HF/6‐31G* energy surfaces for disaccharide analogs

The HF/6‐31G* level of theory was used to calculate relaxed potential energy surfaces for 12 analogs of disaccharides. The analogs were made by replacing glucose with tetrahydropyran and fructose with 2‐methyltetrahydrofuran. Molecules had zero, one or two anomeric carbon atoms, and di‐axial, axial‐equatorial, and di‐equatorial linkages. Despite the absence of hydroxyl groups, the surfaces account well for conformations that are observed in crystals of the parent disaccharides. Thus, torsional energy and the simple bulk of ring structures are major factors in determining disaccharide conformation. The contour shapes around the global minima depend on the number of anomeric carbons involved in the linkage, while the presence of alternative minima that have relative energies less than 4 kcal/mol mostly requires equatorial bonds. However, molecules with two adjacent anomeric centers gave exceptions to these rules. Flexibility values related to a partition function show that the di‐axial trehalose analog is the most rigid. The di‐equatorial pseudodisaccharide analog with no anomeric centers is most flexible. Reproduction of these surfaces is proposed as a simple test of force fields for modeling carbohydrates. Also, these surfaces can be used in a simple hybrid method for calculating disaccharide energy surfaces. © 2000 John Wiley & Sons, Inc. * J Comput Chem 22: 65–78, 2001     

A site-differentiated [4Fe–4S] cluster controls electron transfer reactivity of Clostridium acetobutylicum [FeFe]-hydrogenase I

One of the many functions of reduction–oxidation (redox) cofactors is to mediate electron transfer in biological enzymes catalyzing redox-based chemical transformation reactions. There are numerous examples of enzymes that utilize redox cofactors to form electron transfer relays to connect catalytic sites to external electron donors and acceptors. The compositions of relays are diverse and tune transfer thermodynamics and kinetics towards the chemical reactivity of the enzyme. Diversity in relay design is exemplified among different members of hydrogenases, enzymes which catalyze reversible H₂ activation, which also couple to diverse types of donor and acceptor molecules. The [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI) is a member of a large family of structurally related enzymes where interfacial electron transfer is mediated by a terminal, non-canonical, His-coordinated, [4Fe–4S] cluster. The function of His coordination was examined by comparing the biophysical properties and reactivity to a Cys substituted variant of CaI. This demonstrated that His coordination strongly affected the distal [4Fe–4S] cluster spin state, spin pairing, and spatial orientations of molecular orbitals, with a minor effect on reduction potential. The deviations in these properties by substituting His for Cys in CaI, correlated with pronounced changes in electron transfer and reactivity with the native electron donor–acceptor ferredoxin. The results demonstrate that differential coordination of the surface localized [4Fe–4S]His cluster in CaI is utilized to control intermolecular and intramolecular electron transfer where His coordination creates a physical and electronic environment that enables facile electron exchange between electron carrier molecules and the iron–sulfur cluster relay for coupling to reversible H₂ activation at the catalytic site.

Histidine coordination of the distal [4Fe–4S] cluster in [FeFe]-hydrogenase was demonstrated to tune the cluster spin-states, spin-pairing and surrounding molecular orbitals to enable more facile electron transfer compared to cysteine coordination.     

Luminescent bacteria-based sensing method for methylmercury specific determination

A bacterial biosensor method for the selective determination of a bioavailable organomercurial compound, methylmercury, is presented. A recombinant luminescent whole-cell bacterial strain responding to total mercury content in samples was used. The bacterial cells were freeze-dried and used as robust, reagent-like compounds, without batch-to-batch variations. In this bacteria-based sensing method, luciferase is used as a reporter, which requires no substrate additions, therefore allowing homogenous, real-time monitoring of the reporter gene expression. A noninducible, constitutively light-producing control bacterial strain was included in parallel for determining the overall cytotoxicity of the samples. The specificity of the total mercury sensor Escherichia coli MC1061 (pmerRBlux) bacterial resistance system toward methylmercury is due to a coexpressed specific enzyme, organomercurial lyase. This enzyme mediates the cleavage of the carbon–mercury bond of methylmercury to yield mercury ions, which induce the reporter genes and produce a self-luminescent cell. The selective analysis of methylmercury with the total mercury strain is achieved by specifically chelating the inorganic mercury species from the sample using an optimized concentration of EDTA as a chelating agent. After the treatment with the chelating agent, a cross-reactivity of 0.2% with ionic mercury was observed at nonphysiological ionic mercury concentrations (100 nM). The assay was optimized to be performed in 3 h but results can already be read after 1 h incubation. Total mercury strain E. coli MC1061 (pmerRBlux) has been shown to be highly sensitive and capable of determining methylmercury at a subnanomolar level in optimized assay conditions with a very high dynamic range of two decades. The limit of detection of 75 ng/l (300 pM) allows measurement of methylmercury even from natural samples.     

Synthesis and self‐assembly in water of coil‐rod‐coil amphiphilic block copolymers with central π‐conjugated sequence

The purpose of this study is to correlate the nano‐organization in water of coil‐rod‐coil amphiphilic block copolymers constituted of a conjugated segment to their optoelectronic properties. The ABA block copolymer structures, easily achieved via coupling reactions, are based on conjugated rod of dihexylfluorene and 3,4‐ethylenedioxythiophene units linked to two flexible poly(ethylene oxide) or poly[(ethylene oxide)‐ran‐(propylene oxide)] chains. These well‐defined copolymers exhibited a range of specific morphologies in water, a good solvent of coil blocks and a bad solvent of the conjugated rod. Particularly, vesicles and micelles with spherical, cylindrical, or elongated shape were noticed. Correlations were attempted to be established between the weight percent of the conjugated sequence contained in the copolymers, the morphology of the nanostructures obtained by self‐assembly in solution and the resulting optical properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4602–4616, 2008     

N-Pyridinium imidates as new sources of 2-aminoimidazole and imidazoline derivatives

New 2-aminoimidazole (2-AI) and imidazoline derivatives were obtained in three steps through the reduction of N-pyridinium imidates into 1,2-dihydropyridine imidates and oxidative addition of guanidine derivatives. Among the possible transformations, imidate substitution allows selectivity in the last deprotection step, leading to an original 2-aminoimidazolo-imidazoline skeleton.