Communications
Accordingly, we may assume that at neutral pH the electro-
static repulsion is reduced as a consequence of partial
protonation. This causes reduced solvation, which in turn
leads to much closer packing as is evidenced by the smaller
diameter of the rods relative to that of the globular micelles.
Figure 1b shows the corresponding result of a conformational
simulation of 3 in aqueous environment at neutral pH. The
fact that the rod diameter is constant throughout suggests that
the three-dimensional organization must be based on a
circular-shaped cross-sectional area. Based on the dimensions
and density profile, a double-layered molecular arrangement
in a micellar cylinder can be visualized. According to this
motif an appropriate radius of curvature of ca. 65 ꢀ can be
modeled by using approximately eight molecules. The corre-
sponding model is depicted in Figure 5.
Figure 4. Interpretation of the reconstructed data. a) Eight head
groups of the dendritic fullerene molecule may be fitted in a C -sym-
2
metrical mode into the reconstructed electron-density map. b) View of
(
a) tilted by 908 towards the front side. c) Schematic representation of
the head-group assembly: Two planar U-shaped assemblies consisting
of four molecules each (yellow and blue) are displaced by 908 along
their longitudinal axes and are inserted —though not completely—into
each other at their open ends. d) View of c) tilted by 908 towards the
front side.
Figure 5. Proposed model of a molecular arrangement of 3 in micellar
rods at neutral pH. The employed head-group volume is based on the
partly protonated form shown in Figure 1b. Hydrophobic alkyl chains,
which should reside at the center of the micelle, are not shown for
clarity. They correspond to areas of low density in the electron micro-
scopic images of Figure 2a.
constructed from eight molecules in a C -symmetrical
2
arrangement. The molecular organization of this aggregate
may be described best by considering two planar U-shaped
structures consisting of four molecules each. These are
o
displaced by 90 along the longitudinal axis with respect to
their open ends; however, they are not inserted completely
into each other (Figure 4c and d). This molecular organiza-
tion ensures a nearly spherical shape overall and thus very
efficient shielding of all hydrophobic molecule parts from the
aqueous environment. As was the case for the structurally
persistent micelles derived from calixarene 2, the innermost
core area of the micelle formed by the hydrophobic alkyl
chains is not reproduced in the reconstruction. Since individ-
ual chains do not assume a spatially defined conformation but
are more or less arranged in a statistical way, this part of the
aggregate is averaged out as a result of the statistical
principles of the applied reconstruction method. Only the
head groups, which are located easily due to the high-contrast
fullerene entity, prove to be sterically persistent, thus yielding
the found structural pattern.
We have demonstrated that by suitable substitution
structurally persistent aggregates may be generated in a
reproducible way. This opens up perspectives for the system-
atic investigation of the parameters responsible for aggregate
formation. Fundamental investigations on the stability of the
aggregates, in particular tolerance of chemical modification
and functionalization of the head group, might form the basis
for the synthesis of tailor-made transport systems. These may
have potential for the selective deposition of drugs in living
organisms.
Received: October 29, 2004
Published online: April 13, 2005
The molecular structure of 3 encodes a supramolecular
behavior representing a combination of supramolecular
properties of 1 and 2. On the one hand, and similar to 1, the
spatial demands of the hydrophilic and hydrophobic areas are
well-balanced. On the other hand, and similar to 2, the
dendritic branches are linked by amide rather than ester
linkages, resulting in enhanced rigidity. The significant
structural change when the pH is lowered may be explained
by electrostatic repulsion of the completely deprotonated
carboxy groups, and enhanced solvation is a secondary effect.
Keywords: amphiphiles · dendrimers · electron microscopy ·
fullerenes · micelles · self-assembly
.
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