Supramolecular Polymerizations

520 A. Ciferri
preted [90] in terms of a theory describing the chain-extending
role of labile crosslinkages. [91]
Applications. Applications of SPs have been suggested
in areas that expand the applicability of covalent polymers
(Table 1). The advantage of an easier processing in
spite of a large DP has been commented above. Particularly
significant is the possibility of increasing the DP of
conventional polymers by main-chain supramolecular
bonds. Under study is the extension of the DP of aromatic
polyamides that are usually produced in the desirable
high-molecular-weight range only by cumbersome syntheses.
[92] Polymers based on long covalent segments
extended by supramolecular bonds should be recyclable
with complete recovery of properties. [92] The subsequent
transformation of supramolecular bonds into covalent
ones should allow the stabilization of large DPs and
structures that would have been extremely difficult to
synthesize directly. [38, 93] The thermodynamic control of
DP and of the topology of networks should allow applications
as tunable, smart materials responding to changes in
variables, such as temperature, stress, and solvent permeation.
[2, 52] By using different covalent segments in A-
A/B-B unimers, the tunability could be expressed in
mechanical, conductivity, light emitting, and catalytic
properties. If changes in the above variables were occurring
accidentally, the SP would have the capability of
self-repairing. The high selectivity of molecular recognition
should allow the selection of proper sequences in
mixtures of more than two complementary unimers.
5.2 Helical Chains and Functional Systems
The original Oosawa theory (cf. Section 4.2) is based on
unimers exhibiting the site distribution shown in Figure
3b. Cooperation arises because each unimer makes two
bonds along the linear sequence and two weaker ones
along the helical pattern. The unimers should be large
enough to avoid possible mismatches in the pattern.
There has been no verification of the model using synthetic
SPs. The model was elaborated to describe the formation
of helices and microtubules (G q F transformation)
by biological SPs. [1, 26] However, the verification of
the helical growth model has been problematic even in
the relatively simple case of actin, due to the simultaneous
dephosphorylation of ATP normally bound to the
protein. A study in which growth could be observed without
the complication of ATP e ADP hydrolysis was performed
by Korn on actin-ADP. [27] Actin polymerization
occurs by increasing the ionic strength or temperature in
isotropic solution at unimer concentrations below
0.04 mg/ml. Filaments exceeding 11 lm, corresponding
to a DP larger than 4000, are attained (higher values
were observed with tubulin). [1, 28] The length distribution
conforms to the most probable distribution and chain
stoppers reduce DP, in line with theory. The diagram in
Figure 12. (a) Concentration of helical (0) and oligomeric
(6) actin vs unimer concentration showing the critical concentration
as predicted by Oosawa’s theory (taken from ref. [29] ). (b)
Schematization of translational movement resulting from the
directional growth of F-actin (taken from ref. [25] ).
Figure 12a exhibits the theoretically predicted occurrence
of a critical unimer concentration at which helical growth
begins, and the concentration of unimers and oligomers
attains a constant value. [29] The recent generalization [76] of
Oosawa’s theory to growth processes enhanced by cooperative
effects has not yet been applied to specific chainlike
sequences. It has however been applied to the growth
of columnar systems to be discussed in the following section.
The overall functioning of actin or tubulin as in vivo
systems invites to consider the way in which the helical
growth process is coupled to the dephosphorylation reaction.
This coupling produces a dynamic function of the
polymer [25, 30] that assists in processes, such as motility,
contraction, and cell division. Figure 12b illustrates the
processes believed to occur during the polymerization of
actin filaments. The critical concentration of G-actin
unimers is defined by the condition of equality between
the sum of the assembly and disassembly rates at the two
ends of the filament (the barbed and the pointed ends).
Under ATP hydrolysis, the depolymerization at one end
may be faster than polymerization at the other end. Thus,
a cycling of actin monomers from one end to the other
occurs during the growth of F-filaments, resulting in a
translation of the polymer (tread-milling effect). A
related dynamic instability effect controls the assembly q
disassembly process of tubulin into microtubules. [25]
Applications. The design principle of functional protein
systems is based on the coupling of a supramolecular
polymerization process to a chemical reaction.
[25, 30]
Understanding and possibly reproducing coupled
mechanisms is the challenging road to microengines and
other functional SPs mimicking mechanical properties of
biological systems and engineered for new practical
applications.