Anion recognition by azophenol thiourea-based chromogenic sensors: a combined DFT and molecular dynamics investigation.

The relative binding affinities of several anions towards 2-nitroazophenol thiourea-based receptors were studied using density functional theory (DFT) in the gas phase and in chloroform solvent via PCM calculations. Both receptors have five distinctive NH and OH hydrogen donor atoms. All receptor-anion complexes are characterized by five intermolecular hydrogen bonds. The binding free energies are strongly influenced by a dielectric medium, and the solvation effect alters the trend of anion binding to the receptor. The calculated order of anion binding affinity for the receptor in chloroform, H2PO4->AcO->F->Cl->HSO4->NO3-, is in excellent accord with experimental findings. The overall order of binding affinity is attributed to the basicity of the anion, the effect of solvation, and the number of proton acceptors available. Calculations of the NMR and UV-vis spectra strongly support the experimental characterization of the receptor-anion complexes. Explicit solvent molecular dynamics simulations of selected receptor-anion complexes were also carried out. Analysis of the structural descriptors revealed that the anions were strongly bound within the binding pocket via hydrogen-bonding interactions to the five receptor protons throughout the simulation.