Synthesis and characterisation of Fe6 and Fe12 clusters using bicine

Reaction of bicine {BicH₃, N,N-bis(2-hydroxyethyl)glycine} with an Fe(III) oxo-centered pivalate triangle in MeCN in the presence of Et₂NH yields [Et₂NH₂]₂[Fe₆O₂(OH)₂(Bic)₂(O₂CCMe₃)₈], which possesses an S = 5 ground state.Changing the base to NaOMe produces [Fe₁₂O₄(Bic)₄(HBic)₄(O₂CCMe₃)₈], which contains two Fe₆ units bridged by the carboxylate arms from the bicine ligands. The complex displays strong antiferromagnetic coupling leading to an S = 0 ground state.     

Modulation of ligand-field parameters by heme ruffling in cytochromes c revealed by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of Hydrogenobacter thermophilus cytochrome c(552) (Ht c-552) and Pseudomonas aeruginosa cytochrome c(551) (Pa c-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in Ht c-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (E(m)) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. Biochemistry 2007, 46, 11753-11760]. These mutations are shown here to also increase heme ruffling as E(m) decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d(xy) orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in Pa c-551 and Ht c-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes c is shown to play a role in determining heme conformation and electronic structure.     

Functional initiators for the ring-opening polymerization of polyesters and polycarbonates: An overview

Functional ring-opening polymerization (ROP) initiators can instill a wide array of chemical, physical, and biological effects into a polymeric chain. Highlighting the versatility of this “active” initiator approach, a broad range of characteristics can be achieved through the use of initiators with chemistries spanning from drugs and dyes (key in the case of drug delivery or nanoparticle applications) through to radically active monomers, polymerization transfer agents, and catalysts. The selection of a suitable “active” initiator (monomers for tandem reactions, dyes, drugs, stereo-catalysts, etc.) can not only provide the final polymers with interesting application potential but also facilitate the implementation of ROP reactions in tandem with other polymerization techniques. Overall, this review will highlight that functionalities and properties can be effectively tuned by exploiting simple chemistry approaches, allowing readers to identify how these approaches could be of benefit to their own work in a range of applications including drug/gene delivery, amphiphilic bio/degradable carriers, drug/scent controlled release, and stereo-controlled polymers.     

The influence of the morphology on the dynamics in ordered diblock copolymer melts

We report on polarized and depolarized dynamic light scattering (DLS) measurements on two asymmetric poly(ethylene‐alt‐propylene‐block‐dimethylsiloxane) (PEP‐PDMS) diblock copolymers in bulk. Apart from the disordered phase, the samples form various ordered morphologies below their respective order‐to‐disorder transition temperatures: One of the samples forms the hexagonal structure composed of PEP cylinders in a PDMS matrix, whereas the other one forms three ordered structures as a function of temperature, among them two cubic micellar structures and a non‐cubic structure. We report here on the dynamic processes found in polarized DLS measurements in the disordered, hexagonal, cubic micellar and non‐cubic structure in the same material.     

A method for analysis of dimethyl selenide and dimethyl diselenide by LC-ICP-DRC-MS

The aim of this work was to develop a simple and fast high performance liquid chromatography-inductively coupled argon plasma (ICP) mass spectrometry (MS) method capable of separating and detecting the two volatile selenium species dimethyl selenide (DMeSe) and dimethyl diselenide (DMeDSe) in biological samples. Dimethyl selenide and dimethyl diselenide were separated on a short reversed phase column using an eluent containing 40% methanol and detected by dynamic reaction cell ICP-MS monitoring the (80)Se isotope. The limit of detection was 8 nM for both species (corresponding to 0.6 and 1.3 μg Se/L for DMeDSe and DMeSe, respectively). Both compounds exhibited a linear signal-concentration relationship in the investigated concentration range of 0.1-1 μM with a precision on the determinations better than 3%. The method was applied for analysis of samples from cancer cell lines incubated with methylseleninic acid, selenomethionine, Se-methylselenocysteine, and sodium selenite. DMeDSe were detected in some samples. The method offers a simple and fast analysis of DMeDSe and DMeSe using standard liquid chromatography coupled with ICP-MS equipment and interfacing.     

The reaction mechanism of iron and manganese superoxide dismutases studied by theoretical calculations

We have studied the detailed reaction mechanism of iron and manganese superoxide dismutase with density functional calculations on realistic active-site models, with large basis sets and including solvation, zero-point, and thermal effects. The results indicate that the conversion of O2- to O2 follows an associative mechanism, with O2- directly binding to the metal, followed by the protonation of the metal-bound hydroxide ion, and the dissociation of 3O2. All these reaction steps are exergonic. Likewise, we suggest that the conversion of O2- to H2O2 follows an at least a partly second-sphere pathway. There are small differences in the preferred oxidation and spin states, as well as in the geometries, of Fe and Mn, but these differences have little influence on the energetics, and therefore on the reaction mechanism of the two types of superoxide dismutases. For example, the two metals have very similar reduction potentials in the active-site models, although they differ by 0.7 V in water solution. The reaction mechanisms and spin states seem to have been designed to avoid spin conversions or to facilitate them by employing nearly degenerate spin states.     

Cubic assembly of a geometrically frustrated {Fe12} spin cluster

We describe an S(4)-symmetric {Fe(12)} spin cluster [Fe(12)O(4)(OH)(2)(L)(4)(OAc)(8)][Cl](2) {H(4)L = (HOCH(2)CH(2))(2)NCH(2)CH(2)N(CH(2)CH(2)OH)(2)} where the iron(III) centres describe a squashed hexagonal antiprism. The clusters pack into a large cubic cell with circular cavities, lined by weak C-H···O interactions, and a unit cell volume of over 60,000 ?(3) containing large solvent accessible voids. The core of the cluster is stable in solution, as confirmed by electrospray mass spectrometry. The cluster possesses a non-trivial, frustrated S = 0 ground state, due to the presence of multiple competing antiferromagnetic interactions. The finite temperature Lanczos method has been employed to calculate the temperature dependent magnetic properties of an analogous dodecanuclear S(i) = 3/2 model spin system, in order to reduce the very large Hilbert space. Three archetypal models with two independent exchange coupling parameters have been employed that render a low temperature feature possible, as seen in the χ vs. T plot for the {Fe(12)} spin cluster.     

Stable cation inversion at the MgAl2O4(100) surface

From an interplay of atom-resolved noncontact atomic force microscopy, surface x-ray diffraction experiments, and density functional theory calculations, we reveal the detailed atomic-scale structure of the (100) surface of an insulating ternary metal oxide, MgAl2O4 (spinel). We surprisingly find that the MgAl2O4(100) surface is terminated by an Al and O-rich structure with a thermodynamically favored amount of Al atoms interchanged with Mg. This finding implies that so-called Mg-Al antisites, which are defects in the bulk of MgAl2O4, become a thermodynamically stable and integral part of the surface.     

Chemical degradation of an uncrosslinked pure fluororubber in an alkaline environment

The chemical degradation of an uncrosslinked pure fluoroelastomer (FKM; Viton A) in an alkaline environment (10% NaOH and 80 °C) was investigated. Scanning electron microscopy images showed that on a microscopic level, significant degradation substantially increased the surface roughness after prolonged exposure (e.g., 12 weeks). The molecular mechanisms of the chemical degradation processes at the surface were evaluated with X‐ray photoelectron spectroscopy and attenuated total reflectance/Fourier transform infrared spectroscopy. The results revealed that the early degradation proceeded primarily via dehydrofluorination reactions, creating double bonds in the rubber backbone. This further accelerated the degradation after longer exposure times. Furthermore, the resulting double bonds underwent nucleophilic attack by an aqueous NaOH solution to form several oxygenated species. All these species ultimately recombined to form crosslinks, as evidenced by the increase in the gel fraction and surface hardness (Shore A). The pronounced effect of chemical degradation through a reduction in the thermal stability of the pure FKM rubber upon exposure was also evident from thermogravimetric analysis and differential thermogravimetry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6216–6229, 2004