Flexible receptor architectures for anion recognition via ionic hydrogen bonding

Phosphate anions play key roles in biology, chemistry, the environment, and are important industrial raw materials. However, their recognition, binding, and sensing in water are complicated by their structural diversity, multiple protonation states, and strong hydration. Well-pre-organized macrocyclic receptors are used to increase selectivity and affinity by reducing the entropy associated with the adaptation of the receptor to the phosphate guests. However, the synthesis and purification of macrocyclic sensors is difficult, marred by low yields. Linear (non-macrocyclic) receptors and sensors are easy to synthesize and purify, but may display lower overall affinity and selectivity. We explored the utility of simple linear sensors that contain ionic hydrogen-bonding donors, namely ammonium, isothiouronium, and guanidinium, for phosphorus oxyanions. Anion binding was investigated by NMR spectroscopy and X-ray crystallography. Quantitative binding was determined by fluorescence titrations. Titrations revealed the general order of affinity to be H₂PO₄ ^- > HAsO₄ ^2- > H₂P₂O₇ ^2- > ADP^2- ≥ ATP^3- ≫ AMP^-, and K _assoc ∼ 10⁴-10⁵ M^-1. These sensors could also bind and sense the potentially carcinogenic herbicide glyphosate (K _assoc ∼ 10³ M^-1). Due to their flexible linear structure, these sensors were cross-reactive and, with their distinct ionic hydrogen bonding, they also displayed partial selectivity that could be leveraged in sensor arrays amenable to high-throughput sensing. High-throughput fluorescence data were analyzed using machine learning, linear discriminant analysis (LDA), and support vector machine (SVM)-based linear regressions to obtain 100% correct classification and quantification. Regression analyses allowed the determination of unknown analyte concentrations. LODs were ∼1.0 µM for phosphate and pyrophosphate, while LODs for ATP and ADP were determined to be 1.6-4.5 µM. Overall, these simple linear sensors, despite not being able to leverage the macrocyclic effect, provide a viable alternative at a fraction of the cost and synthetic effort.