Jaroslav Zajicek

Structure and compatibility of a magnesium electrolyte with a sulphur cathode.

Magnesium metal is an ideal rechargeable battery anode material because of its high volumetric energy density, high negative reduction potential and natural abundance. Coupling Mg with high capacity, low-cost cathode materials such as electrophilic sulphur is only possible with a non-nucleophilic electrolyte. Here we show how the crystallization of the electrochemically active species formed from the reaction between hexamethyldisilazide magnesium chloride and aluminum trichloride enables the synthesis of a non-nucleophilic electrolyte. Furthermore, crystallization was essential in the identification of the electroactive species, [Mg2(μ-Cl)3·6THF]+, and vital to improvements in the voltage stability and coulombic efficiency of the electrolyte. X-ray photoelectron spectroscopy analysis of the sulphur electrode confirmed that the electrochemical conversion between sulphur and magnesium sulfide can be successfully performed using this electrolyte.

Electrolyte roadblocks to a magnesium rechargeable battery

Low cost, non-dendritic magnesium metal is an ideal anode for a post lithium ion battery. Currently, development of magnesium electrolytes governs the rate of progress in this field, because electrolyte properties determine the class of cathodes utilized. A review of the latest progress in the area of magnesium battery electrolyte and a perspective on mitigating present challenges is presented herein. Firstly, density functional theory has been shown to predict the potential window of magnesium electrolytes on inert electrodes. Secondly, we report initial efforts aimed to overcome the corrosive property of these magnesium organohaloaluminates towards less noble metals such as stainless steel. This is a major challenge in developing high voltage magnesium electrolytes essential for batteries which operate above 3V. We lastly touch on cathode candidates including the insertion and conversion classes. One conversion cathode we pay particular attention to is electrophilic sulfur which can be married with magnesium metal anodes by utilizing non-nucleophilic electrolytes obtained by simple crystallization of in situ generated magnesium organohaloaluminates. Effectively, non-nucleophilic electrolytes open the door to research on magnesium/sulfur batteries.

Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation

Investigation of the deuterium (2H) nuclear magnetic resonance (NMR) relaxation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R1Z) and quadrupolar order (R1Q) 2H NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 °C. The data for various segmental positions along the DMPC-d54 acyl chain were simultaneously fitted to a composite membrane deformation model, including fast segmental motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d54 in the liquid-crystalline (Lα) phase, for the DMPC-d54:cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupola...

Corrosion of magnesium electrolytes: chlorides - the culprit

Chloride containing magnesium electrolytes are corrosive towards non noble metals. Currently the development of non-corrosive magnesium electrolytes is a key challenge on the road to a rechargeable magnesium battery. The component responsible for corrosion of magnesium electrolytes has not been previously elucidated. Here we clarify that chlorides in the cation (Mg2(μ-Cl)3·6THF)+ are a major culprit for corrosion. We also corroborate the feasibility of ion exchange reactions as a suitable synthetic approach towards magnesium electrolytes which do not contain the cation (Mg2(μ-Cl)3·6THF)+. Our results indicate that magnesium organoborates are an interesting class of magnesium electrolytes which undergo magnesium deposition and dissolution and are non-corrosive in nature at high voltages.

Stereochemistry of the palladium-catalyzed allylic substitution: the syn-anti dichotomy in the formation of (π-allyl)palladium complexes and their equilibration☆

Abstract The mechanism of palladium(0)-catalyzed allylic substitution has been investigated with the aim of finding whether or not the intermediate (π-allyl)palladium complexes can arise in a syn fashion as an alternative to the well known anti-mechanism. Using (diphenylphosphino)acetate as a leaving group and stereochemically biased substrates 30b and 35b evidence for the syn stereochemistry has been acquired (30b → 31 and 35b → 36). This reversal of stereochemistry is facilitated by severe steric congestion in the starting allylic esters (which impairs the ordinary anti-mechanism) and is boosted by the pre-coordination of the Pd(0) reagent to the leaving group. The latter effect apparently lowers the activation entropy. With cyclohexene derivatives 10b, 18b, and 19b and acyclic substrate 25b, where steric hindrance does not operate, the anti-mechanism producing η3-complexes dominates even for (diphenylphosphino)acetates. At elevated temperature, rapid equilibration of η3-complexes (13 ⇄ 14 and 20 ⇄ 21) has been observed prior to the reaction with a nucleophile. This effect has been attributed to the presence of (diphenylphosphino)acetate ion acting as a ligand for palladium.

Fast field‐cycling nuclear magnetic resonance spectrometer

We describe here the design and construction of a modern, state‐of‐the‐art nuclear magnetic resonance (NMR) field‐cycling instrument. Fourier transform NMR spectra of both liquid and solid samples can be measured, and spin‐lattice relaxation times (T1Z) investigated over a broad range of magnetic field strengths ranging from 0 to 2 T. The instrument is based upon an existing personal computer‐based NMR spectrometer [C. Job, R. M. Pearson, and M. F. Brown, Rev. Sci. Instrum. 65, 3354 (1994)] which has been expanded into a fully computer‐controlled field‐cycling instrument. The magnetic field cycling is accomplished electronically by utilizing fast switching thyristors and a storage capacitor based on the Redfield energy storage concept. Unique aspects of the design include the field‐cycling magnet, which can reach fields as high as 2 T; the personal computer‐based NMR spectrometer and associated waveform electronics; and the use of a commercially available pulse width modulation switching current amplifier...

Antifungal terpenoids from Chenopodium ambrosioides.

Abstract Two monoterpenes with antifungal activity were isolated from Chenopodium ambrosioides . Structures were elucidated by spectroscopic methods.

TCR Scanning of Peptide/MHC through Complementary Matching of Receptor and Ligand Molecular Flexibility

Although conformational changes in TCRs and peptide Ags presented by MHC protein (pMHC) molecules often occur upon binding, their relationship to intrinsic flexibility and role in ligand selectivity are poorly understood. In this study, we used nuclear magnetic resonance to study TCR–pMHC binding, examining recognition of the QL9/H-2Ld complex by the 2C TCR. Although the majority of the CDR loops of the 2C TCR rigidify upon binding, the CDR3β loop remains mobile within the TCR–pMHC interface. Remarkably, the region of the QL9 peptide that interfaces with CDR3β is also mobile in the free pMHC and in the TCR–pMHC complex. Determination of conformational exchange kinetics revealed that the motions of CDR3β and QL9 are closely matched. The matching of conformational exchange in the free proteins and its persistence in the complex enhances the thermodynamic and kinetic stability of the TCR–pMHC complex and provides a mechanism for facile binding. We thus propose that matching of structural fluctuations is a component of how TCRs scan among potential ligands for those that can bind with sufficient stability to enable T cell signaling.

PHENYLBUTANOID AND TAXANE-LIKE METABOLITES FROM NEEDLES OF TAXUS-BREVIFOLIA

Abstract The structures for brevifoliol and three baccatin VI derivatives were revised to possess the novel 11(15→1)-abeotaxane tricylic skeleton based on X-ray crystallographic studies. Four more new rearranged taxane derivatives related to brevifoliol and the unusual phenylbutanoid, (−)-rhododendrol, were isolated from the needles of Taxus brevifolia . Their structures were established as 10β-benzoxy-5α-(3′-dimethylamino-3′-phenyl)-propanoxy-1β -hydroxy-7β,9α,13α-triacetoxy-11(15→1)-abeotaxa-4(20), 11-diene; 10β-benzoxy-1β-hydroxy-5α-(3′-methylamino-3′-pheny) -propanoxy-7β,9α,13α-triacetoxy-11(15→1) -abeotaxa-4(20),11-diene; 10β-benzoxy-5α-cinnamoxy-1β-hydroxy-7β, 9α,13α-triacetoxy-11(15→1)-abeotaxa-4(20),11-diene; 10β-benzoxy-1β,5α-dihydroxy-7β,9α,13α-triacetoxy -11(15→1) -abeotaxa-4(20),11-diene and 2( R )-hydroxy-4-(4′-hydroxyphenyl)-butane.

Sequence Requirements for theN-Methyl-d-aspartate Receptor Antagonist Activity of Conantokin-R

Conantokin-R (con-R), a γ-carboxyglutamate-containing 27-residue peptide, is a natural peptide inhibitor of the N-methyl-d-aspartate (NMDA) subtype glutamate receptor. Synthetic analogs of con-R were generated to evaluate the importance of the individual structural elements of this peptide in its NMDA receptor antagonist activity, measured by inhibition of the spermine-enhanced binding of the NMDA receptor-specific channel blocker, [3H]MK-801, to rat brain membranes. Progressive C-terminal truncations of the 27-residue peptide revealed stages of severe activity loss. These occurred at con-R[1–11] and con-R[1–7], corresponding to the deletions of Leu12–Pro27 and Met8–Pro27 respectively. A second set of analogs featured single Ala substitutions in the fully active con-R[1–17] fragment. The replacement of Met8 and Leu12 by Ala resulted in approximate 20- and 55-fold decreases of inhibitor potency, respectively. In addition to these two residues, the only other positions where a single Ala substitution led to substantial losses (from 11-fold to >1000-fold) of activity were those of the first five N-terminal amino acids. Based on the above findings, the binding epitope of con-R was localized to the N-terminal turn of the helix and other residues on one face along two subsequent turns. This contribution pattern of the side chains in activity closely resembles the results obtained with another member of this peptide family, conantokin-T. The secondary structure and metal ion binding properties of the con-R variants were also evaluated using circular dichroism spectroscopy. Divalent cation-dependent increases of α-helix content were observed in most analogs. However, analogs with replacement of Gla11 and Gla15, as well as truncation fragments shorter than 15 residues, lost the ability to be stabilized by metal ions. These results confirmed the location of the primary divalent cation binding locus at Gla11 and Gla15. Additional interactions were indicated by the reduced α-helix stability in the Ala analogs of Gla4, Lys7, and Arg14.