Porphyrin Receptors for Amines, Amino Acids, and Oligopeptides in Water

A series of water-soluble porphyrin receptors having a hydrophobic binding pocket, a Lewis acidic site (Zn), and an electrostatic recognition site (COO- groups) were prepared. All the porphyrin receptors have a [meso-tetrakis(p-carboxyphenyl)porphyrinato]zinc (Zn·TCPP) as a common structural unit, and eight ω-carboxyalkyloxy groups (alkyloxy = methoxy (1), butoxy (2), decyloxy (3)) at the ortho positions of the phenyl groups. These receptors bind amines, α-amino acid esters, and oligopeptides in water with significant selectivity. For binding of hydrophilic guests, 1, 2, and 3 bind histamine in pH 8 buffer at 25 °C with binding constants of 157000, 31000, and 18200 M-1, respectively, where the coordination (Imd−Zn) and the salt bridge (NH3+−-OOC) stabilized the complex. The large dependence of the binding constants on the ionic strength indicated that the electrostatic interaction between the ammonium group of histamine and the carboxylate groups of receptor contributes significantly to tight binding in w...