3 D–A Rotaxane Foldamers in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
3.1 Folding in D–A Polyrotaxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
3.2 Systematic Investigations of Oligorotaxane Foldamers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
4 Computational Evaluation of D–A Oligo-Pseudorotaxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
5 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
1 Introduction
The burgeoning of molecular nanotechnology near the turn of the 21
st century has
brought with it the emergence of new fields in the chemical sciences fueled by the
manipulation of atomic-and molecular-scale matter with ever-increasing dexterity
and precision. Among them, the fields of mechanically interlocked molecules
(MIMs) and synthetic foldamers, both of which assume ‘bottom-up’ approaches
in the construction of functional nano-architectures, have seldom[1–3] crossed
paths.
Foldamers are sequence-specific synthetic oligomers that adopt well-defined,
compact geometries [4, 5]. They represent an attempt to mimic the exquisite control
expressed by natural systems in their three-dimensional arrangement of functional
groups, which underwrites the activity of proteins and biopolymers, for example.
Chemists interested in foldamers have invested their efforts in the rational design of
macromolecules with well-defined structural hierarchies, which involve the assembly of components into stable secondary, tertiary, and quaternary structures. Mastering the synthesis and assembly of foldamers on all these fronts will allow, in
principle, molecules to be engineered with precise three-dimensional shapes and
properties customized for particular applications. The remarkable capabilities of
biomolecules afforded by their complex and diverse structures suggest that this
course will likewise have far-reaching implications for catalysis, sensing, information storage, and so on. Foldamers are most commonly constructed from peptidic
[6–12] or aromatic [13–17] sequences, and the current state-of-the-art is summarized in a number of reviews [18–21] and monographs [22, 23] that have been
published recently.
Mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes,
are molecules with at least two components that are not covalently bound, but
interlocked in such a manner that they cannot be separated without the breaking of a
covalent bond. Since this physical linkage is known as a mechanical bond [24], we
refer to the stereochemistry of MIMs as mechanostereochemistry [25]. MIMs have
been appreciated for their synthetic challenge and aesthetic value [26] as well as
their potential applications. In particular, MIMs have garnered much interest as
artificial molecular switches and machines [27–31] because their internal
noncovalent bonding interactions can be modulated by external stimuli to control
the relative translational and/or circumrotational motions of their interlocked
272
C.J. Bruns and J.F. Stoddart
3.1 Folding in D–A Polyrotaxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
3.2 Systematic Investigations of Oligorotaxane Foldamers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
4 Computational Evaluation of D–A Oligo-Pseudorotaxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
5 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
1 Introduction
The burgeoning of molecular nanotechnology near the turn of the 21
st century has
brought with it the emergence of new fields in the chemical sciences fueled by the
manipulation of atomic-and molecular-scale matter with ever-increasing dexterity
and precision. Among them, the fields of mechanically interlocked molecules
(MIMs) and synthetic foldamers, both of which assume ‘bottom-up’ approaches
in the construction of functional nano-architectures, have seldom[1–3] crossed
paths.
Foldamers are sequence-specific synthetic oligomers that adopt well-defined,
compact geometries [4, 5]. They represent an attempt to mimic the exquisite control
expressed by natural systems in their three-dimensional arrangement of functional
groups, which underwrites the activity of proteins and biopolymers, for example.
Chemists interested in foldamers have invested their efforts in the rational design of
macromolecules with well-defined structural hierarchies, which involve the assembly of components into stable secondary, tertiary, and quaternary structures. Mastering the synthesis and assembly of foldamers on all these fronts will allow, in
principle, molecules to be engineered with precise three-dimensional shapes and
properties customized for particular applications. The remarkable capabilities of
biomolecules afforded by their complex and diverse structures suggest that this
course will likewise have far-reaching implications for catalysis, sensing, information storage, and so on. Foldamers are most commonly constructed from peptidic
[6–12] or aromatic [13–17] sequences, and the current state-of-the-art is summarized in a number of reviews [18–21] and monographs [22, 23] that have been
published recently.
Mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes,
are molecules with at least two components that are not covalently bound, but
interlocked in such a manner that they cannot be separated without the breaking of a
covalent bond. Since this physical linkage is known as a mechanical bond [24], we
refer to the stereochemistry of MIMs as mechanostereochemistry [25]. MIMs have
been appreciated for their synthetic challenge and aesthetic value [26] as well as
their potential applications. In particular, MIMs have garnered much interest as
artificial molecular switches and machines [27–31] because their internal
noncovalent bonding interactions can be modulated by external stimuli to control
the relative translational and/or circumrotational motions of their interlocked
272
C.J. Bruns and J.F. Stoddart