Carmen Anda

A systematic evaluation of molecular recognition phenomena. 2. Interaction between phosphates and nucleotides with hexaazamacrocyclic ligands containing diethylic ether 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 two hexaazamacrocyclic ligands 1,15-dioxa-4,8,12,18,22,26-hexaazacyclooctacosane (Pn) and 1,13-dioxa-4,7,10,16,20,24-hexaazacyclohexacosane (Op) 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 Pn and Op ligands with regard to the different phosphate and nucleotide substrates is discussed and illustrated with total species distribution diagrams. A comparison is also carried out, with the results obtained in this work and those obtained previously with three other closely related hexaazamacrocyclic ligands. This comparison manifests the importance of ligand basicity, rigidity, and pi-stacking capability in order to understand their binding and selectivity.

A thermodynamic and spectrophotometric study of anion binding with a multifunctional dipyridine-based macrobicyclic receptor

Abstract Synthesis and characterisation of a new macrobicycle containing two dipyridine units (L) is reported. Protonated forms of L are efficient receptors for inorganic phosphate and nucleotide anions. The binding properties of L toward these substrates have been investigated in aqueous solution by means of potentiometric, microcalorimetric and 1H NMR measurements. Only 1:1 receptor–anion complexes have been found in solution. The stability of the adducts with inorganic phosphates anions is higher than that found for the nucleotides complexes. The complexation reactions are endothermic, and promoted by invariably favourable entropic contributions, indicating that these pairing processes are mostly determined by the desolvation of the interacting species that occurs upon charge neutralisation.

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

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 receptors 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 receptors cavity.

Cu(II) and Ni(II) complexes with dipyridine-containing macrocyclic polyamines with different binding units

The coordination features of the two dipyridine-containing polyamine macrocycles 2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L1) and 4,4′-(2,5,8,11,14-pentaaza[15]-[15](2,2′)-bipyridylophane) (L2) toward Cu(II) and Ni(II) have been studied by means of potentiometric and spectrophotometric UV-vis titrations in aqueous solutions. While in L1 all the nitrogen donor atoms are convergent inside the macrocyclic cavity, in L2 the heteroaromatic nitrogen atoms are located outside. Ligands L1 and L2 form stable mono- and dinuclear complexes with Cu(II). In the case of Ni(II) coordination, only L1 gives dinuclear complexes, while L2 can form only mononuclear species. In the Cu(II) or Ni(II) complexes with L1 the metal(s) are lodged inside the macrocyclic cavity, coordinated to the heteroaromatic nitrogens. As shown by the crystal structure of the [CuL1]2+ and [NiL1]2+ cations, at least one of the two benzylic nitrogens is not coordinated and facile protonation of the complex takes place at neutral or slightly acidic pH values. The particular molecular architecture of L2, which displays two well-separated binding moieties, strongly affects its coordination behavior. In the mononuclear [CuL2]2+ complex, the metal is encapsulated inside the cavity, not coordinated by the dipyridine unit. Protonation of the complex, however, occurs on the aliphatic polyamine chain and gives rise to translocation of the metal outside the cavity, bound to the heteroaromatic nitrogens. In the [NiL2]2+ complex the metal is coordinated by the dipyridine nitrogens, outside the macrocyclic cavity. Thermodynamic and/or kinetic considerations may explain the different behavior with respect to the corresponding Cu(II) complex.