(1)H NMR investigation of solvent effects in aromatic stacking interactions

J Am Chem Soc. 2001 Aug 8;123(31):7560-3. doi: 10.1021/ja015817m.

Abstract

One of the marquis challenges in modern Organic Chemistry concerns the design and synthesis of abiotic compounds that emulate the exquisite complex structures and/or functions of biological macromolecules. Oligomers possessing the propensity to adopt well-defined compact conformations, or foldamers, have been attained utilizing hydrogen bonding, torsional restriction, and solvophobic interactions.(1) In this laboratory, aromatic electron donor--acceptor interactions have been exploited in the design of aedamers--foldamers that adopt a novel, pleated secondary structure in aqueous solution. Herein is reported detailed (1)H NMR binding studies of aedamer monomers that were carried out in solvents and solvent mixtures covering a broad polarity range. Curve-fitting analysis of the binding data using a model that incorporated the formation of higher order and self-associated complexes yielded a linear free energy relationship between the free energy of complexation and the empirical solvent polarity parameter, E(T)(30). From these studies, the association of electron-rich and electron-deficient aedamer monomers was seen to be driven primarily by hydrophobic interactions in polar solvents. However, the magnitude of these interactions is modulated to a significant extent by the geometry of the donor--acceptor complex, which, in turn, is dictated by the electrostatic complementarity between the electron-deficient and electron-rich aromatic faces of the monomers.