A Systematic Evaluation of Molecular Recognition Phenomena. Part 5. Selective Binding of Tripolyphosphate and ATP to Isomeric Hexaazamacrocyclic Ligands Containing Xylylic Spacers

Abstract

Protonation constants for 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-1(27),11(30),12,14(29),24(28), 25-hexaene (P2) and 3,7,11,18,22,26-hexaazatricyclo[26.2.2.213,16]tetratriaconta-1(31),13(34),14,16(33),28(32),29-hexaene (P3) and their host–guest interactions with tripolyphosphate (Tr) and ATP (At) have been determined and evaluated by 1H NMR and potentiometric equilibrium methods. Ternary complexes were formed in aqueous solution as a result of hydrogen bond formation and Coulombic interactions between the host and the guest. For the case of ATP π-stacking interactions were found. Formation constants for all the species obtained are reported and compared with the isomeric 3,7,11,19,23,27-hexaazatricyclo[27.3.1.113,17]tetratriaconta-1(33),13,15,17(34),29,31-hexaene (Bn) and 3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]triaconta-1(29),11,13,15(30), 25(27)-hexaene (Bd) ligands. Bonding interactions reach a maximum for H6P2Tr+, yielding a value of 12.02. The selectivity of the P3 and P2 ligands with regard to ATP and Tr substrates (S) is discussed and illustrated with global species distribution diagrams showing a strong preference for the latter over the former as a consequence of the much stronger formation constants with Tr. An analysis of the isomeric effect was also carried out by comparing the P3-S vs. Bn-S and P2-S vs. Bd-S systems. For the systems using Tr, a selectivity of more than 97% (pH 5.0) was achieved for its complexation when using the meta (Bd) rather than the para (P2) isomer, due solely to the size and shape of the receptor's cavity. In the case of the P3 and Bn ligands the selectivity toward Tr complexation decreased to 85% (pH 8.0). Molecular recognition of tripolyphosphate and ATP is achieved through the formation of anionic complexes with isomeric hexaazamacrocyclic ligands. A selectivity of more than 97% is achieved for tripolyphosphate complexation when using the meta rather than the para isomer, a result due solely to the size and shape of the receptor's cavity.