Hydrogen transfer in the presence of amino acid radicals

Quantum chemical model studies of hydrogen transfer between amino acids in the presence of radicals have been performed using the density functional theory method B3LYP. These studies were made to investigate alternative mechanisms to the conventional electron transfer-proton transfer mechanisms. The model reactions studied are such that the net result of the reaction is a transfer of one neutral hydrogen atom. Simple models are used for the amino acids. Three different mechanisms for hydrogen transfer were found. In the first of these, a transition state with a protonated intermediate residue is found, in the second, the proton and electron take different paths and in the third, a neutral hydrogen atom can be identified along the reaction pathway. A key feature of these mechanisms is that charge separation is always kept small in contrast to the previous electron transfer-proton transfer mechanisms. It is therefore proposed that the processes normally considered as electron transfer in the biochemical literature could in fact be better explained as hydrogen atom transfer, at least in cases where a suitable hydrogen bonded chain pathway is present in the protein. The presence of such chains in principle allows the protein to define the path of net hydrogen transfer. Another important conclusion is that standard quantum chemical methods can be used to treat these mechanisms for hydrogen transfer, allowing for an accurate representation of the geometric changes during the reactions. Received: 10 February 1997 / Accepted: 11 February 1997     

Molecular Structure of the Bis[tris(2-aminoethyl)aminecyano]dicobalt(III) Ion and its UV/VIS Spectrum

Formation of μ-peroxodicobalt(III) complexes has been studied in solutions containing tris(2-aminoethyl)aminecobalt(II) and additional monodentate ligands X. Depending n the nature and the concentration of X and on pH, singly bridged [(tren)XCoOOCoX(tren)]⁴⁺ and/or doubly bridged [(tren)Co(C₂OH)Co(tren)]³⁺ are formed. The UV/VIS spectra of these complexes are discussed on the basis of a theoretical model which stresses the importance of the dihedral angle of the CoOOCo-group. [(tren)(CN)CoOOCo(CN)(tren)](ClO₄)₂ has been synthesized and its structure determined by single crystal X-ray diffraction. The CoOOCo-group of the cation is planar. Solutions of the complex as well as the solid show two CT bands in the 300–400 nm region.     

Mehrkernige Kobaltkomplexe. I. Darstellung und Struktur von [(tren)Co(O2, OH)Co(tren)](ClO4)3·3H2O

Polynuclear Cobalt Complexes. I. Preparation and Structure of [(tren)Co(O₂, OH) - Co(tren)](ClO₄)₃ · 3H₂O. The title compound is obtained by oxygenation of [Co(tren)(H₂O)₂]²⁺ in alcaline solution. An X-ray structure determination shows that the tertiary nitrogen atoms of both chelate groups are cis to the O₂ bridge.     

Pyridazine and phthalazine derivatives with potential antimicrobial activity

Fifteen new pyridazine and phthalazine derivatives were prepared (in good to excellent yields) and tested in vitro as antimicrobial compounds. All the compounds have proved to have a remarkable activity against Gram positive germs, the results on Sarciria Luteea being spectacular. Correlation structure ‐ biological activity have been done. Stereo‐ and region‐ chemistry involved in these reactions are discussed.     

Paramagnetic Solid-State NMR to Localize the Metal-Ion Cofactor in an Oligomeric DnaB Helicase

Paramagnetic metal ions can be inserted into ATP-fueled motor proteins by exchanging the diamagnetic Mg²⁺ cofactor with Mn²⁺ or Co²⁺. Then, paramagnetic relaxation enhancement (PRE) or pseudo-contact shifts (PCSs) can be measured to report on the localization of the metal ion within the protein. We determine the metal position in the oligomeric bacterial DnaB helicase from Helicobacter pylori complexed with the transition-state ATP-analogue ADP:AlF₄⁻ and single-stranded DNA using solid-state NMR and a structure-calculation protocol employing CYANA. We discuss and compare the use of Mn²⁺ and Co²⁺ in localizing the ATP cofactor in large oligomeric protein assemblies. ³¹P PCSs induced in the Co²⁺-containing sample are then used to localize the DNA phosphate groups on the Co²⁺ PCS tensor surface enabling structural insights into DNA binding to the DnaB helicase.     

Peptide and protein characterization by high-rate electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry

The analytical utility of the electron capture dissociation (ECD) technique, developed by McLafferty and co-workers, has substantially improved peptide and protein characterization using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). The limitations of the first ECD implementations on commercial instruments were eliminated by the employment of low-energy electron-injection systems based on indirectly heated dispenser cathodes. In particular, the ECD rate and reliability were greatly increased, enabling the combination of ECD/FTICR-MS with on-line liquid separation techniques. Further technique development allowed the combination of two rapid fragmentation techniques, high-rate ECD and infrared multiphoton dissociation (IRMPD), in a single experimental configuration. Simultaneous and consecutive irradiations of trapped ions with electrons and photons extended the possibilities for ion activation/dissociation and led to improved peptide and protein characterization. The application of high-rate ECD/FTICR-MS has demonstrated its power and unique capabilities in top-down sequencing of peptides and proteins, including characterization of post-translational modifications, improved sequencing of peptides with multiple disulfide bridges and secondary fragmentation (w-ion formation). Analysis of peptide mixtures has been accomplished using high-rate ECD in bottom-up mass spectrometry based on mixture separation by liquid chromatography and capillary electrophoresis. This paper summarizes the current impact of high-rate ECD/FTICR-MS for top-down and bottom-up mass spectrometry of peptides and proteins.     

Liquid chromatography and electron-capture dissociation in Fourier transform ion cyclotron resonance mass spectrometry

Liquid separation methods in combination with electrospray mass spectrometry as well as the recently introduced fragmentation method electron capture dissociation (ECD) have become powerful tools in proteomics research. This paper presents the results of the first successful attempts to combine liquid chromatography (LC) and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with ECD in the analysis of a mixture of standard peptides and of a bovine serum albumin tryptic digest. A novel electron injection system provided conditions for ECD sufficient to yield extensive sequence information for the most abundant peptides in the mixtures on the time-scale of the chromatographic separation. The results suggest that LC/ECD-FTICRMS can be employed in the characterization of peptides in enzymatic digests of proteins or protein mixtures and identify and localize posttranslational modifications.     

Photoinduced hydrogen atom abstraction in N-(adamantyl)phthalimides: structure-reactivity study in the solid state

Adamantyl-functionalized phthalimides were synthesized and the probability of intramolecular photochemical hydrogen atom abstraction in the solid state analyzed by X-ray crystallographic analyses. These analyses and solid-state photolyses showed that the parameters determining photochemical reactivity for typical carbonyl compounds in the solid state can also be extended to phthalimides. Only N-(2-adamantyl)phthalimide underwent a solid-state photochemical reaction, which is the first example in the phthalimide series. This reaction is regio- and stereoselective, resulting in an endo-alcohol. On the other hand, the photoreaction of N-(2-adamantyl)phthalimide in solution gives an exo-alcohol as the main product together with an endo-alcohol and a benzazepindione.     

Poly[[tetraaquadi‐μ4‐glutarato‐μ2‐terephthalato‐dineodymium(III)] heptadecahydrate]

The title compound, [Nd₂(C₅H₆O₄)₂(C₈H₄O₄)(H₂O)₄]·17H₂O, obtained via hydrothermal reaction of Nd₂O₃ with glutaric acid and terephthalic acid, assembles as a three‐dimensional open framework with ten‐coordinate Nd–O polyhedra. The asymmetric part of the unit cell contains half a glutarate anion, a quarter of a terephthalate dianion, half an Nd^III cation, one coordinated water molecule and 4.25 solvent water molecules. Each [NdO₁₀] coordination polyhedron is comprised of six O atoms originating from four glutarate anions, two others from a terephthalate carboxylate group, which coordinates in a bidentate fashion, and two from water molecules. The Nd—O distances range from 2.4184 (18) to 2.7463 (18) Å. The coordination polyhedra are interconnected by the glutarate anions, extending as a two‐dimensional layer throughout the bc plane. Individual two‐dimensional layers are interlinked via terephthalate anions along the a axis. This arrangement results in rectangular‐shaped cavities with interstices of approximately 3.5 × 6 × 6.5 Å (approximately 140 ų), which are occupied by water molecules. The Nd^III cations, terephthalate anions, glutarate anions and one of the interstitial water molecules are located on special crystallographic positions. The Nd–terephthalate–Nd units are located across twofold rotation axes parallel to [100], with the Nd^III cations located directly on these axes. In addition, the terephthalate anion is bisected by a crystallographic mirror plane perpendicular to that axis, thus creating an inversion centre in the middle of the aromatic ring. The glutarate ligand is bisected by a crystallographic mirror plane perpendicular to (001). One of the solvent water molecules lies on a site of 2/m symmetry, and the symmetry‐imposed disorder of its H atoms extends to the H atoms of the other four solvent water molecules, which are disordered over two equally occupied and mutually exclusive sets of positions.