Amino acid binding by 2-(guanidiniocarbonyl)pyridines in aqueous solvents: A comparative binding study correlating complex stability with stereoelectronic factors

A series of guanidiniocarbonylpyridine receptors has been synthesized, and these compounds bind amino acids (carboxylate forms) in aqueous DMSO with association constants ranging from K = 30 to 460m(-1) as determined by NMR titration experiments. The differences in the complex stabilities can be correlated with stefic and electrostatic effects with the aid of calculated complex structures. For example, the electrostatic repulsion between the pyridine nitrogen lone pair and the bound carboxylate makes anion binding less efficient than with the analogous pyrrole receptors previously introduced by us for carboxylate binding in water. Furthermore, steric interactions between the receptor side chain as in 2b and the bound substrate also disfavor complexation.