Supramolecular Polymerizations

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524 A. Ciferri Figure 17. Shish-kebab composites. Schemes for (a) polyrotaxane, [61] (b) tobacco mosaic virus, [50] and (c) a-cyclodextrin + poly(ethylene oxide) (taken from ref. [104] ). shish-kebab-type architecture. The driving force for the formation of these assemblies is a complex combination of molecular recognition and supramolecular polymerization. In fact, the host polymers often promote the supramolecular polymerization of the guest, or an alteration of its assembly mode. Three examples are illustrated in Figure 17. The primary interaction assisting the threading of a polymer into a single macrocycle cavity, as in the case of pseudopolyrotaxanes (Figure 17a), is attributed to the occurrence of appropriately spaced p-rich hydroquinone rings on the polymer and p-acceptor groups within the tetracationic cyclophane. [59–61] It has been shown that the electron donor/acceptor interaction can be monitored by electrochemically or photochemically induced reduction/ oxidation reactions. Relative motion of the two components can thus be induced, simulating a molecular microengine. [103] The situation of TMV, illustrated in Figure 17b, is more complex. Here the guest is an RNA molecule and the host is a hollow columnar assembly composed of 2130 identical tapered protein molecules. The host/guest systems can be disassembled and reassembled in vitro by pH changes. However, the host can be reassembled even without RNA. A very interesting effect is manifested in the structure of the host when RNA is present. [50] In the absence of RNA the host is a stack of disks of various DPs, each disk comprising 17 protein units. However, formation of a spiral occurs when RNA occupies the cavity. The proteins of the host then follow a helical pattern with 16.3 units per turn, and the assembly assumes definite dimensions (L = 3000, d = 180 Š, X = 16.6) and a DP of 2310. The RNA-induced helix formation in an otherwise stacked systems of disks is reminiscent of the similar effect described in the preceding section (Figure 16a,b). It thus appears that supramolecular interactions between sites on RNA and protein induce spiral formation similar to that of disks connected to a covalent polymer as side chains. The complex role of RNA for the whole structure is evident. RNA acts like a crank-shaft that drives the proteins bound to it into a helical pattern and simultaneously provides the information about the proper length and DP of the host. The assembly mechanism of the overall TMV structure can thus be described in terms of a supramolecular polymerization of the columnar assembly, coupled to the formation of monofunctional sidechain bonds between RNA and proteins. [77] Figure 17c illustrates the assembly of a-cyclodextrin rings over poly(ethylene oxide). This system belongs to the class of inclusion compounds, or clathrates, that have aroused considerable interest for separation processes and for the unique properties of single chains confined in narrow (d L 6 Š) channels. [104] The stability of the crystalline adduct is likely to be assisted by pairwise host/guest interactions, the strength of which is increased within the small cavity. [56] However, the wide variety of systems capable of forming inclusion compounds invites to consider other less specific factors affecting the supramolecular polymerization of a-cyclodextrin rings threaded along the polymer chain. These factors might be: (i) relatively strong contact forces between the surfaces of the host and (ii) a steric-type effect not so far theoretically described. In support of (i) one may note that soluble stoichiometric complexes of the host are known to occur (e.g., head-to-head dimers of cyclodextrin unable to assemble into long channels in the crystalline structure). Concerning (ii) it is plausible that the guest stretches out (loss of conformation entropy) while simultaneously assembling individual host molecules. A suppression of undulation modes of the polymer due to the presence of rings may lead to an entropically induced effective attraction between the threaded rings (a similar Casimir-type effect leads to an attraction between undulating, flexible membranes). Single host/guest channels could form even in isotropic solution of more rigid polymers if there is a favorable balance between the contact energy of cyclodextrin rings and the chain-conformational entropy. In concentrated solutions, the resulting rod-like structure could be favored by the occurrence of a nematic phase. Applications. The possibility of generating relative motion of the assembled surfaces by electrochemical or photochemical stimuli could be exploited in a variety of
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 525 Figure 18. S-layers with hexagonal and square lattice symmetry derived from TEM. (a) Thermoanaerobacter thermohydrosulfuricus, and (b) Desulfotomaculum nigrificans (taken from ref. [46] ). nano/molecular scale engines. [103] The encapsulation of polymer molecules within cavities formed by self-assembling unimers provides systems of interest for separation processes, for recognizing and storing sequential information, [50] for orienting and screening single polymer molecules from similar neighbor interaction. [104] 5.5 Planar Assemblies Figure 3c illustrates an equatorial distribution of four binding sites suitable for the formation of planar assemblies. As discussed in Section 4.2, these assemblies are expected to grow to large sizes by an intra-assembling cooperative mechanism akin to crystallization. At variance with the growth-coupled-to-orientation of linear systems, the growth of a planar assembly does not require the simultaneous formation and orientation of other growing units. Single free-standing, monomolecular layers are possible. An excellent verification of these expectations is provided by self-assembling S-layers forming the protective layer of the external surfaces of bacterial cells, and enabling the maintenance of a closed lattice during cell growth and division. [46] The identical constituent proteins have quasi-spherical form and exhibit an equatorial distribution of donor/acceptor groups capable of H-bonding to adjacent unimers. The proteins also posses a southpole site capable of electrostatic anchoring to the cell surface. S-layers can be disassembled and reassembled in vitro, allowing the preparation of purely H-bonded monolayers standing over an inert surface. The assembly q disassembly process has been described as a crystallization, [46] producing highly organized morphologies such as those shown in Figure 18. Depending upon the lattice type, the center-to-center distance of the morphological units varies from 3 to 30 nm, the thickness of monomolecular lattices vary from 5 to 25 nm, and the pore size is between 2 and 8 nm. Applications. The controllable confinement in definite areas of nanometric dimensions, coupled to the easiness of extraction and re-assembly, has allowed applications in areas of nanotechnology, such as bioanalytical sensors, templates for superlattices with prescribed symmetry, electronic and optical devices, matrices for the immobilization of functional molecules, and biocompatible surfaces. [46] S-layers recrystallized over solid supports have also been successfully patterned by using UV radiation and microlithographic masks. [46] 5.6 Composite and 3D Assemblies Hexagonal cylindrical and lamellar phases (cf. transmission electron microscopy (TEM) photographs in Figure 18) are often seen [48] in diblock and multiblock copolymers, ternary systems, copolymers formed from one unit that can be crystallized, rod-coil copolymers, [49] and some biological fibers. [105] For amorphous diblock copolymers in the cylindrical mode, one block is hexagonally packed within a matrix of the other block. The lamellar mode is instead based on alternating layers of A and B. The lamellar mode is the prevalent feature observed with rodcoil copolymers. [49, 106] In the case of a helical comb-like polymer (poly(b-l-aspartate) with paraffinic side chains), a layered distribution of helices correlated by interdigitation of the side chains was observed. [107] In the case of keratin, the fiber cross-section reveals L1 lm long microfibrils parallel to the fiber axis and hexagonally imbedded in a disordered S-rich matrix. Each microfibril is composed of eight protofibrils that are left-hand cables of two strands, each including two right-hand a-helices. [105] In the case of amorphous block copolymers, the (selfconsistent) mean-field theory [87] (cf. Section 4.2 and Figure 6) describes the occurrence of various phases in terms of parameters pertinent to single copolymer molecules (compatibility, relative length and flexibility of the two blocks). This theory has been an eminently successful one and experimental results for the undiluted melt offer good support to it. [83, 108–110] Even in the case of block copolymer solutions, the predicted [88] sequence of phases upon increasing the concentration (e.g., isotropic e micellar e cubic e hexagonal e lamellar) revealed simi- [97, 98] larities with experimental data. Within the context of supramolecular polymerization it is however useful to explore alternative descriptions of the above structures in terms of a simpler, less sophisticated approach based on the concept of self-assembling of specifically designed building blocks. A similar con-
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