Yun Liu, Abhishek Singharoy, Christopher G. Mayne, Arkajyoti Sengupta, Krishnan
Raghavachari, Klaus Schulten, and Amar H. Flood.
Flexibility coexists with shape persistence in cyanostar macrocycles.
Journal of the American Chemical Society, 138:4843-4851, 2016.
(PMC: PMC4957974)
LIU2016
Shape-persistent macrocycles are attractive functional targets for synthesis, molecular
recognition, and hierarchical self-assembly. Such macrocycles are non-collapsible and
geometrically well-defined, and they are traditionally characterized by having repeat units
and low conformational flexibility. Here, we find it necessary to refine these ideas in the
face of a highly flexible yet shape-persistent macrocycles. A molecule is shape-persistent if
it has a small change in shape when perturbed by external stimuli (e.g., heat, light, and
redox chemistry). In support of this idea we provide the first examination of the
relationships between a macrocycle’s shape persistence, its conformational space, and the
resulting functions. We do this with a star- shaped macrocycle called cyanostar that is
flexible as well as being shape-persistent. We employed molecular dynamics (MD), density
functional theory (DFT), and NMR experiments. Considering a thermal bath as a stimulus we
found a single macrocycle has 332 accessible conformers with olefins undergoing rapid
interconversion by up-down and in-out motions on short time scales (0.2 ns). These many
interconverting conformations classify single cyanostars as flexible. To determine and
confirm that cyanostars are shape-persistent, we show that it has a high 87% shape
similarity across these conformations. To further test the idea, we use the binding of
diglyme to the single macrocycle as guest-induced stimulation. This guest has almost no
effect on the conformational space. However, formation of a 2:1 sandwich complex
involving two macrocycles enhances rigidity and dramatically shifts the conformer
distribution towards perfect bowls. Overall, the present study expands the scope of shape-
persistent macrocycles to include flexible macrocycles if and only if their conformers have
similar shapes.
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