The interaction of glycine, aspartic acid, and lysine by the protonated macrocyclic ligand 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2 11,14]triaconta-11,13,24,26,27,29-hexaene

Abstract

A new hexaaza macrocyclic ligand (L) bearing two 2-hydroxypropyl pendants, 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2 11,14]triaconta-11,13,24,26,27,29-hexaene has been synthesized and characterized. The macrocyclic ligand was isolated as a colorless crystal, monoclinic, P2(1)/n, with a=10.757(2), b=14.214(3), c=13.746(3) Å, β=101.40(3)°, V=2060.3(7) Å 3, Z=2, R1=0.0695, and w R2=0.1538 [I>2σ(I)]. Potentiometric studies of the macrocyclic ligand and three types of amino acids, glycine (equal numbers of carboxylate and amino groups), aspartic acid (more carboxylate groups than amino group), and lysine (more amino groups than carboxylate group) have been performed. The stability constants for the new macrocycle and binary complexes of the amino acid with the macrocyclic ligand are reported. Binary complexes are formed in aqueous solution as a result of hydrogen bonding interaction and electrostatic attraction between the host and the guest. The binding Schemes for the recognition of amino acids are suggested. From the results, it seems that this new macrocyclic ligand is able to bind three different amino acids with selectivity in aqueous solution, and the strength of binding is of the order lysine <glycine <aspartic acid.