Self-Assembling Tubules from Phospholipids - Advances in Chemistry

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 7, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch018

Self-Assembling Tubules from Phospholipids R. Shashidhar and J. M. Schnur* Center for Bio/Molecular Science and Engineering, Code 6900, Naval Research Laboratory, Washington, D C 20375

We present a state-of-the-art review on the fundamental aspects and potential applications of tubules formed from phospholipid molecules. Tubules constitute a unique form of self-assembling microstructures. An understanding of their formation is important to enhance our current understanding of the thermodynamics and intermolecular forces responsible for the self-assembly of amphiphiles into nonspherical structures. Tubules also enable us to explore the potential of developing different types of advanced materials using self-assembling microstructures.

PHOSPHOLIPIDS ARE IMPORTANT COMPONENTS of biological membranes. They are amphiphilic and can self-assemble into a variety of molecular aggregates or microstructures in aqueous solutions. These microstructures are, in general, topologically spherical in shape. The fundamentals of the self-assembly of spherical microstructures formed by amphiphilic molecules have been discussed in terms of intermolecular forces in several recent books and review articles (1-3). Self-assembly of lipid molecules into nonspherical structures is less common. The first observation of helical self-assembled structures was by Nakashima et al. (4). Earlier studies by Kunitake and co-workers (5-7) on some amino acid-derived amphiphiles gave an indication of superstructures with remarkably high circular dichroism. Following-up on this, Nakashima et al. (4) made a direct optical observation of helical superstructures in aqueous dispersions of several double-chain ammonium amphiphiles. This was followed by Kunitake and Yamadas' observation (n

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In Molecular and Biomolecular Electronics; Birge, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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MOLECULAR AND BIOMOLECULAR ELECTRONICS

as the solvent, the formation of tubules is dependent on the size of the vesicles in the L or liquid-crystalline phase. When the vesicles are large (the multilamellar vesicles), tubule formation is seen as a phase transition on cooling from the L phase below T (22). However, when the vesicles are sonicated into small unilamellar vesicles (SUVs) and then cooled below T , no tubules formation is seen. Instead, a stacked bilayer struc ture (referred to as "sheet") has been observed (19) to form at 2 °C. Heating and cooling of this sheetlike structure then yields tubules. The reasons for this dependence of tubules formation on the thermal history and morphology of the sample are not yet clear. On the other hand, when an ethanol and water system is used as the solvent, tubule formation is seen reproducibly as a phase transition without any dependence on sample history. It also has been shown (23) that tubule formation is strongly dependent on the volume fraction of the ethanol in the solvent as well as on the lipid concentration. In fact, a "window" of ethanol fraction (60-80%) exists in which tubule for mation is seen. At lower fractions, the solution transforms on cooling to a nontubular solid form, whereas for higher fractions, the lipid is fully dissolved in the solvent at room temperature. A similar window of tubule formation has been observed (16) for methanol-water and propanolwater solvents as well. The window systematically shifts to a higher volume fraction of the solvent with decreasing length of the alcohol chain. Also, the efficiency of tubule formation is seen to increase with decreasing lipids concentrations for all alcohol-water systems. All these methods of tubule formation involve water as the solvent or as one of the constituents of the solvent systems. Also a report has been made (24) on the formation of cylindrical microstructures from DCg.gPC when acetonitrile is used as the solvent. Although highresolution electron-microscopic studies are needed to establish if these cylindrical structures are similar to tubules, this observation is nev ertheless very important: It shows that lipid molecules can self-assemble without the presence of bulk water. It would be of interest to study the role played by bound water in the self-assembly. a

a

m


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Dependence of Tubule Formation on Intrinsic (Molecular) Parameters Tubule formation has been studied for a series of D C P C lipids (19) varying both m and n, keeping m and η in the range of 15-23. In all these cases, tubules of approximately the same diameter were observed, showing that the location of the diacetylenic unit in the chain is not crucial for the formation of the tubules. However, the presence of this moiety is very important; lipids without a diacetylenic unit in both of the chains have not been observed to form tubules. Also, a lyso lipid m > n


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(i.e., a lipid with only a single chain) does not promote tubule formation. Some recent studies (25) on the effect of the head group modification have been made. These studies indicate that the size and shape of the head group affects the tubule diameter considerably. Another relevant molecular parameter is the nature of the chirality. The D and L forms of D C P C as well as the racemate form tubules (26). Electron micro graphs show that tubules formed from the racemate have both right and left helically wrapped structures, which is likely to be due to molecularlevel phase separation of the D and L forms of the lipid, indicates that the macroscopic chirality of the tubule is probably dictated by the mo lecular chirality. However, the observation of somewhat similar cylin drical morphologies in a nonchiral surfactant, namely, di(hexa cosa12,14-diynyl)dimethyl ammonium bromide (27), raises the question whether molecular chirality is essential for tubule formation. Because all of the above-mentioned tubule-forming molecules do contain the diacetylenic moiety in their chains, it is likely that this is an important factor. We shall discuss this aspect in subsequent sections.


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Structure of Tubules. Tubules consist of bilayers that are heli cally wrapped into the form of hollow cylinders (14, 15). The diameter of the tubule formed from DCs.gPC does not appear to be dependent upon the processing conditions. Typically, tubules grown from DCs.gPC have diameters of about 0.5 μτη regardless of the solvent system used and the method of preparation. The thickness can show small variations depending upon the number of bilayers that constitute the wall. Re cently, it has been shown by Ratna et al. (16) that tubules grown from methanol-water are one bilayer thick, whereas those obtained from ethanol-water or water have multiple bilayers as walls. The methanolgrown tubule is therefore a very special instance of a self-assembled finite-size 2D system. High-resolution studies on these single-walled tubules are expected to enhance our understanding of melting and phase transitions in finite-size systems. Several investigations to elucidate the structure of tubules have been conducted. Infrared studies (28, 29) on tubules have shown that tubules have highly ordered acyl chains. Raman studies (28, 30, 31) have led to the observation of the longitudinal-acoustic modes indicating that the chains are fully ordered and are in an all-trans configuration. Also, the infrared spectra of tubules, the stacked bilayer sheets obtained on cooling the sonicated unilamellar residues (32), and of nontubular solids (22) are found to be similar. Although these studies do indicate that the chains of the tubule are as highly ordered as in a nontubular solid, this cannot be taken as proof of the existence of true long-range crystalline order in the tubule. This aspect will be discussed in detail in a subsequent section.




460

The first X-ray difiraction studies on tubules are attributed to Rhodes et al. (32). A multilayer stack of flattened " d r y " tubules were obtained by centrifugation and were then mounted on a curved glass surface. Low-angle X-ray diffraction data collected on such samples shows 16 orders-of-diffraction maxima in the equatorial direction, the layer spacing being in the range of 68 to 61 À depending upon the temperature and relative humidity. The observation of so many higher harmonics clearly shows that the lamellar or bilayer order is extremely high in the tubules. These data also revealed a very small unit-cell size. The corresponding one-dimensional (ID) electron-density profile yielded a chain tilt of 28° with respect to layer normal. A partial-chain interdigitation was also suggested by this profile. However, subsequent data as well as more refined computations by Blechner et al. (33) show that this is not the case. Blechner et al. have also carried out low-angle X-ray diffraction studies on D C P C and D C P C . Their data indicate that the acyl chains in D C P C are more disordered than in D C P C . 8 1 3

9 1 2

9 1 2

8 ) 9

Caffrey et al. (34) carried out X-ray studies on hydrated tubules as well as on sheets formed from SUVs (the lipid being D C P C ) . The X ray diffractograms of tubules (Figure 3) show 15 orders-of-diffraction maxima. These data, which have about the same resolution (4 À) as Rhodes et al. (32), are consistent with a 32° tilt of the chain, minimalchain interdigitation, and fully extended ûl-trans methylene segments (Figure 4). The sample conditions of Caffrey et al. (34) are different from those of Rhodes et al. (32), who have studied hydrated tubules, whereas Caffrey et al. studied dry tubules. Despite this difference, the results are essentially the same. Caffrey et al. also have assigned the thermodynamic phase of tubules as L or a lamellar-crystalline phase with a lamellar spacing of 66.4 À. Upon heating L , it is found to transform into the liquid-crystalline L phase with d = 72.5 Â and with a 8 9

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20 Figure 3. Θ-2Θ diffraction scans of DC PC tubules. Fifteen orders of dif fraction maxima are seen that show the high degree of lamellar order. (Re produced from reference 30. Published 1987 Gordon b- Breach.) 89



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Figure 4. One-dimensional electron-density profile ofDCs^PC in tubule morphology at 57% RH and 22 ° C (A). The origin of the various peaks and valleys in the profile is indicated above the profile and is referenced to the proposed molecular conformation of the lipid in the tubule morphology as shown in B. The lamellar repeat distance is 59 Â and the tilt of the long axis is 32°. (Reproduced from reference 34. Published 1991 American Chemical Society.)

diffuse scattering around 4.5 Â due to the fluidlike packing of the hydrocarbon chains. Reversing the temperature retrieves the L . phase with the same periodicity, showing that the transition to the L phase (tubules) from L is reversible. The diffraction patterns of the hydrated tubules are identical to those of sheets (obtained by cooling SUVs) showing that the sheets and the tubules should be structurally similar. This result is consistent with the Fourier transform infrared studies (29). There has also been an electron-diffraction study (35) on dry tubules formed from D C P G . The diffraction spots have been indexed as belonging to a monoclinic unit cell. It has also been found that a six-layer Langmuir-Blodgett film of the same materials also has the same unit cell. The question may be asked whether tubules have a true long-range c

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crystalline order. None of the experiments carried out so far have suf ficient resolution to probe the extent of the correlation length. The observation of a large number of higher harmonics of the (001) spacing only indicates that the lamellar order is extremely well-defined, but does not necessarily prove the crystalline nature of the tubules. [For instance, the liquid-crystal L phase, which has fluidlike in-plane order, is known to show several orders of diffraction maxima (36)]. Also, no data are available on tubules in both their dry and hydrated states. Some of these questions have been addressed in the recent exper iments by Thomas et al. (37). These experiments have been carried out on dry tubules as well as on tubules hydrated in the ethanol-water sol vent system. The diffraction pattern for the dry tubule consists of a large number of peaks including combination reflections. The data have been indexed as belonging to a monoclinic unit cell, in conformity with the electron-diffraction results of Lando and Sudiwala (35). On the other hand, the tubules hydrated in ethanol-water have a much smaller num ber of diffraction peaks and, more significantly, the combination reflec tions are absent. This, in turn, shows that interlayer correlations, which exist in dry tubules, are definitely absent in the hydrated tubules. The in-plane structure of the hydrated tubules has also been studied. These results indicate that the in-plane order for the hydrated tubules is finite, but is much greater than that expected for a fluid. Thus, we can infer that the hydrated tubule is either a 2D crystal with a finite correlation length or a hexatic structure of the kind seen in both thermotropic and lyotropic liquid crystals (38, 39). Further studies are needed to clarify the situation.


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Effect of Electric and Magnetic Fields on Tubules. Rosenblatt and co-workers (40-42) studied the effects of both electric and magnetic fields on tubules. They measured the magnetic birefringence of dilute sample of tubules of D C P C in magnetic fields of up to 4 Τ (40). The tubules were found to orient with their long axes parallel to the field direction. The order parameter S > 0.95 for fields H > 2 T. Here S =3 0 or a left helix with —45° for k < 0, where k is the cholesteric chiral-curvature modulus. In addition, the theory explains the formation of another type of helical structure, which was not con sidered by any of the previous treatments, twisted strips or helicoids. 2

2

2

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By including additional terms in the elastic free energy, Ou-Yang and Liu made predictions on the sizes of tubules as well as of helical structures that appear to be in reasonable agreement with experiments. Chappell and Yager (49) recently proposed a model for crystalline order within helical and tubular structures of chiral bilayers. In this model, the packing interactions transverse to the helical chains is very important for the stability of a crystalline-cylindrical bilayer. Theoretical approaches (47, 48, 50) consider the formation of a cylindrical tubule from a spherical surface. Because of the cooperative tilt of the molecules in the bilayer, a closed-spherical surface constituted by such a tilted bilayer will have two singular points. In the presence of sufficiently strong long-range positional order, these defects cost too much energy and the structure opens spontaneously to form a hollow cylinder. OuYang and Liu have also discussed (47,48) the transitions from a spherical surface to helical structures and then to tubules. In all these approaches, the anisotropic packing within the bilayer plays an important role in stabilizing the cylindrical curvature. However, a completely different theoretical approach has been adopted by deGennes (51) to explain tubules formation. Recognizing that the bilayers forming the tubule have the same symmetry as that of the ferroelectricsmectic C* phase, deGennes has suggested electrooptic buckling as the mechanism by which tubules form. The curvature in this case is expected to arise from an electrostatic attraction of the edges of a bilayer. This approach predicts that the addition of salt should suppress tubule formation. However, recent experiments (52) using zwitterionic headgroups do not seem to support this prediction. It is possible that deGennes' predictions might be perturbed by the zwitterionic charges. Therefore, similar experiments on tubules from phospholipids with a charged head group would be of interest. Thus, the different theoretical approaches are successful in explaining several aspects of geometry of tubules and their formation. However, none of these explains all the experimental observations. Some key questions, for example, what are the factors that determine the width of the bilayer constituting the tubules, are yet to be addressed quantitatively. Also, none of the theoretical models predict precisely the molecular aspects that may be responsible for tubule formation. Potential Applications of Tubules. Tubules have many unique features associated with their geometry. First, they are hollow cylindrical microstructures with micrometer diameters and a wide range of aspect ratios, leading to a large degree of shape anisotropy. Second, the dimensions of tubules can be controlled by manipulating either the molecular structure of the lipid or the processing conditions under which the tubules form, enabling us to "tune" the shape anisotropy. Third, a



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tubule, due to its cylindrical symmetry and a small hollow core, is essentially a "microvial" in which a solid or liquid can be encapsulated. As already mentioned, tubules can be coated with a variety of metals, and therefore become "templates" for fabricating anisotropically shaped metallic particles of different sizes. A combination of all these features makes tubules extremely attractive from the point of view for many technological applications. In particular, the tubules become building blocks for fabricating novel materials with unique anisotropic electromagnetic properties. Tubules can also be used to fabricate special types of devices. In addition, tubules can be used for many controiled-release applications. The proof of principle regarding many of the areas of potential applications of tubules have already been demonstrated. We briefly discuss some of these areas. Materials with Anisotropic Electric and Magnetic Properties. Several types of novel materials with highly anisotropic electric and magnetic properties can be fabricated with tubules coated with a suitable metal (53). The metalized tubules (Figure 6) are usually dispersed in an epoxy and aligned with a small magnetic field, the tubules orienting in the field direction. This alignment is then locked-in by curing the epoxy. The resultant composite can be sectioned parallel or perpendicular to the long axis of the tubule. Tubules coated with nickel and cast into a composite as discussed previously exhibit extremely interesting dielectric properties. Both the real and imaginary part of the dielectric permittivity have been measured (54) for giga Hertz frequencies up to a weight fraction of 0-4% tubules in the composite (Figure 7). The real part (e ), measured at 9.5 G H z , R

Figure 6. tubules.

Scanning electron-microscope photograph of nickel-coated lipid



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T U B U L E WEIGHT F R A C T I O N

Figure 7. Real and imaginary parts of the dielectric permittivity of a composite consisting of aligned nickel-coated tubules plotted against the weight fraction of the tubules in the composite. The measuring field is parallel to the axis of the tubules. (Reproduced from reference 54. Published 1990 American Institute of Physics.)

shows that a strong increase with increasing weight fraction, from about 2 for the pure epoxy (without tubules) to about 55 at 0.3 weight% loading of metalized tubules. For the same loading fraction, the imaginary part of the dielectric permittivity (e^) also increases considerably (Figure 7). The essentially linear dependence of both e and e on the loading fraction is in good agreement with the curves evaluated from a simple theoretical treatment of the interaction of electromagnetic radiation with a dilute dielectric medium consisting of noninteracting metalized tubules. Such a dielectric medium with very low losses and large anisotropic dielectric constant has great promise in many areas of technological applications, such as development of microwave capacities and waveguides. R


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It is also possible to achieve anisotropic magnetic properties for a tubule-based composite by using tubules coated with a ferromagnetic material such as nickel and permalloy (an iron-nickel alloy) (55). Hys teresis curves measured for field directions parallel and perpendicular to the long axis of the tubules show pronounced differences (56). The values of permeability evaluated from the initial slopes of the B - H curve are found to be highly anisotropic with μ| « 50 and μ± « 5. Another interesting feature was the small coercivity (width of the hysteresis curve at zero field) for both parallel and perpendicular directions. Also, the magnetization direction for a bulk-sample, sectioned perpendicular to the tubule axis, lies normal to the surface of the sample (along the tubule direction). These properties are usually typical of thin magnetic films. It is unique that a bulk composite of magnetic tubules exhibits magnetic properties that are characteristic of thin magnetic films. Yet another attractive feature associated with tubules with regards to their magnetic properties exists because tubules can have different aspect ratios (length-to-diameter ratio), it is possible to control the shape anisotropy of the medium. In addition, it is possible to metalize tubules in the presence of large magnetic fields. Consequently, it should be possible to control the crystalline anisotropy when the tubules are coated with materials such as cobalt with a large inherent anisotropy. Thus, tubules provide the unique opportunity to manipulate both the shape anisotropy and the crystalline anisotropy of a medium. A combination of all these features makes a tubule-based magnetic composite attractive for many areas of applications such as magnetic recording, electromag netic induction shielding, mode filters, and high-density packaging applications.


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Devices and Systems Using Metalized Tubules. As discussed pre viously, metalized tubules have several features: They have small di ameters (0-5 μηι), their aspect ratios can be varied from about 20 to over 200, and they are ferromagnetic and electrically conducting. In addition, tubules can be oriented in an epoxy matrix and can be cured in this aligned state to form an uniaxially conducting composite. These features can be used to fabricate a variety of tubule-based devices and systems that will be used in many technological areas. In this section, a specific type of device, namely a microcathode fabricated from metalized tubules, is discussed. Many technological areas like high-energy particle accelerators, laser-pumping systems, and microwave devices use elec tron-beam sources. Presently available electron sources can be cate gorized into three types: typical thermally excited cathodes (vacuum electronics), laser-activated photo-emitters, and field emitters. Two types of field emitters, exploding field emitters (or plasma cathodes) and vac uum field emitters, exist. The generation of macroscopic electron-beam



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currents through vacuum-field emission from a large number of emission sites requires a surface with a complex microstructure. Fabrication of surfaces suitable for vacuum-field emission is usually done using microlithographic techniques. Recently, a novel way of fabricating a microcathode array using metalized tubules has been demonstrated (57, 58). The structure essentially consists of hollow metallic tubules protruding by uniform distance from the base electrode (Figure 8). Because the edges of the tubules are thin and sharp, a very-large local electricfield enhancement results allowing for the vacuum-field emission of significant current densities. It has also been demonstrated (57, 58) that the observed continuous field emission is in accordance with the process of quantum-field emission described by the Fowler-Nordheim expression (59) (Figure 9). It is remarkable that an array of microcathodes, fabricated with self-assembled tubule structures as templates, offers an interesting alternative to the microlithographic techniques. Similar possibilities are likely in many other areas like cross talk reduction in multichip modules and high-density interconnects for pyroelectric detectors. Tubules for Controlled Release Applications. As previously noted, the tubules formed from phospholipids become mechanically rugged and solvent-resistant when coated with metals like gold, copper, and nickel. Freeze-drying these metalized tubules leads to a dispersible powder of hollow metallic cylinders that are "microcapillary" tubes; these microcapillaries can capture and retain a chemical encapsulant or colloidal suspension. The encapsulated material is released at a controlled rate over periods of time extending from hours to several years. Recently,

Figure 8. Schematic diagram of a field-emission cathode fabricated from tubules. (Reproduced from reference 57. Published 1991 The Institute of Electrical and Electronics Engineers, Inc.)



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Price and Patchan (60) demonstrated the encapsulation of a mixture of tetracycline, epoxy monomers polymers inside metal-coated tubules. The in vitro release kinetics of the tetracycline were studied in both static and dynamic environments. A more or less linear-release rate was observed over a period of 30 days. It has also been shown (61) that when employed in paint coatings, tubules can deliver the encapsulated tet racycline over a long period extending to over 26 months. Controlled release over a long period has also been confirmed (61) by the obser vations on metal structures coated with a tubule-based paint and im mersed in the sea. These structures have not shown any evidence of barnacle growth for over a year. It is clear that the tubules, because of their length, can indeed serve as controlled-release systems for many other areas of engineering and biomedical applications.

Summary Tubules constitute a unique form of self-assembling microstructures. Understanding their structure and formation is of considerable funda-



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mental interest. Tubules also have considerable potential in many areas of technological applications such as high-dielectric materials, magnetic materials for recording and shielding, high-density packaging, intercon nect technology, microcathodes, and controlled-release applications. Another interesting and important aspect of tubules is that they dem onstrate how the principle of self-assembly of molecules into well-de fined structures can be harnessed to develop a variety of advanced ma terials. This field is indeed fertile, and work on tubules is an important step in this direction.

Acknowledgments The financial assistance of Defense Advanced Research Projects Agency (DARPA), the Office of Naval Technology, and the Naval Research Lab oratory is gratefully acknowledged.

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RECEIVED for review M a r c h 12, 1992. ACCEPTED revised manuscript A p r i l 2, 1993.


Self-Assembling Tubules from Phospholipids - Advances in Chemistry

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