NMR-based analysis of aminoglycoside recognition by the resistance enzyme ANT(4'): the pattern of OH/NH3(+) substitution determines the preferred antibiotic binding mode and is critical for drug inactivation

The most significant mechanism of bacterial resistance to aminoglycosides is the enzymatic inactivation of the drug. Herein, we analyze several key aspects of the aminoglycoside recognition by the resistance enzyme ANT(4') from Staphylococcus aureus, employing NMR complemented with site-directed mutagenesis experiments and measurements of the enzymatic activity on newly synthesized kanamycin derivatives. From a methodological perspective, this analysis provides the first example reported for the use of transferred NOE (trNOE) experiments in the analysis of complex molecular recognition processes, characterized by the existence of simultaneous binding events of the ligand to different regions of a protein receptor. The obtained results show that, in favorable cases, these overlapping binding processes can be isolated employing site-directed mutagenesis and then independently analyzed. From a molecular recognition perspective, this work conclusively shows that the enzyme ANT(4') displays a wide tolerance to conformational variations in the drug. Thus, according to the NMR data, kanamycin-A I/II linkage exhibits an unusual anti-Psi orientation in the ternary complex, which is in qualitative agreement with the previously reported crystallographic complex. In contrast, closely related, kanamycin-B is recognized by the enzyme in the syn-type arrangement for both glycosidic bonds. This observation together with the enzymatic activity displayed by ANT(4') against several synthetic kanamycin derivatives strongly suggests that the spatial distribution of positive charges within the aminoglycoside scaffold is the key feature that governs its preferred binding mode to the protein catalytic region and also the regioselectivity of the adenylation reaction. In contrast, the global shape of the antibiotic does not seem to be a critical factor. This feature represents a qualitative difference between the target A-site RNA and the resistance enzyme ANT(4') as aminoglycoside receptors.     

New Alkaloids from Hippeastrum papilio (Ravenna) Van Scheepen

A new phytochemical study of the indigenous Brazilian species Hippeastrum papilio is reported herein. Three novel Amaryllidaceae alkaloids were isolated, including hippapiline ( 1 ), papiline ( 2 ), and 3‐O‐demethyl‐3‐O‐(3‐hydroxybutanoyl)haemanthamine ( 3 ). Their structures were determined by physical and spectroscopic methods. In addition, the known alkaloids, haemanthamine ( 4 ), galanthamine ( 5 ), narwedine ( 6 ), 11β‐hydroxygalanthamine ( 7 ), apogalanthamine ( 8 ), and 9‐O‐demethyllycosinine B ( 9 ) were identified. The unusual cis‐B/C‐ring fusion for the new homolycorine representative hippapiline was ratified by NMR and CD spectroscopy.     

On the use of optimal internal vibrational coordinates for symmetrical bent triatomic molecules

The use of generalized internal coordinates for the variational calculation of excited vibrational states of symmetrical bent triatomic molecules is considered with applications to the SO2, O3, NO2, and H2O molecules. These coordinates depend on two external parameters which can be properly optimized. We propose a simple analytical method to determine the optimal internal coordinates for this kind of molecules based on the minimization with respect to the external parameters of the zero-point energy, assuming only quadratic terms in the Hamiltonian and no quadratic coupling between the optimal coordinates. The optimal values of the parameters thus obtained are shown to agree quite well with those that minimize the sum of a number of unconverged energies of the lowest vibrational states, computed variationally using a small basis function set. The unconverged variational calculation uses a basis set consisting of the eigenfunctions of the uncoupled anharmonic internal coordinate Hamiltonian. Variational calculations of the excited vibrational states for the four molecules considered carried out with an increasing number of basis functions, also evidence the excellent convergence properties of the optimal internal coordinates versus those provided by other normal and local coordinate systems.     

Co(II), Ni(II) and Cu(II) metal complexes with a N8 azamacrocyclic ligand

Hydrated nitrate and perchlorate salts of the transitional metal ions Co²⁺, Ni²⁺ and Cu²⁺ have been used to investigate the coordination capability of the octaaza macrocycle L derived from 2,6-diformylpyridine and diethylenetriamine. The synthesis of the metal complexes was carried out in 1:1 and 2:1 metal:ligand molar ratios, but dinuclear complexes were obtained in all cases due to the size of the 24-membered ligand. The complexes have been characterized by elemental analysis, molar conductivity, mass spectrometry, IR spectroscopy, diffuse reflectance and magnetic measurements. The dinuclear nature of the compounds was confirmed by X-ray diffraction. The crystal structures of [Ni₂L(NO₃)₂](NO₃)₂, [Cu₂L(NO₃)₄] and [Cu₂L(ClO₄)₄], were determined.     

A hexaaza macrocyclic ligand containing acetohydrazide pendants for Ln(III) complexation in aqueous solution. Solid-state and solution structures and DFT calculations

Lanthanide complexes of a hexaaza macrocyclic ligand containing acetohydrazide pendants (L) have been synthesised (Ln = La-Er, except Pm), and structural studies have been carried out both in the solid state and in aqueous solution. Attempts to isolate the complexes of the heaviest Ln(iii) ions (Ln = Tm-Lu) were unsuccessful. The crystal structures of the ligand and its lanthanum complex have been determined by single-crystal X-ray crystallography. The X-ray crystal structure of [La(L)](3+) shows the metal ion being ten-coordinate, with the acetohydrazide pendants situated alternatively above and below the plane of the macrocycle. The two five membered chelate rings formed by the ethylenediamine moieties adopt (deltadelta) [or (lambdalambda)] conformations. The [Ln(L)](3+) complexes have been characterised by means of density-functional theory (DFT) calculations (B3LYP model). The structures obtained from these theoretical calculations are in very good agreement with the experimental solution structures, as obtained from paramagnetic NMR measurements on the Ce(iii), Pr(III), Nd(III) and Eu(III) complexes. The complexes adopt in aqueous solution a D(2) structure with the ligand adopting a (deltadelta) [or (lambdalambda)] conformation.     

Dinuclear cobalt(II) and copper(II) complexes with a Py2N4S2 macrocyclic ligand

The interaction between Co(II) and Cu(II) ions with a Py(2)N(4)S(2)-coordinating octadentate macrocyclic ligand (L) to afford dinuclear compounds has been investigated. The complexes were characterized by microanalysis, conductivity measurements, IR spectroscopy and liquid secondary ion mass spectrometry. The crystal structure of the compounds [H(4)L](NO(3))(4), [Cu(2)LCl(2)](NO(3))(2) (5), [Cu(2)L(NO(3))(2)](NO(3))(2) (6), and [Cu(2)L(μ-OH)](ClO(4))(3)·H(2)O (7) was also determined by single-crystal X-ray diffraction. The [H(4)L](4+) cation crystal structure presents two different conformations, planar and step, with intermolecular face-to-face π,π-stacking interactions between the pyridinic rings. Complexes 5 and 6 show the metal ions in a slightly distorted square-pyramidal coordination geometry. In the case of complex 7, the crystal structure presents the two metal ions joined by a μ-hydroxo bridge and the Cu(II) centers in a slightly distorted square plane or a tetragonally distorted octahedral geometry, taking into account weak interactions in axial positions. Electron paramagnetic resonance spectroscopy is in accordance with the dinuclear nature of the complexes, with an octahedral environment for the cobalt(II) compounds and square-pyramidal or tetragonally elongated octahedral geometries for the copper(II) compounds. The magnetic behavior is consistent with the existence of antiferromagnetic interactions between the ions for cobalt(II) and copper(II) complexes, while for the Co(II) ones, this behavior could also be explained by spin-orbit coupling.     

Synthesis, characterization, and fluorescence behavior of four novel macrocyclic emissive ligands containing a flexible 8-hydroxyquinoline unit

Four emissive macrocyclic ligands mono-substituted with an 8-hydroxyquinoline pendant arm are presented. The new compounds have been used for metal-ion detection, which results from the competition between PET (photo-induced electron transfer) and PPT (photo-induced proton transfer) mechanisms. Solid metal complexes with divalent Cu(II), Zn(II), and Cd(II), and trivalent metal ions Al(III) and Cr(III) have been also synthesized and characterized. The compounds have been isolated as mononuclear or dinuclear (Cu(II)) complexes, confirming the stoichiometry observed in solution.     

Two hydro­chlorides of 7‐methyl‐3,7,11,17‐tetra­aza­bi­cyclo­[11.3.1]­heptadeca‐1(17),13,15‐triene

The X‐ray structure determinations of the two title com­pounds, namely 7‐methyl‐7,17‐di­aza‐3,11‐diazo­niabi­cyclo[11.3.1]­hep­ta­deca‐1(17),13,15‐triene dichloride monohydrate, C₁₄H₂₆N₄²⁺·2Cl^?·H₂O, (I), and 7‐methyl‐17‐aza‐3,7,11‐triazo­niabi­cyclo­[11.3.1]­heptadeca‐1(17),13,15‐triene 2.826‐chloride 0.174‐nitrate, C₁₄H₂₇N₄³⁺·2.826Cl^?·0.174NO₃^?, (II), are re­ported. Protonation occurs at the secondary amine N atoms in (I) and at all three amine N atoms in (II) to which the Cl^? ions are linked via N—H?Cl hydrogen bonds. The macrocyclic hole is quite different in both structures, as is observed by comparing particularly the N3?N4 distances [2.976 (4) and 4.175 (4) Å for (I) and (II), respectively]. In (II), a Cl^? ion alternates with an NO₃^? ion in a disordered structure.     

Qualitative analysis of phenolic compounds in apple pomace using liquid chromatography coupled to mass spectrometry in tandem mode

The occurrence of phenolic compounds in apple residues resulting from the juice industry was investigated to provide an alternative use for this raw material. For the identification of these compounds, liquid chromatography coupled to ionspray mass spectrometry in tandem mode (LC/MS/MS) with negative ion detection was used. The residues were first extracted and then chromatographed on Sephadex LH-20 to yield 13 fractions. Positive identification of the compounds was based on their retention times and mass spectra in full scan mode (MS), and in different MS/MS modes (product ion scan, precursor ion scan and neutral loss scan). In this way, 60 compounds, including cinnamic and benzoic acid derivatives and flavonoids, were identified, some of them not previously reported in apple waste.     

Manganese(II), Cobalt(II), and Nickel(II) Perchlorate Complexes Containing N,O Donor Macrocyclic Ligands

Mononuclear macrocyclic complexes of manganese(II ), cobalt(II ) and nickel(II ) perchlorate using 10 different oxaazamacrocyclic ligands (L¹ — L¹⁰) have been prepared and characterized. The complexation reactions with the diiminic ligands were obtained by template condensation of the appropriate dialdehyde and diamine precursors; the reduced macrocycle complexes were synthesized using a direct route. The complexes have been characterized by elemental analyses, molar conductivity, mass spectrometry, IR, UV‐vis spectroscopy, diffuse reflectance and magnetic susceptibility measurements.