Synthetic Receptors as Models for Alkali Metal cation-π Binding Sites in Proteins

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 97; no. 12; pp. 6271 - 6276
• Main Authors: De Wall, Stephen L., Meadows, Eric S., Barbour, Leonard J., Gokel, George W.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 06.06.2000; National Acad Sciences; National Academy of Sciences
• Subjects: Alkali metals; Alkalies; Atoms; Binding Sites; Biological Sciences; Cations; Chemical reactions; Chemistry; Chlorides; Crystal structure; Cyclic amino acids; Indoles; Metals; Physical Sciences; Potassium - metabolism; Protein Conformation; Proteins; Proteins - metabolism; Receptors; Sodium - metabolism
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.97.12.6271

The alkali metal cations Na+and K+have several important physiological roles, including modulating enzyme activity. Recent work has suggested that alkali metal cations may be coordinated by π systems, such as the aromatic amino acid side chains. The ability of K+to interact with an aromatic ring has been assessed by preparing a family of synthetic receptors that incorporate the aromatic side chains of phenylalanine, tyrosine, and tryptophan. These receptors are constructed around a diaza-18-crown-6 scaffold, which serves as the primary binding site for an alkali metal cation. The ability of the aromatic rings to coordinate a cation was determined by crystallizing each of the receptors in the presence of K+and by solving the solid state structures. In all cases, complexation of K+by the π system was observed. When possible, the structures of the unbound receptors also were determined for comparison. Further proof that the aromatic ring makes an energetically favorable interaction with the cation was obtained by preparing a receptor in which the arene was perfluorinated. Fluorination of the arene reverses the electrostatics, but the aromaticity is maintained. The fluorinated arene rings do not coordinate the cation in the solid state structure of the K+complex. Thus, the results of the predicted electrostatic reversal were confirmed. Finally, the biological implications of the alkali metal cation-π interaction are addressed.