Formation Mechanism of Nanostructured Metal Carbides via Salt-Flux Synthesis

Nanostructured metal carbides are of particular interest because of their potential as high surface area, low-cost catalysts. By taking advantage of a salt-flux synthesis method, multiple carbide compounds were synthesized at low temperatures providing a pathway to nanosized materials. To better understand the reaction mechanism, vanadium carbide (V₈C₇) synthesis was monitored by quenching samples at 100 °C intervals and analyzed by multiple spectroscopic methods. The reaction was determined to occur through the formation of metal halide and acetylide carbide intermediates, which were repeatedly observed by X-ray diffraction and further supported by IR and Raman spectroscopies. Control experiments were also employed to further verify this mechanism of formation by using different salt compositions and a solid-state metathesis reaction. The reaction mechanism was also verified by applying these techniques to other metal carbide systems, which produced similar intermediate compounds

\u3ci\u3eDrosophila\u3c/i\u3e Muller F Elements Maintain a Distinct Set of Genomic Properties Over 40 Million Years of Evolution

The Muller F element (4.2 Mb, ~80 protein-coding genes) is an unusual autosome of Drosophila melanogaster; it is mostly heterochromatic with a low recombination rate. To investigate how these properties impact the evolution of repeats and genes, we manually improved the sequence and annotated the genes on the D. erecta, D. mojavensis, and D. grimshawi F elements and euchromatic domains from the Muller D element. We find that F elements have greater transposon density (25–50%) than euchromatic reference regions (3–11%). Among the F elements, D. grimshawi has the lowest transposon density (particularly DINE-1: 2% vs. 11–27%). F element genes have larger coding spans, more coding exons, larger introns, and lower codon bias. Comparison of the Effective Number of Codons with the Codon Adaptation Index shows that, in contrast to the other species, codon bias in D. grimshawi F element genes can be attributed primarily to selection instead of mutational biases, suggesting that density and types of transposons affect the degree of local heterochromatin formation. F element genes have lower estimated DNA melting temperatures than D element genes, potentially facilitating transcription through heterochromatin. Most F element genes (~90%) have remained on that element, but the F element has smaller syntenic blocks than genome averages (3.4–3.6 vs. 8.4–8.8 genes per block), indicating greater rates of inversion despite lower rates of recombination. Overall, the F element has maintained characteristics that are distinct from other autosomes in the Drosophila lineage, illuminating the constraints imposed by a heterochromatic milieu