Project Details
Projekt
Experimental dissection of dispersion energies from electrostatic contributions and solvent effects in face-to-face pi-stacking complexes
Applicant
Dr. Frank Biedermann
Subject Area
Organic Molecular Chemistry - Synthesis and Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term
from 2018 to 2022
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 397985541
We propose a novel experimental toolbox for evaluating the binding energies of face-to-face “stacking” aromatic systems, which will be of help to separate electrostatic from dispersive contributions for typical pi-pi complexes. Our approach is complementary to existing experimental approaches, e.g. studies with association complexes of small molecules, the molecular balance technique or the double mutant cycle analysis, which have to date not been fully conclusive in scrutinizing competing theoretical models about pi-pi stacking interactions.Stable and geometrically well-defined rotaxanated complexes of the large macrocycle cucurbit[8]uril and a suitable aromatic component will serve as the receptor moiety for binding a second aromatic guest. In this design, both aromatic compounds, i.e. the rotaxanated one, and the incoming second aromatic guest, are hold in a face-to-face orientation (=> high geometric control) and are each almost completely shielded from contact with solvent molecules (=> minimization of solvation effects on the overall pi-pi stacking energy). We also propose a path how the desolvation energy cost of the binding partners can be experimentally accounted for without the need for computational treatments. Both the high geometric control and the subtraction of solvation effects are important advantages over other association-complex-based studies. Furthermore, it will be straightforward to generate a data library, because the second aromatic binding partner can be readily mixed in and does not need to be covalently tethered, as is the case for molecular-balances. In fact, our supramolecular approach also avoids probing a “non-optimal” binding geometry, which can be a shortcoming for covalent molecular-balance setups.In the proposed line of research, we will first prepare the rotaxanated complexes and will then measure by isothermal titration calorimetry (ITC) the binding enthalpies and free enthalpies of their pi-pi-complex formation with aromatic second guests. Having direct access to binding enthalpies will facilitate the comparison to computed pi-pi interaction energies, as the computationally difficult to treat entropic component can be disregarded. Having access to free energies (and thus entropies) will provide a valuable data sets to test future improved theoretical models.Information about the pi-pi stacking geometry will be obtained by structure-based methods such as NMR spectroscopy in solution and X-ray diffraction structure analysis of crystallized complexes. By systematically varying both the aromatic binding partners, including systems with typical “polar” substituents or with “dispersion donors”, we will aim towards an experimental separation of electrostatic and dispersive effects for the face-to-face pi-pi interaction motif. This study will complement the emerging picture of the importance of dispersive interactions for molecular recognition and self-assembly.
DFG Programme
Priority Programmes
