Purse Byron W, Butterfield Sara M, Ballester Pablo, Shivanyuk Alexander, Rebek Julius
The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
J Org Chem. 2008 Sep 5;73(17):6480-8. doi: 10.1021/jo8008534. Epub 2008 Aug 2.
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.
本文简要回顾了胶囊和空穴配体在物理有机化学中的应用，并介绍了它们在盐桥研究中的应用。羧酸根/铵离子对在一个或多或少围绕着合成固定内向型溶剂球内官能团的环境中产生。这是由围绕胺折叠并为其提供羧酸功能的空穴配体实现的。制备了有机和水溶性两种形式，并通过核磁共振方法测定了它们与奎宁环碱的平衡亲和力。缔合常数范围从水中的约10³ M⁻¹到有机溶剂中的超过10⁵ M⁻¹。对内向型酸中[2.2.2]-二氮杂双环辛烷的氮翻转和翻滚的研究也说明了酸碱相互作用的强度。几种胺客体进出交换的势垒在15至24 kcal mol⁻¹范围内。文中还提到了与酶的一些相似之处：受体围绕客体物种折叠；为它们提供向内的官能团；并提供一个通常疏水的环境和二级酰胺键的外围。
