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Structural and Thermodynamic Characterization of the New Binding Motif

As expected from the modeling, guanidiniocarbonyl pyrrole receptors indeed strongly bind carboxylates even in aqueous solvents, by a combination of ion pairing and multiple hydrogen bonding, as can be seen in the NMR spectrum. For example, addition of acetate to a solution of the ethylamide-substituted guanidinio-carbonyl pyrrole 2 in dimethyl sulfoxide causes significant complexation-induced shifts (CIS) of the various protons of 2 in the 1H NMR spectrum (Figure 2.3.3).

The 1H NMR spectrum of the receptor itself contains the ''normal'' signals expected for an acyl guanidinium cation (Figure 2.3.3, front) [11]. On addition of the carboxylate, large downfield complexation-induced shifts of some of the NH protons of the guanidiniocarbonyl pyrrole are observed (Figure 2.3.3, back). Such shift changes indicate that these protons form hydrogen bonds to the carboxylate. On formation of an H-bond the electron density around the proton is further diminished and this deshielding causes a downfield shift of the corresponding signal in

aceiate 10.0 ao 8.0

Fig. 2.3.3. 1 H NMR spectrum of receptor 2 (picrate salt) with (back) and without (front) acetate (NMe4+ salt) in [D6]DMSO, showing the CIS of the guanidinium NH protons and the amide NH.

aceiate 10.0 ao 8.0

Fig. 2.3.3. 1 H NMR spectrum of receptor 2 (picrate salt) with (back) and without (front) acetate (NMe4+ salt) in [D6]DMSO, showing the CIS of the guanidinium NH protons and the amide NH.

144 | 2.3 Bioorganic Receptors for Amino Acids and Peptides 2 11 1

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