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Arturo Espinosa
Departamento de Química Orgánica, Facultad de Química, Campus de Espinardo, Universidad de Murcia, 30100 Murcia, SPAIN.
Intra- and intermolecular NCIs balance in biscarbazole-trisurea (BC3U) receptors and their pyrophosphate complexes
As a part of recent studies on selective (optical, fluorescent or electrochemical) sensing of ions or neutral molecules, a number of derivatives of the novel biscarbazole-trisurea (BC3U) family of receptors have been prepared [1]. The prototypical peripherycally bis-p-nitrophenyl substituted term (Figure 1, left) shows a deeply coiled conformation in DMSO solution, according to the most stable computed [2] geometry obtained with the ORCA electronic structure package [3]. Addition of the biologically highly significant pyrophosphate anion selectively promotes remarkable experimental changes (optical, NMR shifts) that agree with the proposed computed (same level) structure (Figure 1, right). Complex formation requires a favourable energy balance between cleavage of several intramolecular NCIs in the receptor, involving both H-bonding and aromatic stacking (of either π,π- and T-types) and formation of new intermolecular (supramolecular) NCIs of mainly H-bonding type. All NCIs are displayed by means of colour-coded RDG (reduced density gradient) isosurfaces generated with the NCIplot program [4] and conveniently quantified by several typical bond-strength parameters.
Figure 1: Example of computed structures of a BC3U receptor and its pyrophosphate complex.
References
[1] G. Sánchez, A. Espinosa, D. Curiel, A. Tárraga, P. Molina, manuscript in preparation.
[2] COSMO(DMSO)/B3LYP-D3/def2-TZVP level of theory.
[3] ORCA An ab initio, DFT and semiempirical SCF-MO package. Written by F. Neese, Max Planck Institute for Bioinorganic Chemistry, D-45470 Mülheim/Ruhr, 2012. Version 2.9.1. Website: http://www.mpibac.mpg.de/bac/logins/neese/description.php. F. Neese, "The ORCA program system", WIREs Comput Mol Sci 2012, 2, 73–78.
[4] J. Contreras-García, E. R. Johnson, S. Keinan, R. Chaudret, J-P. Piquemal, D. N. Beratan, W. Yang. J. Chem. Theory Comput. 2011, 7, 625-632.