A Systematic Evaluation of Molecular Recognition Phenomena. 1. Interaction between Phosphates and Nucleotides with Hexaazamacrocyclic Ligands Containing m-Xylylic Spacers

The host−guest interactions between ortho- (Ph), pyro- (Pp), and tripolyphosphate (Tr) anions together with ATP (At), ADP (Ad), and AMP (Am) nucleotides and the hexaazamacrocyclic ligand 3,7,11,19,23,27-hexaazatricyclo[27.3.1.113,17]triaconta-1(32),13,15,17(34),29(33),30-hexaene (Bn) have been investigated by potentiometric equilibrium methods. Ternary complexes are formed in aqueous solution as a result of hydrogen bond formation and Coulombic attraction between the host and the guest. Formation constants for all the species obtained are reported. The selectivity of the Bn ligand with regard to the phosphate and nucleotide substrates is discussed and illustrated with species distribution diagrams. A comparison of the present results with those obtained for the similar but smaller macrocyclic ligand 3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]triaconta-1(29),11(30),12,14,25,27-hexaene (Bd) is also discussed. It is found that the competition of the Bd and Bn ligands for the formation of ternary species with a specific substrate is strongly dependent on the p[H]. The crystal structure of the compound [(H6Bn)(H2PO4)6]·2H2O with empirical formula C28H68N6O26P6 has been solved by means of X-ray diffraction analysis. The compound belongs to the triclinic P1̄ space group with Z = 1, a = 8.892(2) Å, b = 9.369(4) Å, c = 16.337(8) Å, α = 73.72(4)°, β = 83.01(4)°, and γ = 64.81(3)°. The phosphate counterions are found to bridge adjacent layers of macrocyclic molecules through an extensive hydrogen-bonding network.