A Multi-State,
Allosterically-Regulated Molecular
Receptor With Switchable Selectivity
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Abstract
A biomimetic, ion-regulated molecular
receptor was synthesized
via the Weak-Link Approach (WLA). This structure features both a calix[4]arene
moiety which serves as a molecular recognition unit and an activity
regulator composed of hemilabile phosphine alkyl thioether ligands
(P,S) chelated to a Pt(II) center. The host–guest properties
of the ion-regulated receptor were found to be highly dependent upon
the coordination of the Pt(II) center, which is controlled through
the reversible coordination of small molecule effectors. The environment
at the regulatory site dictates the charge and the structural conformation
of the entire assembly resulting in three accessible binding configurations:
one closed, inactive state and two open, active states. One of the
active states, the semiopen state, recognizes a neutral guest molecule,
while the other, the fully open state, recognizes a cationic guest
molecule. Job plots and <sup>1</sup>H NMR spectroscopy titrations
were used to study the formation of these inclusion complexes, the
receptor binding modes, and the receptor binding affinities (<i>K</i><sub>a</sub>) in solution. Single crystal X-ray diffraction
studies provided insight into the solid-state structures of the receptor
when complexed with each guest molecule. The dipole moments and electrostatic
potential maps of the structures were generated via DFT calculations
at the B97D/LANL2DZ level of theory. Finally, we describe the reversible
capture and release of guests by switching the receptor between the
closed and semiopen configurations via elemental anion and small molecule
effectors