Interaction energies and dynamics of acid-base pairs isolated in cavitands.
The use of capsules and cavitands in physical organic chemistry is briefly reviewed, and their application to the study of salt bridges is introduced. Carboxylate/ammonium ion pairs are generated within an environment that more or less surrounds the functional groups within a synthetic fixed introverted solvent sphere. This is provided by cavitands that fold around amines and present them with a carboxylic acid function. Both organic and water-soluble versions were prepared, and their equilibrium affinities with quinuclidine bases were determined by NMR methods. The association constants range from approximately 10(3) M(-1) in water to more than 10(5) M(-1) in organic solvents. Studies of nitrogen inversion and tumbling of [2.2.2]-diazabicyclooctane within the introverted acids also illustrate the strength of the acid-base interactions. The barriers to in-out exchange of several amine guests were determined to be in the range from 15 to 24 kcal mol(-1). Some parallels with enzymes are drawn: the receptor folds around the guest species; presents them with inwardly directed functionality; and provides a generally hydrophobic environment and a periphery of secondary amide bonds.
Purse
Byron W
BW
The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
Butterfield
Sara M
SM
Ballester
Pablo
P
Shivanyuk
Alexander
A
Rebek
Julius
J
D000577:Amides / Q000737:chemistry; D000588:Amines / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D004988:Ethers, Cyclic / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D012118:Resorcinols / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics
D000577:Amides; D000588:Amines; D002264:Carboxylic Acids; D004988:Ethers, Cyclic; D012118:Resorcinols; D012997:Solvents; C442939:cavitand