as 14 °C. Although tubule formation appeared to be more facile in the presence of copper(II) and nickel(II) ions, the presence of metal ions was not necessary. The metal ion coated superstructures were treated with sodium borohydride to effect reduction to the native metal thus coating the structures. Reduction of the nickel(II)-coated structures in the presence of the magnetic field provided by a permanent magnet (0.20.6 T) resulted in deposition of tubules predominantly aligned with the north-south field of the magnet (Figure 2A). In addition to this, several long parallel tubules originating on the edge of the lipid system and running parallel to the magnetic field were observed (Figure 2B). It has been reported that metallized tubules can be orientated in electric or magnetic fields, but strong magnetic fields of the order of several tesla are usually used.24 The three largest obstacles to the preparation of tubules from chiral lipid systems are the difficulty in isolating or synthesizing significant amounts of materials, the long times and stringent conditions required for tubule formation, and the fragility of the final products. Since the discovery of this new type of lipid microsctructure,10,23 there has been an ongoing effort to find lipid systems that can form tubules and that allow the fabrication of tubules in large quantities. Many potential tubule-forming lipids from natural phospholipids to various synthetic lipid systems12-15,25-29 and the optimal conditions for forming tubules from them have been investigated. The tubuleforming lipid we described here can be prepared in two (23) Yager, P.; Schoen, P. E.; Davies, C.; Price, R.; Singh, A. B. Biophys. J. 1985, 48, 899-906. (24) Rosenblatt, C.; Yager, P.; Schoen, P. E. Biophys. J. 1987, 52, 295-301. (25) (a) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1991, 113, 7436-7437. (b) Fuhrhop, J.-H.; Blumtritt, P.; Lehmann, C.; Luger. J. Am. Chem. Soc. 1991, 113, 7437-7439. (c) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1994, 116, 10057-10069. (26) (a) Markowitz, M.; Singh, A. Langmuir 1991, 7, 16-18. (b) Markowitz, M.; Schnur, J. M.; Singh, A. Chem. Phys. Lipids 1992, 62, 193-204. (27) Giulieri, F.; Guillod, F.; Greiner, J.; Krafft, M.-P.; Riess, J. G. Chem. Eur. J. 1996, 2, 1335-1339. (28) Thoms, B. N.; Corcorn, R. C.; Cotant, C. L.; Lindemann, C. M.; Kirsch, J. E.; Persichini, P. J. J. Am. Chem. Soc. 1998, 120, 1217812186. (29) Kodama, M.; Miyata, T.; Yokoyama, T. Biochim. Biophys. Acta 1993, 1168, 243-248.
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Figure 1. (A) A phase contrast laser scanning confocal microscopy image of tubules formed by 1 in the presence of acetic acid after 2 days. The ratio of acetic acid to lipid was 10:1. (B) A dark field image of very long tubules formed in the presence of trifluoroacetic acid (1:1 equiv to lipid) after 6 days. The tubules were treated with NiII ions and left for one additional day. (C) A phase contrast image by confocal microscopy of spirals and tubules formed in the presence of 1 equiv of acetic acid. (D) A phase contrast image by laser scanning confocal microscopy optical section of tubules formed under neutral condition in the absence of metal ions. The inner dark region is the long axial center part of the tubule. The tubule growth time was 2-3 days. (E) A phase contrast image by confocal microscopy of spirals and tubules formed in the presence of 0.5 equiv of acetic acid after growth at room temperature for 2 days. Note the nested superstructures in which spirals resided inside of hollow tubes. (F) A scanning electron micrograph of the tubules formed under neutral condition and absence of metals ions at room temperature after 4 days.
simple steps from a readily available chiral lactone. The propensity of lipid 1 to form tubules is very high. This probably stems from the strong dipole of the carbonyl group, which should exert a tremendous organizational
effect. A spiral offset between adjacent molecules should reduce the unfavorable alignment of the dipoles while facilitating the spiral architecture necessary for tubule formation. The formation of spirals and tubules is also
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facilitated by charges in the headgroup. Some charge component is also provided by the dimethylamino group especially at low pH. The increase in diameter (both internal and external) of the tubules under acidic conditions and in the presence of metal cations is expected because of increased repulsion of the headgroups. This phenomenon affords a practical method for controlling the length, width, and stability of tubules. The amino functions in the headgroup are good ligands for transition metals thus allowing the tubules to be metal-coated by in situ reduction by a hydride donor after allowing them to adsorb ions from solution. The diacetylene functions in tubules can be polymerized by treating with UV light. Metallized tubules or spiral-in-tubule superstructures might allow the fabrication of composite devices with highly conducting metallic and moderately conducting polydiacetylene elements. The facile orientation of nickel-coated tubules in a weak magnetic field have significance in the area of microfabrication. The tubular diameter and length are significantly greater than other tubule systems reported so far. This might present advantages in the construction of microelectronics devices. The ready availability of new versatile chiral lipid analogues such as 1 always represents a significant development in the chemistry of new advanced materials.
Figure 2. (A) Light micrograph showing preferential alignment of nickel-coated tubules formed in a magnetic field. (B) A dark field image by confocal microscopy of nickel-coated filaments from an acetic acid containing solution. The metallization was performed in a magnetic field.
Acknowledgment. This work was supported by the State of Michigan Research Excellence Fund. We thank Dr. Joanne Whallon and Dr. Shirley Owens at the Laser Scanning Confocal Microscopy Center and Dr. Ewa Danielewicz at the Center for Electron Optics for technical assistance. LA990724S
