Abstract
The binding affinity for pyrophosphate (PPi) in aqueous media by N-alkyl ammonium resorcinarene salts can be enhanced via using weakly coordinating counter anions and modifying the upper rims of the receptors and the lower rim hydroxyl groups.
Highlights
Ring-opening of the six-membered tetrabenzoxazine ring is effected by refluxing in the presence of an acid (HCl, HBr, and triflic acid) to provide crude R-NARX4s (Fig. 1); these are purified through recrystallization to give the final products in 50–85% yields
Thermodynamic parameters (K, ΔH, ΔS, and ΔG) between the R-NARX4 receptors and the different phosphate anions were measured by a series of isothermal titration calorimetry (ITC) experiments in 90% H2O/10% DMSO
Counter anion identity showed the same trend; the K1 value for the complexation of PPi to Np-NARX4 increased by 52.5% and 68.2% (Table 1) upon switching the chloride counterion to bromide and triflate, respectively
Summary
A significant challenge in supramolecular chemistry is developing high-affinity receptors for anions in biologically relevant solvents,[1,2,3,4,5] and the design of such receptors has elicited considerable effort from the research community.[6,7,8,9,10,11,12,13,14,15] Primarily, this challenge has been addressed by leveraging the cooperative effect of non-covalent attractions.[16,17,18] One such anion is pyrophosphate (PPi), produced as a side product during ATP metabolism.[14]. The favorable ΔH term hints at the significant strength of the salt bridges formed in the binding pocket.[37,38] Affinity is maintained in these new receptors: the K1 values for PPi are generally more than for AMP, ADP, and ATP (Fig. S23–S33 and Tables S1–S11†).
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