Conformational changes of functionalised indole receptors upon their interaction with anions

The conformational analysis of four C2-amido and C7-ureido functionalised indole anion receptors was performed by a combination of heteronuclear NMR spectroscopy and ab initio quantum mechanical calculations. NOE experiments showed that anti-anti conformation across C2-C2 and C7-N7 bonds is predominant in acetone solution in the absence of anions. Upon anion binding to receptors, syn-syn conformation becomes predominant. The conformational changes upon anion binding are in good agreement with energetic preferences established by ab initio calculations. Chemical shift changes induced by interaction of anions suggest that binding of chloride and bromide anions occurs primarily to H1 and H7 protons. Nitrate anions favour interaction with H7 and H7 ureido protons, whereas acetate anions interact strongly with all four available hydrogen bond donor groups.