Synthesis and Recognition Properties of Aromatic Amide Oligomers: Molecular Zippers
摘要:
A series of amide oligomers have been prepared from isophthalic acid and a bisaniline derivative. These compounds assemble into double-stranded zipper complexes in solution via hydrogen-bonding and edge-to-face aromatic interactions. The stability and structures of the complexes have been determined by H-1 NMR spectroscopy in chloroform solution. The stability of the complexes increases with increasing chain length, indicating cooperativity between the individual recognition sites in the oligomers. Oligomers which are complementary form more stable complexes than non-complementary systems with overhanging ends. Addition of polar solvents such as methanol destabilizes the complexes, because it competes for hydrogen-bonding interactions which appear to be the main driving force for binding in this system.
Synthesis and Recognition Properties of Aromatic Amide Oligomers: Molecular Zippers
摘要:
A series of amide oligomers have been prepared from isophthalic acid and a bisaniline derivative. These compounds assemble into double-stranded zipper complexes in solution via hydrogen-bonding and edge-to-face aromatic interactions. The stability and structures of the complexes have been determined by H-1 NMR spectroscopy in chloroform solution. The stability of the complexes increases with increasing chain length, indicating cooperativity between the individual recognition sites in the oligomers. Oligomers which are complementary form more stable complexes than non-complementary systems with overhanging ends. Addition of polar solvents such as methanol destabilizes the complexes, because it competes for hydrogen-bonding interactions which appear to be the main driving force for binding in this system.
Chemical double-mutant cycles have been used to quantify intermolecular functional-group interactions in H-bonded zipper complexes in chloroform. If the same interaction is measured in zippers of different overall stability, the double-mutant cycles can be combined to produce a triple-mutant box. This construct quantifies cooperativity between the functional group interaction of interest and the other interactions that are used to change the overall stability of the complexes. The sum of two edge-to-face aromatic interactions (-2.9 +/- 0.5 kJ mol(-1)) is shown to be insensitive to changes of up to 13.7 +/- 0.2 kJ mol(-1) in the overall stability of the complex. In principle, enthalpic cooperative effects caused by entropy - enthalpy compensation could perturb the measurement of intermolecular interactions when using the double-mutant cycle approach, but these experiments show that, for this system, the magnitude of the effect lies within the error of the measurements.
Quantitative Measurements of Edge‐to‐Face Aromatic Interactions by Using Chemical Double‐Mutant Cycles
作者:Fiona J. Carver、Christopher A. Hunter、Philip S. Jones、David J. Livingstone、James F. McCabe、Eileen M. Seward、Pascale Tiger、Sharon E. Spey
Synthetic H-bonded zipper complexes have been used to quantify the magnitude of an edge-to-face aromatic interaction between a benzoyl group and an aniline ring. Four chemical double-mutant cycles were constructed by using a matrix of nine closely related complexes in which the aromatic rings were sequentially substituted for alkyl substituents. The stability constants and three-dimensional structures of the complexes were determined by using 1H NMR titrations in deuterochloroform at room temperature. The value of the interaction energy is similar in all cases, the average is -1.4 +/- 0.5 kJ mol(-1). The scope and limitations of the double-mutant approach are explored, and the consequences of conformational equilibria are discussed.
Quantitative Determination of Intermolecular Interactions with Fluorinated Aromatic Rings
作者:Harry Adams、Jose-Luis Jimenez Blanco、Gianni Chessari、Christopher A. Hunter、Caroline M. R. Low、John M. Sanderson、Jeremy G. Vinter
The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl pi -systems. The complexes have been characterised using H-1 and F-19 NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl pi -system. The interactions are generally repulsive and this repulsion increases with more electron-withdrawing substituents up to a limit of +2.2 kJ mol(-1), when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a chance in the geometry of the aromatic interaction and attractive stacking interactions are found (- 1.6 kJ mol(-1) for NMe2). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high pi -electron density there is a large electrostatic driving force in favour of the stacked geometry which places this pi -electron density over the centre of the positive charge on the pentafluorophenyl group.