Electrostatic and Allosteric Cooperativity in Ion-Pair Binding: A Quantitative and Coupled Experiment鈥揟heory Study with Aryl鈥揟riazole鈥揈ther Macrocycles

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• 作者：Bo Qiao ; Arkajyoti Sengupta ; Yun Liu ; Kevin P. McDonald ; Maren Pink ; Joseph R. Anderson ; Krishnan Raghavachari ; Amar H. Flood
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 5, 2015
• 年：2015
• 卷：137
• 期：30
• 页码：9746-9757
• 全文大小：861K
• ISSN：1520-5126

文摘

Cooperative binding of ion pairs to receptors is crucial for the manipulation of salts, but a comprehensive understanding of cooperativity has been elusive. To this end, we combine experiment and theory to quantify ion-pair binding and to separate allostery from electrostatics to understand their relative contributions. We designed aryl鈥搕riazole鈥揺ther macrocycles (MC) to be semiflexible, which allows ion pairs (NaX; X = anion) to make contact, and to be monocyclic to simplify analyses. A multiequilibrium model allows us to quantify, for the first time, the experimental cooperativity, 伪, for the equilibrium MC路Na⁺ + MC路X^{鈥?/sup> 鈬?MC路NaX + MC, which is associated with contact ion-pair binding of NaI (伪 = 1300, 螖G_伪 = 鈭?8 kJ mol鈥?) and NaClO₄ (伪 = 400, 螖G_伪 = 鈭?5 kJ mol鈥?) in 4:1 dichloromethane鈥揳cetonitrile. We used accurate energies from density functional theory to deconvolute how the electrostatic effects and the allosteric changes in receptor geometry individually contribute to cooperativity. Computations, using a continuum solvation model (dichloromethane), show that allostery contributes 鈭?0% to overall positive cooperativity. The calculated trend of electrostatic cooperativity using pairs of spherical ions (NaCl > NaBr > NaI) correlates to experimental observations (NaI > NaClO₄). We show that intrinsic ionic size, which dictates charge separation distance in contact ion pairs, controls electrostatic cooperativity. This finding supports the design principle that semiflexible receptors can facilitate optimal electrostatic cooperativity. While Coulomb鈥檚 law predicts the size-dependent trend, it overestimates electrostatic cooperativity; we suggest that binding of the individual anion and cation to their respective binding sites dilutes their effective charge. This comprehensive understanding is critical for rational designs of ion-pair receptors for the manipulation of salts.}