Supramolecular Polymerizations

522 A. Ciferri
Figure 14. (a) Disk-shaped, three blades molecule prepared by
Meijer and coworkers [4] with side chains having polar, nonpolar,
chiral, achiral character. [32, 33] (b) Formation of a helical assembly
when the blades assume a propeller-like conformation. [100]
(c) Schematic representation of the transition from dispersed
disks to partially ordered and to fully ordered columns. [76]
(i Am. Chem. Soc. 2000 and 2001.)
mation of long columns was shown to occur in very dilute
solution in hexane (10 –6 m), and a large association constant
(10 8 m –1 ) was reported. [32] It was suggested that such
an aggregation reflects the formation of helical columns
through a cooperative process attributed to a conformational
transition from flat to propeller shape for the blades
of each disk, resulting in a maximization of interaction
for the chiral helical assembly (Figure 14b). [100] Experimental
data for the more polar molecules in dilute butanol
(2.4610 –4 m) [33] revealed a sequence of two association
steps upon temperature changes. The postulated process
is schematized in Figure 14c: starting with an isotropic
dispersion of disks, a decrease in the temperature causes
the formation of low-DP achiral aggregates stabilized by
non-cooperative interaction, followed at a lower critical
temperature by the cooperative formation of helical columns
with DP attaining the 1000 range. [76]
The theoretical description of the processes described
above was formulated by van der Schoot and coworkers. [76]
The occurrence of the two regimes was described in terms
of the cooperativity parameter r. The situation r =1is
equivalent to the binding of unimers into disordered aggregates
(i.e. MSOA), while r s 1 describes the subsequent
cooperative formation of ordered aggregates by a HG
mechanism. Essentially, the treatment is a generalization
of the Zimm-Bragg and the Oosawa theories (Kh A K) without
necessarily specifying a detailed molecular model and
a critical nucleus. With respect to Oosawa’s treatment that
focused on the critical concentration C* (cf. Figure 4c),
Figure 15. (a) Deoxyguanosine, its oligomers, and folic acid.
Their assembly in tetrameric disks. [36] (b) Variation of the number
of stacked tetrameric disks with folate concentration in (1)
pure H2O and (2) 1 m NaCl at 308 C. The vertical broken line
indicates the I e H transition (replotted using data taken from
ref. [26] ).
the van der Schoot treatment emphasizes the fractions of
aggregated material and helical bonds as a function of
both temperature and concentration. Rigidity and excluded
volume effects are not introduced and, therefore, liquid
crystallinity does not direct the aggregation of the stacks.
5.3.3 Supermolecules. Tubular Assemblies
The formation of disk-like supermolecules from two or
more complementary components was discussed by several
authors. [2, 34–36, 43, 44] Disk-like supermolecules often
show liquid-crystalline behavior even though the separate
components do not. Moreover, the discotic supermolecules
can form columnar stacks in the melt and in solution
just as the molecular discotics discussed above do.
The relative contributions of MSOA, HG and SLC
mechanisms have not always been characterized adequately.
Gottarelli et al. [35, 36] have investigated the most
interesting assembly of the nucleotide deoxyguanosine,
its oligomers and alkaline folates (Figure 15a). These
compounds form hydrogen-bonded disk-like tetramers in
solution and are able to assemble in columnar stacks of
discrete length and DP. Small-angle neutron scattering
techniques were used to determine the length of the
aggregates in water and in salt solutions. The critical concentrations
for the appearance of the nematic (cholesteric)
and the hexagonal phases were determined by
means of X-ray diffraction. Selected data for the deoxyguanosine
dimer (d(GpG); Figure 15a) and the folate is