Daniela Donghi

Highly Emitting Concomitant Polymorphic Crystals of a Dinuclear Rhenium Complex

The dinuclear complex [Re(2)(μ-Cl)(2)(CO)(6)(μ-4,5-(Me(3)Si)(2)pyridazine)] gives in the solid state two polymorphs (yellow, 1Y, and orange, 1O), which can be either concomitantly or separately obtained on varying the crystallization rate. Both crystal phases exhibit intense photoluminescence from the lowest lying triplet metal-to-ligand charge transfer state, much stronger than in solution (quantum yields 0.56 and 0.52, for 1O and 1Y respectively, vs 0.06 in toluene), likely due to the restricted rotation of the Me(3)Si groups in the solid state. A clean, irreversible 1O → 1Y single-crystal-to-single-crystal phase transition occurs at 443 K, as revealed by variable temperature X-ray diffraction analysis. In spite of the absence of any strong intermolecular interactions in both forms, 1O and 1Y show very different absorption and emission maxima (λ(abs) 370 and 393 nm, λ(em) 534 and 570 nm, for 1Y and 1O, respectively). This behavior highlights the importance of the local organization of molecular dipoles in perturbing the photophysical properties of the molecule in the crystal.

Luminescent conjugates between dinuclear rhenium(I) complexes and peptide nucleic acids (PNA) for cell imaging and DNA targeting

New luminescent dinuclear rhenium(I) tricarbonyl complex-PNA conjugates have been synthesized through a reliable solid-phase synthetic methodology. Their photophysical properties have been measured. The most luminescent Re-PNA conjugate 7 showed interesting two-photon absorption (TPA) properties, that were exploited for imaging experiments, to demonstrate its easy uptake into living cells.

A new class of luminescent tricarbonyl rhenium(I) complexes containing bridging 1,2-diazine ligands: electrochemical, photophysical, and computational characterization.

A novel class of luminescent tricarbonyl rhenium(I) complexes of general formula [Re2(mu-X)2(CO)6(mu-diaz)] (X=halogen and diaz=1,2-diazine) was prepared by reacting [ReX(CO)5] with 0.5 equiv of diazine (seven different ligands were used). The bridging coordination of the diazine in these dinuclear complexes was confirmed by single-crystal X-ray analysis. Cyclic voltammetry in acetonitrile showed for all the complexes (but the phthalazine derivative) a chemically and electrochemically reversible ligand-centered reduction, as well as a reversible metal-centered bielectronic oxidation. With respect to the prototypical luminescent [ReCl(CO)3(bpy)] complex, the oxidation is more difficult and the reduction easier (about +0.3 V), so that a similar highest occupied molecular orbital-lowest unoccupied molecular orbital gap is observed. All of the complexes exhibit photoluminescence at room temperature in solution, with broad unstructured emission from metal-to-ligand charge-transfer states, at lambda in the range 579-620 nm. Lifetimes (tau=20-2200 ns) and quantum yields (Phi up to 0.12) dramatically change upon varying the bridging ligand X and the diazine substituents: in particular, quantum yields decrease in the series Cl, Br, and I and in the presence of substituents at the alpha positions of the pyridazine ring. A combined density functional and time-dependent density functional study of the geometry, relative stability, electronic structure, and photophysical properties of all the pyridazine derivatives was performed. The nature of the excited states involved in the electronic absorption spectra was ascertained, and trends in the energy of the highest occupied and lowest unoccupied molecular orbitals upon changing the pyridazine substituents and the bridging halogen ligands were discussed. The observed emission properties of these complexes were shown to be related to a combination of steric and electronic factors affecting their ground-state geometry and their stability.

Bovine β-lactoglobulin acts as an acid-resistant drug carrier by exploiting its diverse binding regions

Abstract Binding of fluorine-containing drugs to bovine β-lactoglobulin, the most abundant whey protein in bovine milk, was investigated by means of 19F NMR and mass spectrometry. The stoichiometry of the binding and its stability in acidic medium, where β-lactoglobulin is folded and stable, were also studied, along with competition from molecules that can be regarded as analogs of physiological ligands to bovine β-lactoglobulin. Conditional binding data were combined with protein structural information derived from circular dichroism and limited proteolysis studies. Spectroscopic techniques were also used to assess whether the bound drugs stabilize the protein structure against denaturation by chaotropes or temperature at various pH values. The results obtained provide evidence for the presence of multiple binding regions on the protein, with a specific and different affinity for structurally different classes of hydrophobic drugs and, more generally, that bovine β-lactoglobulin can bind and protect against low pH values various classes of drugs of pharmaceutical relevance.

The structural stabilization of the κ three-way junction by Mg(II) represents the first step in the folding of a group II intron

Folding of group II introns is characterized by a first slow compaction of domain 1 (D1) followed by the rapid docking of other domains to this scaffold. D1 compaction initiates in a small subregion encompassing the κ and ζ elements. These two tertiary elements are also the major interaction sites with domain 5 to form the catalytic core. Here, we provide the first characterization of the structure adopted at an early folding step and show that the folding control element can be narrowed down to the three-way junction with the κ motif. In our nuclear magnetic resonance studies of this substructure derived from the yeast mitochondrial group II intron Sc.ai5γ, we show that a high affinity Mg(II) ion stabilizes the κ element and enables coaxial stacking between helices d′ and d′′, favoring a rigid duplex across the three-way junction. The κ-element folds into a stable GAAA-tetraloop motif and engages in A-minor interactions with helix d′. The addition of cobalt(III)hexammine reveals three distinct binding sites. The Mg(II)-promoted structural rearrangement and rigidification of the D1 core can be identified as the first micro-step of D1 folding.

Tricarbonyl rhenium(I) complexes containing a bridging 2,5-diphenyl-1,3,4-oxadiazole ligand: structural, spectroscopic, electrochemical, and computational characterization.

The three complexes [Re2(mu-X1)(mu-X2)(CO)6(mu-ppd-kappaN3:kappaN4)] (X1, X2 ) H, 1; X1 ) H, X2 ) Cl, 2; X1, X2 ) Cl, 3; ppd) 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re4(mu3-H)4(CO)12]with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)5] with ppd for 3. The three complexes possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes 1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series)and in the stability of the complexes in the coordinating solvent acetonitrile (t1/2 for ppd displacement 13.6, 4.5, and 3.7 h,for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd,which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4 degrees, 23.3 degrees, and 45.0 degrees, respectively). The UV spectra show pi-pi* transitions of the oxadiazole ligand (which shift to higher energy on increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from 300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm(phi)0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation.Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction peaks (from -1.83 to -1.93 V vs Fc+|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of the heterocycle ligand upon coordination. The complexes also show a metal-centered, bi-electronic, irreversible oxidation peak (from 1.05 to 1.37 V vs Fc+/Fc). A combined density functional and time-dependent density functional (TD DFT)study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical and photophysical properties in terms of their electronic structure.

Protonation‐Dependent Base Flipping at Neutral pH in the Catalytic Triad of a Self‐Splicing Bacterial Group II Intron

NMR spectroscopy has revealed pH-dependent structural changes in the highly conserved catalytic domain 5 of a bacterial group II intron. Two adenines with pK(a) values close to neutral pH were identified in the catalytic triad and the bulge. Protonation of the adenine opposite to the catalytic triad is stabilized within a G(syn)-AH(+) (anti) base pair. The pH-dependent anti-to-syn flipping of this G in the catalytic triad modulates the known interaction with the linker region between domains 2 and 3 (J23) and simultaneously the binding of the catalytic Mg(2+) ion to its backbone. Hence, this here identified shifted pK(a) value controls the conformational change between the two steps of splicing.

Tricarbonyl−Rhenium Complexes of a Thiol-Functionalized Amphoteric Poly(amidoamine)

An amphoteric thiol-functionalized poly(amidoamine) nicknamed ISA23SH(10%) was synthesized. Rhenium complexes 1 and 2, containing 0.5 and 0.8 equiv of rhenium, respectively, were easily obtained by reacting ISA23SH(10%) with [Re(CO)(3)(H(2)O)(3)](CF(3)SO(3)) in aqueous solution at pH 5.5. Both ISA23SH(10%), and its rhenium complexes were soluble in water under physiological conditions. The resultant solutions were stable, even in the presence of cysteine. Rhenium chelation occurred through the S and N atoms of the cysteamine moiety, as demonstrated by (1)H, (13)C, and (15)N NMR spectroscopy. The diffusion coefficients and the hydrodynamic radii of ISA23SH(10%) and complex 1 were determined by pulsed gradient spin echo (PGSE) NMR experiments. The radius of the rhenium complexes 1 and 2 was always slightly larger than that of the parent polymer. TEM analysis showed that both complexes form spherical nanoparticles with narrow size distributions. Consistent results were obtained by dynamic light scattering. The observed sizes were in good agreement with those evaluated by PGSE. Preliminary in vitro and in vivo biological studies have been performed on complexes 1 and 2 as well as on the parent ISA23SH(10%). Neither hemolytic activity of the two rhenium complexes and the parent polymer, up to a concentration of 5 mg/mL, nor cytotoxic effects were observed on Hela cell after 48 h at a concentration of 100 ng/mL. In vivo toxicological tests showed that ISA23SH(10%) is highly biocompatible, with a maximum tolerated dose (MTD) of 500 mg/kg. No toxic side effects were apparent after the intravenous injection in mice of the two rhenium complexes in doses up to 20 mg/kg.

Metal ion-RNA interactions studied via multinuclear NMR.

Metal ions are indispensable for ribonucleic acids (RNAs) folding and activity. First they act as charge neutralization agents, allowing the RNA molecule to attain the complex active three dimensional structure. Second, metal ions are eventually directly involved in function. Nuclear magnetic resonance (NMR) spectroscopy offers several ways to study the RNA-metal ion interactions at an atomic level. Here, we first focus on special requirements for NMR sample preparation for this kind of experiments: the practical aspects of in vitro transcription and purification of small (<50 nt) RNA fragments are described, as well as the precautions that must be taken into account when a sample for metal ion titration experiments is prepared. Subsequently, we discuss the NMR techniques to accurately locate and characterize metal ion binding sites in a large RNA. For example, (2) J-[(1)H,(15)N]-HSQC (heteronuclear single quantum coherence) experiments are described to qualitatively distinguish between different modes of interaction. Finally, part of the last section is devoted to data analysis; this is how to calculate intrinsic affinity constants.

Covalent and non-covalent binding of metal complexes to RNA.

Targeting nucleic acids with metal complexes is an exciting and widely explored field of research. Following the discovery of the anticancer drug cisplatin, a number of metal complexes have been designed, synthesised, and tested for their DNA binding properties. On the contrary, the interaction of metal complexes with RNA has been much less investigated. RNA is an essential biomolecule, involved in a variety of crucial cellular functions, which offers a much wider structural diversity than DNA. As such, RNA represents an attractive target for the design and the development of structure-selective therapeutic and diagnostic agents. A few recent publications describe the ability of various metal complexes to interact with RNA, and the binding of cisplatin and derivatives to RNA is being currently investigated. This short review offers an overview of some recent advances on both covalent and non-covalent interactions of metal complexes with RNA and addresses the potential of targeting RNA non-duplex structures.