Switchable electronic coupling in model oligoporphyrin molecular wires examined through the measurement and assignment of electronic absorption spectra

The use of a quinone functionality in the linkage unit of laterally bridged oligoporphyrins as a switch for controlling electronic coupling between the termini is examined. The quinone-bridged bisporphyrin P(2)TA-O(2) was synthesized by condensation of 2 equiv of the dione 2,3-dioxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin with 2,3,5,6-tetraamino-1,4-benzoquinone. The electronic absorption spectra of P(2)TA-O(2) and its conjugated benzenoid analogue P(2)TA are measured and assigned, in conjunction with the spectra of the fragment monomers and porphyrin-bridge compounds. Band homologies and CASPT2 calculations are used to make the assignments. Chemically, the dimer in one case is bridged by a through-conjugated, pi-delocalized 1,4,5,8-tetraazaanthracene molecule. This is shown to display significant inter-porphyrin coupling, with an observed difference in the exciton couplings of the B(x) and B(y) bands being ca. 0.18 eV. However, the other dimer is bridged using a derivative in which the central ring is converted to a cross-conjugated, pi-localized quinonoid form; this molecule displays no observable inter-porphyrin coupling. This scenario provides a paradigm for the use of molecular electronic devices in sensing, control, and high-capacity relatively low-speed data storage applications.     

Polyacidic multiloading metal extractants

Novel polynucleating, di- and tri-acidic ligands have been designed to increase the molar and mass transport efficiencies for the recovery of base metals by solvent extraction.     

Stable aqueous nanoparticle film assemblies with covalent and charged polymer linking networks

The construction of highly stable and efficiently assembled multilayer films of purely water soluble gold nanoparticles is reported. Citrate-stabilized nanoparticles (CS-NPs) of average core diameter of 10 nm are used as templates for stabilization-based exchange reactions with thioctic acid to form more robust aqueous NPs that can be assembled into multilayer films. The thioctic acid stabilized nanoparticles (TAS-NPs) are networked via covalent and electrostatic linking systems, employing dithiols and the cationic polymer poly(L-lysine), respectively. Multilayer films of up to 150 nm in thickness are successfully grown at biological pH with no observable degradation of the NPs within the film. The characteristic surface plasmon band, an optical feature of certain NP film assemblies that can be used to report the local environment and core spacing within the film, is preserved. Growth dynamics and film stability in solution and in the air are examined, with poly(L-lysine) linked films showing no evidence of aggregation for at least 50 days. We believe these films represent a pivotal step toward exploring the potential of aqueous NP film assemblies as a sensing apparatus.     

Consistent predictable patterns in the hydrogen and oxygen stable isotope ratios of animal proteins consumed by modern humans in the USA

Published datasets of proteinaceous animal tissues suggest that co-variation between amino acid hydrogen (δ2H) and oxygen (δ1?O) isotope ratios is a common feature in systems where isotopic variation is driven by geographic or temporal variation in the δ2H and δ1?O values of environmental water. This has led to the development of models relating tissue δ2H and δ1?O values to those of water, with potential application in a number of fields. However, the strength and ubiquity of the influence of environmental water on protein isotope ratios across taxonomic groups, and thus the relevance of predictive models, is an open question. Here we report strong co-variation of δ2H and δ1?O values across a suite of terrestrial and aquatic animal meats purchased in American food markets, including beef, poultry (chicken and turkey), chicken eggs, pork, lamb, freshwater fish, and marine fish. Significant isotope co-variation was not found for small collections of marine bivalves and crustaceans. These results imply that isotopic signals from environmental water were propagated similarly through most of the diverse natural and human-managed foodwebs represented by our samples. Freshwater fish had the largest variation in δ2H and δ1?O values, with ranges of 121‰ and 19.2‰, respectively, reflecting the large isotopic variation in environmental freshwaters. In contrast marine animals had the smallest variation for both δ2H (7‰ range, crustaceans) and δ1?O (3.0‰ range, bivalves) values. Known-origin beef samples demonstrated direct relationships between the variance of environmental water isotope ratios and that of collected meats.     

Computational comparison of the stacking interactions between the aromatic amino acids and the natural or (cationic) methylated nucleobases

The strongest gas-phase MP2/6-31G*(0.25) stacking energies between the aromatic amino acids and the natural or methylated nucleobases were considered. The potential energy surfaces of dimers were searched as a function of the vertical separation, angle of rotation and horizontal displacement between monomers stacked according to their centers of mass. Our calculations reveal that the stacking interactions of adducts for a given nucleobase are dependent on the methylation site (by up to 20 kJ mol(-1)), where the relative magnitudes of the interactions are determined by the dipole moments of the adducts and the proton affinities of nucleobase methylation sites. Nevertheless, the differences in the (gas-phase) stacking of methylated adducts are small compared with the differences between the stacking of the corresponding natural and methylated nucleobases. Indeed, methylation increases the stacking energy by up to 40 kJ mol(-1) (or 135%). Although immersing the dimers in different solvents decreases the gas-phase stacking energies with an increase in the polarity of the environment, base methylation still has a significant effect on the nucleobase stacking ability in solvents with large dipole moments, and, perhaps more importantly, environments that mimic enzyme active sites. Our results shed light on the workings of DNA repairs enzymes that selectively remove a wide variety of alkylated nucleobases over the natural bases.     

Engineered biosynthesis of hybrid macrolide polyketides containing D-angolosamine and D-mycaminose moieties

The glycosylation of natural product scaffolds with highly modified deoxysugars is often essential for their biological activity, being responsible for specific contacts to molecular targets and significantly affecting their pharmacokinetic properties. In order to provide tools for the targeted alteration of natural product glycosylation patterns, significant strides have been made to understand the biosynthesis of activated deoxysugars and their transfer. We report here efforts towards the production of plasmid-borne biosynthetic gene cassettes capable of producing TDP-activated forms of D-mycaminose, D-angolosamine and D-desosamine. We additionally describe the transfer of these deoxysugars to macrolide aglycones using the glycosyl transferases EryCIII, TylMII and AngMII, which display usefully broad substrate tolerance.     

Molecular approaches to the electrochemical reduction of carbon dioxide

This article reviews recent progress in the exploitation of carbon dioxide as a chemical feedstock. In particular, the design and development of molecular complexes that can act as catalysts for the electrochemical reduction of CO(2) is highlighted, and compared to other biological, metal- and non-metal-based systems.     

Marked difference in the electronic structure of cyanide-ligated ferric protoglobins and myoglobin due to heme ruffling

Electron paramagnetic resonance experiments reveal a significant difference between the principal g values (and hence ligand-field parameters) of the ferric cyanide-ligated form of different variants of the protoglobin of Methanosarcina acetivorans (MaPgb) and of horse heart myoglobin (hhMb). The largest principal g value of the ferric cyanide-ligated MaPgb variants is found to be significantly lower than for any of the other globins reported so far. This is at least partially caused by the strong heme distortions as proven by the determination of the hyperfine interaction of the heme nitrogens and mesoprotons. Furthermore, the experiments confirm recent theoretical predictions [Forti, F.; Boechi, L., Bikiel, D., Martí, M.A.; Nardini, M.; Bolognesi, M.; Viappiani, C.; Estrin, D.; Luque, F. J. J. Phys. Chem. B2011, 115, 13771-13780] that Phe(G8)145 plays a crucial role in the ligand modulation in MaPgb. Finally, the influence of the N-terminal 20 amino-acid chain on the heme pocket in these protoglobins is also proven.