Carbon nitride as a ligand: edge-site coordination of ReCl(CO)(3)-fragments to g-C3N4

IR spectroscopy and model structural studies show binding of ReCl(CO)3-fragments to carbon nitride (g-C3N4) occurs via κ2 N,N′ bidentate coordination

The Pneumococcal Iron Uptake Protein a (PiuA) Specifically Recognizes Tetradentate FeIIIbis- and Mono-Catechol Complexes

Streptococcus pneumoniae (Spn) is an important Gram-positive human pathogen that causes millions of infections worldwide with an increasing occurrence of antibiotic resistance. Fe acquisition is a crucial virulence determinant in Spn; further, Spn relies on exogenous FeIII-siderophore scavenging to meet nutritional Fe needs. Recent studies suggest that the human catecholamine stress hormone, norepinephrine (NE), facilitates Fe acquisition in Spn under conditions of transferrin-mediated Fe starvation. Here we show that the solute binding lipoprotein PiuA from the piu Fe acquisition ABC transporter PiuBCDA, previously described as an Fe-hemin binding protein, binds tetradentate catechol FeIII complexes, including NE and the hydrolysis products of enterobactin. Two protein-derived ligands (H238, Y300) create a coordinately-saturated FeIII complex, which parallel recent studies in the Gram-negative intestinal pathogen Campylobacter jejuni. Our in vitro studies using NMR spectroscopy and 54Fe LC-ICP-MS confirm the FeIII can move from transferrin to apo-PiuA in a NE-dependent manner. Structural analysis of PiuA FeIII-bis-catechol and GaIII-bis-catechol and GaIII-(NE)2 complexes by NMR spectroscopy reveals only localized structural perturbations in PiuA upon ligand binding, largely consistent with recent descriptions of other solute binding proteins of type II ABC transporters. We speculate that tetradentate FeIII complexes formed by mono- and bis-catechol species are important Fe sources in Gram-positive human pathogens, since PiuA functions in the same way as SstD from Staphylococcus aureus

The iron-binding properties of aminochelin, the mono(catecholamide) siderophore of Azotobacter vinelandii

Azotobacter vinelandii produces siderophores with different metal-binding properties, depending on the concentration of Fe(III) and molybdate in the growth medium. The three protonation constants of the mono(catecholamide) siderophore aminochelin were determined by simultaneous spectrophotometric and potentiometric titrations as log K 1=12.1, log K 2=10.22 and log K 3=7.04. Based on the two catechol protonation constants, log K 1 and log K 3, the overall stability constant of the aminochelin iron 3:1 complex was found to be log ß 3=41.3, resulting in a pFe 3+ value of 17.6 at pH 7.45. In order to further investigate the properties of the siderophore, the solubilization of Fe(III) hydroxide by a 8×10 –4 M solution of aminochelin at pH 7 and 25 °C was followed spectrophotometrically in the absence and in the presence of molybdate. It was observed that the addition of molybdate resulted in a significant delay in the solubilization

An [{Fe(mecam)}2]6- bridge in the crystal structure of a ferric enterobactin binding protein

Bridging the gap: The dinuclear ferric enterobactin mimic [{Fe(mecam)}2]6- (H6-mecam=1,3,5-N,N,N-tris(2,3-dihydroxybenzoyl)triaminomethylbenzene) is recognized by two periplasmic binding protein molecules (yellow and blue; see structure). The assembly is stabilized through hydrophobic interactions between the ligand backbones of the dinuclear iron complex

Atropisomerisation in sterically hindered α,β-disubstituted cyclopentenones derived from an intermolecular cobalt(0)-mediated Pauson-Khand reaction

4-(2-Phenylethynyl)-2H-chromen-2-one reacts with norbornene and Co ₂(CO)₈ in an intermolecular Pauson-Khand reaction by focused microwave dielectric heating. Two regioisomeric products are formed; the electron-deficient coumarin moiety preferentially occupies the β-position of the cyclopentenone ring system, whereas the phenyl occupies the α-position. The sterically hindered α,β-(2,3)-disubstituted cyclopentenone regioisomeric products exhibit pronounced atropisomerisation, and the magnitude of the energetic barrier to interconversion between these atropisomers is dependent on the relative position of the coumarin moieties. Interconversion is slow when the coumarin is found in the α-position, whereas interconversion is relatively fast when found in the β-position. © 2010 The Royal Society of Chemistry

Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor

A heteroditopic ligand H 2-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me 2-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H 2-L+, H 2-L, and H-L-, showed distinct 1 H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re-(bpy)(CO) 3(H 2-L)]+. It was found that upon deprotonation of the 2-hydroxy group of H 2-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)-bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dn)→ bpy 3MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO) 3(H 2-L)]+ with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO 2{Re(CO) 3-(bpy)(L)} 2], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO 2 center

Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor

A heteroditopic ligand H 2-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me 2-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H 2-L+, H 2-L, and H-L-, showed distinct 1 H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re-(bpy)(CO) 3(H 2-L)]+. It was found that upon deprotonation of the 2-hydroxy group of H 2-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)-bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dn)→ bpy 3MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO) 3(H 2-L)]+ with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO 2{Re(CO) 3-(bpy)(L)} 2], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO 2 center

Synthesis of citrate-ciprofloxacin conjugates.

noTwo regioisomeric citrate-functionalized ciprofloxacin conjugates have been synthesized and their antimicrobial activities against a panel of clinically-relevant bacteria have been determined. Cellular uptake mechanisms were investigated using wild-type and ompF deletion strains of Escherichia coli K-12