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Pyrrolic Tripodal Receptors Effectively Recognizing Monosaccharides. Affinity Assessment through a Generalized Binding Descriptor.

Nativi C, Cacciarini M, Francesconi O, Vacca A, Moneti G, Ienco A, Roelens S

Contribution from the Dipartimento di Chimica Organica, Dipartimento di Chimica, Centro Risonanze Magnetiche (CERM), and Centro Interdipartimentale di Spettrometria di Massa (CISM), Università di Firenze, and Istituto di Chimica dei Composti OrganoMetallici (ICCOM) and Istituto di Metodologie Chimiche (IMC), Consiglio Nazionale delle Ricerche (CNR), Polo Scientifico e Tecnologico, I-50019 Sesto Fiorentino, Firenze, Italy.

Pyrrolic and imino (3) or amino (4) H-bonding ligands were incorporated into a benzene-based tripodal scaffold to develop a new generation of receptors for molecular recognition of carbohydrates. Receptors 3 and 4 effectively bound a set of octylglycosides of biologically relevant monosaccharides, including glucose (Glc), galactose (Gal), mannose (Man), and N-acetyl-glucosamine (GlcNAc), showing micromolar affinities in CDCl3 and millimolar affinities in CD3CN by NMR titrations. Both receptors selectively recognized Glc among the investigated monosaccharides, with 3 generally less effective than 4 but showing selectivities for the all-equatorial beta-glycosides of Glc and GlcNAc among the largest reported for H-bonding synthetic receptors. Selectivities in CDCl3 spanned a range of nearly 250-fold for 3 and over 30-fold for 4. Affinities and selectivities were univocally assessed through the BC50 descriptor, for which a generalized treatment is described that extends the scope of the descriptor to include any two-reagent host-guest system featuring any number of binding constants. ITC titrations of betaGlc in acetonitrile evidenced, for both receptors, a strong enthalpic contribution to the binding interaction, suggesting multiple H bonding. Selectivity trends toward alphaGlc and betaGlc analogous to those obtained in solution were also observed in the gas phase for 3 and 4 by collision-induced dissociation experiments. From comparison with appropriate reference compounds, a substantial contribution to carbohydrate binding emerged for both the imino/amino and the pyrrolic H-bonding groups but not for the amidic group. This previously undocumented behavior, supported by crystallographic evidence, has been discussed in terms of geometric, functional, and coordinative complementarity between H-bonding groups and glycosidic hydroxyls and opens the way to a new designer strategy of H-bonding receptors for carbohydrates.

Published 4 April 2007 in J Am Chem Soc, 129(14): 4377-4385.
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