Oxoanion binding by flexible guanidiniocarbonyl pyrrole-ammonium bis-cations in water

The syntheses of several bis-cations 1-5 with a simple primary ammonium cation attached via flexible linkers of varying length to a guanidiniocarbonyl pyrrole oxo anion binding site are reported. In UV-binding studies in aqueous buffer solution these bis-cations showed efficient binding of various N-acetyl amino acid carboxylates. However, complex affinity is significantly depending on both the anion and the length of the linker in the bis-cation. With increasing linker length, complex stability first increases until an optimum is reached for bis-cation 3 with a C4-linker. Then the complex stability decreases again. The best binding substrate in this series is N-acetyl phenyl alanine, most likely due to additional cation-pi-interactions between the aromatic ring and the guanidiniocarbonyl pyrrole cation. The formation of the complex between bis-cation 3 and N-acetyl phenyl alanine carboxylate was investigated further by fluorescence titrations and NOE studies, as well as molecular mechanics calculations.