Supramolecular Polymerizations
Supramolecular Polymerizations
Supramolecular Polymerizations
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524 A. Ciferri<br />
Figure 17. Shish-kebab composites. Schemes for (a) polyrotaxane,<br />
[61] (b) tobacco mosaic virus, [50] and (c) a-cyclodextrin +<br />
poly(ethylene oxide) (taken from ref. [104] ).<br />
shish-kebab-type architecture. The driving force for the<br />
formation of these assemblies is a complex combination<br />
of molecular recognition and supramolecular polymerization.<br />
In fact, the host polymers often promote the supramolecular<br />
polymerization of the guest, or an alteration of<br />
its assembly mode. Three examples are illustrated in Figure<br />
17.<br />
The primary interaction assisting the threading of a<br />
polymer into a single macrocycle cavity, as in the case of<br />
pseudopolyrotaxanes (Figure 17a), is attributed to the<br />
occurrence of appropriately spaced p-rich hydroquinone<br />
rings on the polymer and p-acceptor groups within the tetracationic<br />
cyclophane. [59–61] It has been shown that the<br />
electron donor/acceptor interaction can be monitored by<br />
electrochemically or photochemically induced reduction/<br />
oxidation reactions. Relative motion of the two components<br />
can thus be induced, simulating a molecular microengine.<br />
[103]<br />
The situation of TMV, illustrated in Figure 17b, is<br />
more complex. Here the guest is an RNA molecule and<br />
the host is a hollow columnar assembly composed of<br />
2130 identical tapered protein molecules. The host/guest<br />
systems can be disassembled and reassembled in vitro by<br />
pH changes. However, the host can be reassembled even<br />
without RNA. A very interesting effect is manifested in<br />
the structure of the host when RNA is present. [50] In the<br />
absence of RNA the host is a stack of disks of various<br />
DPs, each disk comprising 17 protein units. However,<br />
formation of a spiral occurs when RNA occupies the cavity.<br />
The proteins of the host then follow a helical pattern<br />
with 16.3 units per turn, and the assembly assumes definite<br />
dimensions (L = 3000, d = 180 Š, X = 16.6) and a<br />
DP of 2310.<br />
The RNA-induced helix formation in an otherwise<br />
stacked systems of disks is reminiscent of the similar<br />
effect described in the preceding section (Figure 16a,b).<br />
It thus appears that supramolecular interactions between<br />
sites on RNA and protein induce spiral formation similar<br />
to that of disks connected to a covalent polymer as side<br />
chains. The complex role of RNA for the whole structure<br />
is evident. RNA acts like a crank-shaft that drives the<br />
proteins bound to it into a helical pattern and simultaneously<br />
provides the information about the proper length<br />
and DP of the host. The assembly mechanism of the overall<br />
TMV structure can thus be described in terms of a<br />
supramolecular polymerization of the columnar assembly,<br />
coupled to the formation of monofunctional sidechain<br />
bonds between RNA and proteins. [77]<br />
Figure 17c illustrates the assembly of a-cyclodextrin<br />
rings over poly(ethylene oxide). This system belongs to<br />
the class of inclusion compounds, or clathrates, that have<br />
aroused considerable interest for separation processes and<br />
for the unique properties of single chains confined in narrow<br />
(d L 6 Š) channels. [104] The stability of the crystalline<br />
adduct is likely to be assisted by pairwise host/guest<br />
interactions, the strength of which is increased within the<br />
small cavity. [56] However, the wide variety of systems<br />
capable of forming inclusion compounds invites to consider<br />
other less specific factors affecting the supramolecular<br />
polymerization of a-cyclodextrin rings threaded<br />
along the polymer chain. These factors might be: (i) relatively<br />
strong contact forces between the surfaces of the<br />
host and (ii) a steric-type effect not so far theoretically<br />
described. In support of (i) one may note that soluble stoichiometric<br />
complexes of the host are known to occur<br />
(e.g., head-to-head dimers of cyclodextrin unable to<br />
assemble into long channels in the crystalline structure).<br />
Concerning (ii) it is plausible that the guest stretches out<br />
(loss of conformation entropy) while simultaneously<br />
assembling individual host molecules. A suppression of<br />
undulation modes of the polymer due to the presence of<br />
rings may lead to an entropically induced effective attraction<br />
between the threaded rings (a similar Casimir-type<br />
effect leads to an attraction between undulating, flexible<br />
membranes). Single host/guest channels could form even<br />
in isotropic solution of more rigid polymers if there is a<br />
favorable balance between the contact energy of cyclodextrin<br />
rings and the chain-conformational entropy. In<br />
concentrated solutions, the resulting rod-like structure<br />
could be favored by the occurrence of a nematic phase.<br />
Applications. The possibility of generating relative<br />
motion of the assembled surfaces by electrochemical or<br />
photochemical stimuli could be exploited in a variety of