Recognition of quaternary ammonium salts with tetrapeptides containing alpha-aminoisobutyric acid as a conformational constraint

Tetrapeptides Trp-Aib-Gly-Leu-NH-Ar (Aib: alpha-aminoisobutyric acid, 2-ami no-2-methylpropanoic acid, Ar=phenyl or 3,5-dimethylphenyl) were synthesize d. The peptides bound quaternary ammonium salts as guests in CDCl3. For eve ry guest, the binding constant K of the peptide host which has a 3,5-dimeth ylphenyl group was larger than that of the host which has a phenyl group. R OESY analysis of the complex revealed that the N+-CH3 groups of the guests were close to the aromatic moieties of the host in the complex. The charge in cation guests, the pi-basicity of the host, and the turn conformation of the peptides were important factors for the complexation.