This critical review describes mechanisms by which guest molecules enter and depart molecular capsules. The discussion focuses on presenting gated molecular encapsulation, i.e., trapping and releasing of guest molecules at rates that are controlled by conformational changes in the host's structure. Developing quantitative rules that describe the gating are, at present, a matter of scientific curiosity but could play an important role in building more effective catalysts, drug-delivery devices or membranes (105 references).
Synthetic supramolecular systems
can provide insight into how complex
biological systems organize as well as produce self-organized systems
with functionality comparable to their biological counterparts. Herein,
we study the assembly into superstructures of a system composed of
diketopyrrolopyrrole (DPP) donors with chiral and achiral side chains
that can form triple hydrogen bonds with perylene diimide (PDI) acceptors
into superstructures. The homoaggregation of the individual components
as well as the heteroaggregate formation, as a result of π···π
stacking and H-bonding, were studied by variable-temperature UV/vis
and CD spectroscopies and electronic structure theory calculations.
It was found that, upon cooling, the achiral PDIs bind to disordered
DPP stacks, which drives the formation of chiral superstructures.
A new thermodynamic model was developed for this unprecedented assembly
that is able to isolate the thermodynamic binding parameters (ΔH°, ΔS°) for all the different
noncovalent contacts that drive the assembly. This novel assembly
as well as the quantitative model described in this work may help
researchers develop complex self-assembled systems with emergent properties
that arise as a direct result of their supramolecular structures.
Some highly efficient enzymes, e.g., acetylcholinesterase, use gating as a tool for controlling the rate by which substrates access their active site to direct product formation. Mastering gated molecular encapsulation could therefore be important for manipulating reactivity in artificial environments, albeit quantitative relationships that describe these processes are unknown. In this work, we examined the interdependence between the thermodynamics (DeltaG(o)) and the kinetics (DeltaG(in)(double dagger) and DeltaG(out)(double dagger)) of encapsulation as mediated by gated molecular basket 1. For a series of isosteric guests (2-6, 106-107 A(3)) entering/exiting 1, we found a linear correlation between the host-guest affinities (DeltaG(o)) and the free energies of the activation (DeltaG(in)(double dagger) and DeltaG(out)(double dagger)), which was fit to the following equation: DeltaG(double dagger) = rhoDeltaG(o) + delta. Markedly, the kinetics for the entrapment of smaller guest 7 (93 A(3)) and bigger guest 8 (121 A(3)) did not follow the free energy trends observed for 2-6. Thus, it appears that the kinetics of the gated encapsulation mediated by 1 is a function of the encapsulation's favorability (DeltaG(o)) and the guest's profile. When the size/shape of guests is kept constant, a linear dependence between the encapsulation potential (DeltaG(o)) and the rate of guests' entering/departing basket (DeltaG(in/out)(double dagger)) holds. However, when the potential (DeltaG(o)) is fixed, the basket discriminates guests on the basis of their size/shape via dynamic modulation of the binding site's access.
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