Circular Dichroism Reveals Positive Cooperativity through Compensatory Motion in Stepwise Cage Metalation.

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Chemical Spectroscopy

Background:

• Biological allosteric systems utilize long-range communication for functional control.
• Synthetic systems can be designed to mimic these complex biological mechanisms.

Purpose of the Study:

• To investigate allosteric-like behavior in a synthetic chiral cage with two tris(2-pyridylmethyl)amine (TPMA) binding sites.
• To explore the communication between distant binding sites during metal coordination.

Main Methods:

• Nuclear Magnetic Resonance (NMR) titrations to study metalation kinetics.
• Circular Dichroism (CD) spectroscopy to track structural evolution.
• Time-Dependent Density Functional Theory (TD-DFT) calculations for spectral analysis.

Main Results:

• Stepwise coordination of Zn(II) to the TPMA cage showed sequential metalation kinetics.
• The second coordination step was slower in mixed solvents, indicating electrostatic effects and preorganization.
• CD spectroscopy revealed clean interconversion between species with five isodichroic points.
• TD-DFT calculations correlated spectral changes with geometric reorganization over >10 Å.

Conclusions:

• The synthetic chiral cage exhibits structural communication analogous to allosteric systems.
• CD spectroscopy is a valuable tool for probing allosteric-like mechanisms in synthetic supramolecular assemblies.
• Findings have implications for designing responsive materials and synthetic receptors for catalysis.