Conformational changes of functionalised indole receptors upon their interaction with anions

Abstract

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–C2α and C7–N7α 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 H7α protons. Nitrate anions favour interaction with H7α and H7γ ureido protons, whereas acetate anions interact strongly with all four available hydrogen bond donor groups.

Keywords:

• anion recognition
• conformation analysis
• host–guest systems
• NMR spectroscopy

Acknowledgements

We gratefully acknowledge the financial support of the Slovenian Research Agency, Ministry of Higher Education, Science and Technology of Republic Slovenia (program no. P1-0242), EN-FIST Centre of Excellence and COST d-31.