Palladium(II)-Mediated Assembly of a M2L2 Macrocycle and M3L6 Cage from a Cyclopeptide-Derived Ligand.

A cyclic tetrapeptide L comprising two l-proline and two 3-amino-5-(pyridin-4-yl)benzoic acid subunits assembles into a dimetallic metallamacrocycle Pd2L2 and a trimetallic coordination cage Pd3L6 in the presence of suitable palladium(II) precursors. Pd2L2 recognizes organic anions in aqueous media, forming a particularly stable complex with sodium 2,6-naphthalenedisulfonate, in which two dianionic guest molecules reside in the cavity surrounded by the cyclopeptide ligands.

Recognition of phosphopeptides by a dinuclear copper(ii) macrocyclic complex in a water : methanol 50 : 50 v/v solution.

A new triethylbenzene-derived tetraazamacrocycle containing pyridyl spacers, L, was prepared and its dinuclear copper(ii) complex was used as a receptor for the recognition of phosphorylated peptides in aqueous solution. A detailed study of the acid-base behaviour of L and its copper(ii) complexation properties as well as of the cascade species with phosphorylated anions including two peptidic substrates was carried out in a H2O/MeOH (50 : 50 v/v) solution using different techniques, such as potentiometry, X-band EPR and DFT calculations. The association constants of the dinuclear receptor with the phosphorylated peptides and other anionic species revealed a clear preference towards phenylic phosphorylated substrates, with values ranging 3.96-5.35 log units. Single-crystal X-ray diffraction determination of the dicopper(ii) complex of L showed the copper centres at a distance of 5.812(1) Å from one another, with the phosphate group of the PhPO42- substrate well accommodated between them. X-band EPR studies indicated a similar structure for this cascade complex and for the other cascade complexes with the phosphorylated anions studied. DFT studies of the [Cu2L(µ-OH)]3+ complex revealed a different conformation of the ligand that brings the two copper centres at a very short distance of 3.94 Å aided by the presence of a bridging hydroxide anion that provides a CuOCu angle of 167.3°. This complex is EPR silent, in line with the singlet ground state obtained using CASSCF(2,2) calculations and DFT calculations with the broken-symmetry approach. This species coexists in solution with a complex in a different conformation, and having a CuCu distance of 6.63 Å, in lower percentage.

Inhibition of the STAT3 Protein by a Dinuclear Macrocyclic Complex.

A new diethylenetriamine-derived macrocycle bearing 2-methylpyridyl arms and containing m-xylyl spacers, L, was prepared, and its dinuclear copper(II) and zinc(II) complexes were used as receptors for the recognition in aqueous solution of a phosphorylated peptide derived from a sequence of the STAT3 protein. A detailed study of the acid-base behavior of L and of its complexation properties as well as of the association of the phosphorylated peptide to the receptor was carried out by potentiometry in aqueous solution at 298.2 K and I = 0.10 M in KNO3. The data revealed that the receptor forms stable associations with several protonated forms of the substrate, with constant values ranging from 3.32 to 4.25 log units. The affinity of the receptor for the phosphorylated substrate studied is higher at a pH value where the receptor is mainly in the [Cu2L](4+) form and the pY residue of the substrate is in the dianionic form (pH 6.55). These results, also supported by (31)P NMR studies, showed that the phosphopeptide is bound through the phosphoryl group in a bridging mode. Additionally, the receptor inhibited binding between active (phosphorylated) STAT3 and its target DNA sequence in a dose-dependent manner (IC50 63 ± 3.4 µM) in human nuclear extracts in vitro. Treatment of whole cells with the inhibitor revealed that it is bioactive in living cells and has oncostatic properties that could be interesting for the fight against cancer and other pathologies involving the STAT3 protein.

Dinuclear Zinc(II) Macrocyclic Complex as Receptor for Selective Fluorescence Sensing of Pyrophosphate.

A new diethylenetriamine-derived macrocycle known as L, bearing 2-methylquinoline arms and containing m-xylyl spacers, was prepared in good yield by a one-pot [2 + 2] Schiff base condensation procedure, followed by reduction with sodium borohydride. Up to now this is the first hexaazamacrocycle with appended fluorophore units. Single-crystal X-ray diffraction determination of the dinuclear zinc(II) complex of L showed that metal centers are located at about 7.20(2) Å from one another. This complex exhibits only weak fluorescence in aqueous solution, but the addition of 1 equiv of pyrophosphate (PPi) caused a 21-fold enhancement of the fluorescence intensity. The sensor response is linear up to a value of 10 µM HPPi(3-) and has a detection limit of 300 nM. The receptor behaves as a highly selective sensor for pyrophosphate as other anions, including phosphate, phenylphosphate (PhP), adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP), failed to induce any fluorescence change and practically do not affect the fluorescence intensity of the sensor in the presence of HPPi(3-). Competition titrations carried out in aqueous solution at pH 7.4 [in 20 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer] by spectrofluorometry revealed a high association constant value of 6.22 log units for binding of PPi by the dinuclear zinc(II) receptor, one of the highest reported values for colorimetric/fluorometric sensors able to work under real aqueous physiological conditions, while association constant values for binding of the other phosphorylated substrates are in the 5.51-4.03 log unit range.