Coordination-induced O-H bond weakening in Sm(ii)-water complexes.

The combination of Sm(ii)-based reductants with water provides reagent systems that promote a range of reductions and bond-forming reactions that are endergonic based on known redox potentials. Despite the utility of these reagent systems, little work has been done to elucidate the basis for this unusual reactivity. Herein we present recent work designed to explore the underlying mechanistic basis for the unexpected reactivity of the Sm(ii)-water reducing system and demonstrate that O-H bond weakening induced by coordination of water to low valent samarium enables proton coupled electron transfer to substrates. We also demonstrate that coordination-induced bond weakening can be exploited and used to create extremely powerful reductants capable of reducing functional groups typically recalcitrant to reduction through single electron transfer alone.

Influence of HMPA on reducing power and reactivity of SmBr(2).

[reaction: see text] Addition of HMPA to SmBr(2) in THF produces a reactant with an estimated redox potential of -2.63 +/- 0.01 V (vs Ag/AgNO(3)). This powerful reductant is capable of reducing ketimines and alkyl chlorides at room temperature. Although the structure of the reductant has not been established, it is nonetheless a powerful addition to the arsenal of samarium-based reductants currently utilized.

Sequence specificity of hydrogen-bonded molecular duplexes.

Hydrogen-bonded molecular duplexes, 1.3 and 1.4, each of which contains a mismatched binding site (acceptor-to-acceptor in 1.3, and donor-to-donor in 1.4), were designed and synthesized based on duplex 1.2. One- and two-dimensional NMR studies demonstrated that, despite their single mismatched binding sites, the backbones of duplexes 1.3 and 1.4 still stayed in register through the formation of the remaining five H-bonds. The backbones of 1.3 and 1.4 adjusted to the presence of the mismatched binding sites by slightly twisting around these sites, which alleviate any head-on repulsive interactions between two H-bond donors (amide O) or between two acceptors (amide H). After 1 equiv of single strand 2, which forms a perfectly matched duplex 1.2 with single strand 1, was added into the solution of either 1.3 or 1.4, only 1.2 and single strand 3 or 4, were detected. Isothermal titration calorimetry (ITC, in chloroform containing 5% DMSO) indicated that duplexes 1.3 and 1.4 were significantly (>40 times) less stable than the corresponding perfectly hydrogen-bonded duplex 1.2. These NMR and ITC results indicate that the pairing of two complementary single strands is not affected by another very similar single strand that contains only one wrong H-bond donor or acceptor, which demonstrates that the self-assembly of this class of H-bonded duplexes is a highly sequence-specific process. The role of these H-bonded duplexes as predictable and programmable molecular recognition units for directing intermolecular interactions has thus been established.

Protein renaturation by the liquid organic salt ethylammonium nitrate.

The room-temperature liquid salt, ethylammonium nitrate (EAN), has been used to enhance the recovery of denatured-reduced hen egg white lysozyme (HEWL). Our results show that EAN has the ability to prevent aggregation of the denatured protein. The use of EAN as a refolding additive is advantageous because the renaturation is a one-step process. When HEWL was denatured reduced using routine procedures and renatured using EAN as an additive, HEWL was found to regain 75% of its activity. When HEWL was denatured and reduced in neat EAN, dilution resulted in over 90% recovery of active protein. An important aspect of this process is that renaturation of HEWL occurs at concentrations of 1.6 mg/mL, whereas other renaturation processes occur at significantly lower protein concentrations. Additionally, the refolded-active protein can be separated from the molten salt by simple desalting methods. Although the use of a low-temperature molten salt in protein renaturation is unconventional, the power of this approach lies in its simplicity and utility.

Guest and subunit exchange in self-assembled ionophores.

[reaction: see text] Self-assembled ionophores, formed by hydrogen bonding of isoG 1 around a cation, are dynamic structures. Multinuclear NMR spectroscopy in CD(3)CN-CDCl(3) showed that cation exchange is >10(4) faster than exchange of isoG 1 ligand in (isoG 1)(10)-Cs(+) Ph(4)B(-). The cationic guest also affected the kinetic stability of the complex. 2D-EXSY NMR experiments in CDCl(3) showed that ligand exchange was 2 orders of magnitude faster for the Li(+)-decamer than for the Cs(+)-decamer.

Ethylammonium nitrate: a protein crystallization reagent.

Ethylammonium nitrate (EAN) is a liquid organic salt that has many potential applications in protein chemistry. Because this solvent has hydrophobic and ionic character as well as the ability to hydrogen bond, it is especially well suited for broad use in protein crystallography. For example, EAN may be used as an additive, a detergent, a precipitating agent or to deliver ligands into protein crystals. A discussion of the crystallization of lysozyme using EAN as a precipitating agent is given here.

Structural and thermochemical characterization of lipoxygenase-catechol complexes.

A complex between native, iron(II) soybean lipoxygenase 3 and 4-nitrocatechol, a known inhibitor of the enzyme, has been detected by isothermal titration calorimetry and characterized by X-ray crystallography. The compound moors in the central cavity of the protein close to the essential iron atom, but not in a bonding arrangement with it. The iron ligands experience a significant rearrangement upon formation of the complex relative to their positions in the native enzyme; a water molecule becomes bound to iron in the complex, and one histidine ligand moves away from the iron to become involved in a hydrogen bonding interaction with the catechol. These changes in position result in a trigonal pyramid coordination geometry for iron in the complex. Molecular modeling and force field calculations predict more than one stable complex between 4-nitrocatechol and the central cavity of lipoxygenase 3, but the interaction having the small molecule in the same orientation as the one found in the crystal structure was the most favorable. These observations reveal specific details of the interaction between lipoxygenase and a small molecule and raise the possibility that changes in the ligand environment of the iron atom could be a feature of the product activation reaction or the catalytic mechanism.

Necrotizing pneumonia after pharyngitis due to Fusobacterium necrophorum.

A case of necrotizing pneumonia secondary to Fusobacterium necrophorum is reported. This anaerobic infection commonly originates in the upper respiratory tract and is often accompanied by multiple system disease due to hematogenous seeding. When the lungs are involved, diffuse necrotizing pneumonia with pleural effusions and cavitation results. The course is prolonged, and the diagnosis is frequently delayed. With appropriate antibiotics, the prognosis is good.